Import GNU readline 5.0.

[dragonfly.git] / contrib / readline-5.0 / doc / rltech.texi

@comment %**start of header (This is for running Texinfo on a region.)
@setfilename rltech.info
@comment %**end of header (This is for running Texinfo on a region.)
@setchapternewpage odd

@ifinfo
This document describes the GNU Readline Library, a utility for aiding
in the consitency of user interface across discrete programs that need
to provide a command line interface.

Copyright (C) 1988-2004 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
pare preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end ifinfo

@node Programming with GNU Readline
@chapter Programming with GNU Readline

This chapter describes the interface between the @sc{gnu} Readline Library and
other programs.  If you are a programmer, and you wish to include the
features found in @sc{gnu} Readline
such as completion, line editing, and interactive history manipulation
in your own programs, this section is for you.

@menu
* Basic Behavior::      Using the default behavior of Readline.
* Custom Functions::    Adding your own functions to Readline.
* Readline Variables::                  Variables accessible to custom
                                        functions.
* Readline Convenience Functions::      Functions which Readline supplies to
                                        aid in writing your own custom
                                        functions.
* Readline Signal Handling::    How Readline behaves when it receives signals.
* Custom Completers::   Supplanting or supplementing Readline's
                        completion functions.
@end menu

@node Basic Behavior
@section Basic Behavior

Many programs provide a command line interface, such as @code{mail},
@code{ftp}, and @code{sh}.  For such programs, the default behaviour of
Readline is sufficient.  This section describes how to use Readline in
the simplest way possible, perhaps to replace calls in your code to
@code{gets()} or @code{fgets()}.

@findex readline
@cindex readline, function

The function @code{readline()} prints a prompt @var{prompt}
and then reads and returns a single line of text from the user.
If @var{prompt} is @code{NULL} or the empty string, no prompt is displayed.
The line @code{readline} returns is allocated with @code{malloc()};
the caller should @code{free()} the line when it has finished with it.
The declaration for @code{readline} in ANSI C is

@example
@code{char *readline (const char *@var{prompt});}
@end example

@noindent
So, one might say
@example
@code{char *line = readline ("Enter a line: ");}
@end example
@noindent
in order to read a line of text from the user.
The line returned has the final newline removed, so only the
text remains.

If @code{readline} encounters an @code{EOF} while reading the line, and the
line is empty at that point, then @code{(char *)NULL} is returned.
Otherwise, the line is ended just as if a newline had been typed.

If you want the user to be able to get at the line later, (with
@key{C-p} for example), you must call @code{add_history()} to save the
line away in a @dfn{history} list of such lines.

@example
@code{add_history (line)};
@end example

@noindent
For full details on the GNU History Library, see the associated manual.

It is preferable to avoid saving empty lines on the history list, since
users rarely have a burning need to reuse a blank line.  Here is
a function which usefully replaces the standard @code{gets()} library
function, and has the advantage of no static buffer to overflow:

@example
/* A static variable for holding the line. */
static char *line_read = (char *)NULL;

/* Read a string, and return a pointer to it.
   Returns NULL on EOF. */
char *
rl_gets ()
@{
  /* If the buffer has already been allocated,
     return the memory to the free pool. */
  if (line_read)
    @{
      free (line_read);
      line_read = (char *)NULL;
    @}

  /* Get a line from the user. */
  line_read = readline ("");

  /* If the line has any text in it,
     save it on the history. */
  if (line_read && *line_read)
    add_history (line_read);

  return (line_read);
@}
@end example

This function gives the user the default behaviour of @key{TAB}
completion: completion on file names.  If you do not want Readline to
complete on filenames, you can change the binding of the @key{TAB} key
with @code{rl_bind_key()}.

@example
@code{int rl_bind_key (int @var{key}, rl_command_func_t *@var{function});}
@end example

@code{rl_bind_key()} takes two arguments: @var{key} is the character that
you want to bind, and @var{function} is the address of the function to
call when @var{key} is pressed.  Binding @key{TAB} to @code{rl_insert()}
makes @key{TAB} insert itself.
@code{rl_bind_key()} returns non-zero if @var{key} is not a valid
ASCII character code (between 0 and 255).

Thus, to disable the default @key{TAB} behavior, the following suffices:
@example
@code{rl_bind_key ('\t', rl_insert);}
@end example

This code should be executed once at the start of your program; you
might write a function called @code{initialize_readline()} which
performs this and other desired initializations, such as installing
custom completers (@pxref{Custom Completers}).

@node Custom Functions
@section Custom Functions

Readline provides many functions for manipulating the text of
the line, but it isn't possible to anticipate the needs of all
programs.  This section describes the various functions and variables
defined within the Readline library which allow a user program to add
customized functionality to Readline.

Before declaring any functions that customize Readline's behavior, or
using any functionality Readline provides in other code, an
application writer should include the file @code{<readline/readline.h>}
in any file that uses Readline's features.  Since some of the definitions
in @code{readline.h} use the @code{stdio} library, the file
@code{<stdio.h>} should be included before @code{readline.h}.

@code{readline.h} defines a C preprocessor variable that should
be treated as an integer, @code{RL_READLINE_VERSION}, which may
be used to conditionally compile application code depending on
the installed Readline version.  The value is a hexadecimal
encoding of the major and minor version numbers of the library,
of the form 0x@var{MMmm}.  @var{MM} is the two-digit major
version number; @var{mm} is the two-digit minor version number. 
For Readline 4.2, for example, the value of
@code{RL_READLINE_VERSION} would be @code{0x0402}. 

@menu
* Readline Typedefs::   C declarations to make code readable.
* Function Writing::    Variables and calling conventions.
@end menu

@node Readline Typedefs
@subsection Readline Typedefs

For readabilty, we declare a number of new object types, all pointers
to functions.

The reason for declaring these new types is to make it easier to write
code describing pointers to C functions with appropriately prototyped
arguments and return values.

For instance, say we want to declare a variable @var{func} as a pointer
to a function which takes two @code{int} arguments and returns an
@code{int} (this is the type of all of the Readline bindable functions).
Instead of the classic C declaration

@code{int (*func)();}

@noindent
or the ANSI-C style declaration

@code{int (*func)(int, int);}

@noindent
we may write

@code{rl_command_func_t *func;}

The full list of function pointer types available is

@table @code
@item typedef int rl_command_func_t (int, int);

@item typedef char *rl_compentry_func_t (const char *, int);

@item typedef char **rl_completion_func_t (const char *, int, int);

@item typedef char *rl_quote_func_t (char *, int, char *);

@item typedef char *rl_dequote_func_t (char *, int);

@item typedef int rl_compignore_func_t (char **);

@item typedef void rl_compdisp_func_t (char **, int, int);

@item typedef int rl_hook_func_t (void);

@item typedef int rl_getc_func_t (FILE *);

@item typedef int rl_linebuf_func_t (char *, int);

@item typedef int rl_intfunc_t (int);
@item #define rl_ivoidfunc_t rl_hook_func_t
@item typedef int rl_icpfunc_t (char *);
@item typedef int rl_icppfunc_t (char **);

@item typedef void rl_voidfunc_t (void);
@item typedef void rl_vintfunc_t (int);
@item typedef void rl_vcpfunc_t (char *);
@item typedef void rl_vcppfunc_t (char **);

@end table

@node Function Writing
@subsection Writing a New Function

In order to write new functions for Readline, you need to know the
calling conventions for keyboard-invoked functions, and the names of the
variables that describe the current state of the line read so far.

The calling sequence for a command @code{foo} looks like

@example
@code{int foo (int count, int key)}
@end example

@noindent
where @var{count} is the numeric argument (or 1 if defaulted) and
@var{key} is the key that invoked this function.

It is completely up to the function as to what should be done with the
numeric argument.  Some functions use it as a repeat count, some
as a flag, and others to choose alternate behavior (refreshing the current
line as opposed to refreshing the screen, for example).  Some choose to
ignore it.  In general, if a
function uses the numeric argument as a repeat count, it should be able
to do something useful with both negative and positive arguments.
At the very least, it should be aware that it can be passed a
negative argument.

A command function should return 0 if its action completes successfully,
and a non-zero value if some error occurs.

@node Readline Variables
@section Readline Variables

These variables are available to function writers.

@deftypevar {char *} rl_line_buffer
This is the line gathered so far.  You are welcome to modify the
contents of the line, but see @ref{Allowing Undoing}.  The
function @code{rl_extend_line_buffer} is available to increase
the memory allocated to @code{rl_line_buffer}.
@end deftypevar

@deftypevar int rl_point
The offset of the current cursor position in @code{rl_line_buffer}
(the @emph{point}).
@end deftypevar

@deftypevar int rl_end
The number of characters present in @code{rl_line_buffer}.  When
@code{rl_point} is at the end of the line, @code{rl_point} and
@code{rl_end} are equal.
@end deftypevar

@deftypevar int rl_mark
The @var{mark} (saved position) in the current line.  If set, the mark
and point define a @emph{region}.
@end deftypevar

@deftypevar int rl_done
Setting this to a non-zero value causes Readline to return the current
line immediately.
@end deftypevar

@deftypevar int rl_num_chars_to_read
Setting this to a positive value before calling @code{readline()} causes
Readline to return after accepting that many characters, rather
than reading up to a character bound to @code{accept-line}.
@end deftypevar

@deftypevar int rl_pending_input
Setting this to a value makes it the next keystroke read.  This is a
way to stuff a single character into the input stream.
@end deftypevar

@deftypevar int rl_dispatching
Set to a non-zero value if a function is being called from a key binding;
zero otherwise.  Application functions can test this to discover whether
they were called directly or by Readline's dispatching mechanism.
@end deftypevar

@deftypevar int rl_erase_empty_line
Setting this to a non-zero value causes Readline to completely erase
the current line, including any prompt, any time a newline is typed as
the only character on an otherwise-empty line.  The cursor is moved to
the beginning of the newly-blank line.
@end deftypevar

@deftypevar {char *} rl_prompt
The prompt Readline uses.  This is set from the argument to
@code{readline()}, and should not be assigned to directly.
The @code{rl_set_prompt()} function (@pxref{Redisplay}) may
be used to modify the prompt string after calling @code{readline()}.
@end deftypevar

@deftypevar int rl_already_prompted
If an application wishes to display the prompt itself, rather than have
Readline do it the first time @code{readline()} is called, it should set
this variable to a non-zero value after displaying the prompt.
The prompt must also be passed as the argument to @code{readline()} so
the redisplay functions can update the display properly.
The calling application is responsible for managing the value; Readline
never sets it.
@end deftypevar

@deftypevar {const char *} rl_library_version
The version number of this revision of the library.
@end deftypevar

@deftypevar int rl_readline_version
An integer encoding the current version of the library.  The encoding is
of the form 0x@var{MMmm}, where @var{MM} is the two-digit major version
number, and @var{mm} is the two-digit minor version number.
For example, for Readline-4.2, @code{rl_readline_version} would have the
value 0x0402.
@end deftypevar

@deftypevar {int} rl_gnu_readline_p
Always set to 1, denoting that this is @sc{gnu} readline rather than some
emulation.
@end deftypevar

@deftypevar {const char *} rl_terminal_name
The terminal type, used for initialization.  If not set by the application,
Readline sets this to the value of the @env{TERM} environment variable
the first time it is called.
@end deftypevar

@deftypevar {const char *} rl_readline_name
This variable is set to a unique name by each application using Readline.
The value allows conditional parsing of the inputrc file
(@pxref{Conditional Init Constructs}).
@end deftypevar

@deftypevar {FILE *} rl_instream
The stdio stream from which Readline reads input.
If @code{NULL}, Readline defaults to @var{stdin}.
@end deftypevar

@deftypevar {FILE *} rl_outstream
The stdio stream to which Readline performs output.
If @code{NULL}, Readline defaults to @var{stdout}.
@end deftypevar

@deftypevar {rl_command_func_t *} rl_last_func
The address of the last command function Readline executed.  May be used to
test whether or not a function is being executed twice in succession, for
example.
@end deftypevar

@deftypevar {rl_hook_func_t *} rl_startup_hook
If non-zero, this is the address of a function to call just
before @code{readline} prints the first prompt.
@end deftypevar

@deftypevar {rl_hook_func_t *} rl_pre_input_hook
If non-zero, this is the address of a function to call after
the first prompt has been printed and just before @code{readline}
starts reading input characters.
@end deftypevar

@deftypevar {rl_hook_func_t *} rl_event_hook
If non-zero, this is the address of a function to call periodically
when Readline is waiting for terminal input.
By default, this will be called at most ten times a second if there
is no keyboard input.
@end deftypevar

@deftypevar {rl_getc_func_t *} rl_getc_function
If non-zero, Readline will call indirectly through this pointer
to get a character from the input stream.  By default, it is set to
@code{rl_getc}, the default Readline character input function
(@pxref{Character Input}).
@end deftypevar

@deftypevar {rl_voidfunc_t *} rl_redisplay_function
If non-zero, Readline will call indirectly through this pointer
to update the display with the current contents of the editing buffer.
By default, it is set to @code{rl_redisplay}, the default Readline
redisplay function (@pxref{Redisplay}).
@end deftypevar

@deftypevar {rl_vintfunc_t *} rl_prep_term_function
If non-zero, Readline will call indirectly through this pointer
to initialize the terminal.  The function takes a single argument, an
@code{int} flag that says whether or not to use eight-bit characters.
By default, this is set to @code{rl_prep_terminal}
(@pxref{Terminal Management}).
@end deftypevar

@deftypevar {rl_voidfunc_t *} rl_deprep_term_function
If non-zero, Readline will call indirectly through this pointer
to reset the terminal.  This function should undo the effects of
@code{rl_prep_term_function}.
By default, this is set to @code{rl_deprep_terminal}
(@pxref{Terminal Management}).
@end deftypevar

@deftypevar {Keymap} rl_executing_keymap
This variable is set to the keymap (@pxref{Keymaps}) in which the
currently executing readline function was found.
@end deftypevar 

@deftypevar {Keymap} rl_binding_keymap
This variable is set to the keymap (@pxref{Keymaps}) in which the
last key binding occurred.
@end deftypevar 

@deftypevar {char *} rl_executing_macro
This variable is set to the text of any currently-executing macro.
@end deftypevar

@deftypevar {int} rl_readline_state
A variable with bit values that encapsulate the current Readline state.
A bit is set with the @code{RL_SETSTATE} macro, and unset with the
@code{RL_UNSETSTATE} macro.  Use the @code{RL_ISSTATE} macro to test
whether a particular state bit is set.  Current state bits include:

@table @code
@item RL_STATE_NONE
Readline has not yet been called, nor has it begun to intialize.
@item RL_STATE_INITIALIZING
Readline is initializing its internal data structures.
@item RL_STATE_INITIALIZED
Readline has completed its initialization.
@item RL_STATE_TERMPREPPED
Readline has modified the terminal modes to do its own input and redisplay.
@item RL_STATE_READCMD
Readline is reading a command from the keyboard.
@item RL_STATE_METANEXT
Readline is reading more input after reading the meta-prefix character.
@item RL_STATE_DISPATCHING
Readline is dispatching to a command.
@item RL_STATE_MOREINPUT
Readline is reading more input while executing an editing command.
@item RL_STATE_ISEARCH
Readline is performing an incremental history search.
@item RL_STATE_NSEARCH
Readline is performing a non-incremental history search.
@item RL_STATE_SEARCH
Readline is searching backward or forward through the history for a string.
@item RL_STATE_NUMERICARG
Readline is reading a numeric argument.
@item RL_STATE_MACROINPUT
Readline is currently getting its input from a previously-defined keyboard
macro.
@item RL_STATE_MACRODEF
Readline is currently reading characters defining a keyboard macro.
@item RL_STATE_OVERWRITE
Readline is in overwrite mode.
@item RL_STATE_COMPLETING
Readline is performing word completion.
@item RL_STATE_SIGHANDLER
Readline is currently executing the readline signal handler.
@item RL_STATE_UNDOING
Readline is performing an undo.
@item RL_STATE_DONE
Readline has read a key sequence bound to @code{accept-line}
and is about to return the line to the caller.
@end table

@end deftypevar

@deftypevar {int} rl_explicit_arg
Set to a non-zero value if an explicit numeric argument was specified by
the user.  Only valid in a bindable command function.
@end deftypevar

@deftypevar {int} rl_numeric_arg
Set to the value of any numeric argument explicitly specified by the user
before executing the current Readline function.  Only valid in a bindable
command function.
@end deftypevar

@deftypevar {int} rl_editing_mode
Set to a value denoting Readline's current editing mode.  A value of
@var{1} means Readline is currently in emacs mode; @var{0}
means that vi mode is active.
@end deftypevar


@node Readline Convenience Functions
@section Readline Convenience Functions

@menu
* Function Naming::     How to give a function you write a name.
* Keymaps::             Making keymaps.
* Binding Keys::        Changing Keymaps.
* Associating Function Names and Bindings::     Translate function names to
                                                key sequences.
* Allowing Undoing::    How to make your functions undoable.
* Redisplay::           Functions to control line display.
* Modifying Text::      Functions to modify @code{rl_line_buffer}.
* Character Input::     Functions to read keyboard input.
* Terminal Management:: Functions to manage terminal settings.
* Utility Functions::   Generally useful functions and hooks.
* Miscellaneous Functions::     Functions that don't fall into any category.
* Alternate Interface:: Using Readline in a `callback' fashion.
* A Readline Example::          An example Readline function.
@end menu

@node Function Naming
@subsection Naming a Function

The user can dynamically change the bindings of keys while using
Readline.  This is done by representing the function with a descriptive
name.  The user is able to type the descriptive name when referring to
the function.  Thus, in an init file, one might find

@example
Meta-Rubout:    backward-kill-word
@end example

This binds the keystroke @key{Meta-Rubout} to the function
@emph{descriptively} named @code{backward-kill-word}.  You, as the
programmer, should bind the functions you write to descriptive names as
well.  Readline provides a function for doing that:

@deftypefun int rl_add_defun (const char *name, rl_command_func_t *function, int key)
Add @var{name} to the list of named functions.  Make @var{function} be
the function that gets called.  If @var{key} is not -1, then bind it to
@var{function} using @code{rl_bind_key()}.
@end deftypefun

Using this function alone is sufficient for most applications.
It is the recommended way to add a few functions to the default
functions that Readline has built in.
If you need to do something other than adding a function to Readline,
you may need to use the underlying functions described below.

@node Keymaps
@subsection Selecting a Keymap

Key bindings take place on a @dfn{keymap}.  The keymap is the
association between the keys that the user types and the functions that
get run.  You can make your own keymaps, copy existing keymaps, and tell
Readline which keymap to use.

@deftypefun Keymap rl_make_bare_keymap (void)
Returns a new, empty keymap.  The space for the keymap is allocated with
@code{malloc()}; the caller should free it by calling
@code{rl_discard_keymap()} when done.
@end deftypefun

@deftypefun Keymap rl_copy_keymap (Keymap map)
Return a new keymap which is a copy of @var{map}.
@end deftypefun

@deftypefun Keymap rl_make_keymap (void)
Return a new keymap with the printing characters bound to rl_insert,
the lowercase Meta characters bound to run their equivalents, and
the Meta digits bound to produce numeric arguments.
@end deftypefun

@deftypefun void rl_discard_keymap (Keymap keymap)
Free the storage associated with @var{keymap}.
@end deftypefun

Readline has several internal keymaps.  These functions allow you to
change which keymap is active.

@deftypefun Keymap rl_get_keymap (void)
Returns the currently active keymap.
@end deftypefun

@deftypefun void rl_set_keymap (Keymap keymap)
Makes @var{keymap} the currently active keymap.
@end deftypefun

@deftypefun Keymap rl_get_keymap_by_name (const char *name)
Return the keymap matching @var{name}.  @var{name} is one which would
be supplied in a @code{set keymap} inputrc line (@pxref{Readline Init File}).
@end deftypefun

@deftypefun {char *} rl_get_keymap_name (Keymap keymap)
Return the name matching @var{keymap}.  @var{name} is one which would
be supplied in a @code{set keymap} inputrc line (@pxref{Readline Init File}).
@end deftypefun

@node Binding Keys
@subsection Binding Keys

Key sequences are associate with functions through the keymap.
Readline has several internal keymaps: @code{emacs_standard_keymap},
@code{emacs_meta_keymap}, @code{emacs_ctlx_keymap},
@code{vi_movement_keymap}, and @code{vi_insertion_keymap}.
@code{emacs_standard_keymap} is the default, and the examples in
this manual assume that.

Since @code{readline()} installs a set of default key bindings the first
time it is called, there is always the danger that a custom binding
installed before the first call to @code{readline()} will be overridden.
An alternate mechanism is to install custom key bindings in an
initialization function assigned to the @code{rl_startup_hook} variable
(@pxref{Readline Variables}).

These functions manage key bindings.

@deftypefun int rl_bind_key (int key, rl_command_func_t *function)
Binds @var{key} to @var{function} in the currently active keymap.
Returns non-zero in the case of an invalid @var{key}.
@end deftypefun

@deftypefun int rl_bind_key_in_map (int key, rl_command_func_t *function, Keymap map)
Bind @var{key} to @var{function} in @var{map}.
Returns non-zero in the case of an invalid @var{key}.
@end deftypefun

@deftypefun int rl_bind_key_if_unbound (int key, rl_command_func_t *function)
Binds @var{key} to @var{function} if it is not already bound in the
currently active keymap.
Returns non-zero in the case of an invalid @var{key} or if @var{key} is
already bound.
@end deftypefun

@deftypefun int rl_bind_key_if_unbound_in_map (int key, rl_command_func_t *function, Keymap map)
Binds @var{key} to @var{function} if it is not already bound in @var{map}.
Returns non-zero in the case of an invalid @var{key} or if @var{key} is
already bound.
@end deftypefun

@deftypefun int rl_unbind_key (int key)
Bind @var{key} to the null function in the currently active keymap.
Returns non-zero in case of error.
@end deftypefun

@deftypefun int rl_unbind_key_in_map (int key, Keymap map)
Bind @var{key} to the null function in @var{map}.
Returns non-zero in case of error.
@end deftypefun

@deftypefun int rl_unbind_function_in_map (rl_command_func_t *function, Keymap map)
Unbind all keys that execute @var{function} in @var{map}.
@end deftypefun

@deftypefun int rl_unbind_command_in_map (const char *command, Keymap map)
Unbind all keys that are bound to @var{command} in @var{map}.
@end deftypefun

@deftypefun int rl_bind_keyseq (const char *keyseq, rl_command_func_t *function)
Bind the key sequence represented by the string @var{keyseq} to the function
@var{function}, beginning in the current keymap.
This makes new keymaps as necessary.
The return value is non-zero if @var{keyseq} is invalid.
@end deftypefun

@deftypefun int rl_bind_keyseq_in_map (const char *keyseq, rl_command_func_t *function, Keymap map)
Bind the key sequence represented by the string @var{keyseq} to the function
@var{function}.  This makes new keymaps as necessary.
Initial bindings are performed in @var{map}.
The return value is non-zero if @var{keyseq} is invalid.
@end deftypefun

@deftypefun int rl_set_key (const char *keyseq, rl_command_func_t *function, Keymap map)
Equivalent to @code{rl_bind_keyseq_in_map}.
@end deftypefun

@deftypefun int rl_bind_keyseq_if_unbound (const char *keyseq, rl_command_func_t *function)
Binds @var{keyseq} to @var{function} if it is not already bound in the
currently active keymap.
Returns non-zero in the case of an invalid @var{keyseq} or if @var{keyseq} is
already bound.
@end deftypefun

@deftypefun int rl_bind_keyseq_if_unbound_in_map (const char *keyseq, rl_command_func_t *function, Keymap map)
Binds @var{keyseq} to @var{function} if it is not already bound in @var{map}.
Returns non-zero in the case of an invalid @var{keyseq} or if @var{keyseq} is
already bound.
@end deftypefun

@deftypefun int rl_generic_bind (int type, const char *keyseq, char *data, Keymap map)
Bind the key sequence represented by the string @var{keyseq} to the arbitrary
pointer @var{data}.  @var{type} says what kind of data is pointed to by
@var{data}; this can be a function (@code{ISFUNC}), a macro
(@code{ISMACR}), or a keymap (@code{ISKMAP}).  This makes new keymaps as
necessary.  The initial keymap in which to do bindings is @var{map}.
@end deftypefun

@deftypefun int rl_parse_and_bind (char *line)
Parse @var{line} as if it had been read from the @code{inputrc} file and
perform any key bindings and variable assignments found
(@pxref{Readline Init File}).
@end deftypefun

@deftypefun int rl_read_init_file (const char *filename)
Read keybindings and variable assignments from @var{filename}
(@pxref{Readline Init File}).
@end deftypefun

@node Associating Function Names and Bindings
@subsection Associating Function Names and Bindings

These functions allow you to find out what keys invoke named functions
and the functions invoked by a particular key sequence.  You may also
associate a new function name with an arbitrary function.

@deftypefun {rl_command_func_t *} rl_named_function (const char *name)
Return the function with name @var{name}.
@end deftypefun

@deftypefun {rl_command_func_t *} rl_function_of_keyseq (const char *keyseq, Keymap map, int *type)
Return the function invoked by @var{keyseq} in keymap @var{map}.
If @var{map} is @code{NULL}, the current keymap is used.  If @var{type} is
not @code{NULL}, the type of the object is returned in the @code{int} variable
it points to (one of @code{ISFUNC}, @code{ISKMAP}, or @code{ISMACR}).
@end deftypefun

@deftypefun {char **} rl_invoking_keyseqs (rl_command_func_t *function)
Return an array of strings representing the key sequences used to
invoke @var{function} in the current keymap.
@end deftypefun

@deftypefun {char **} rl_invoking_keyseqs_in_map (rl_command_func_t *function, Keymap map)
Return an array of strings representing the key sequences used to
invoke @var{function} in the keymap @var{map}.
@end deftypefun

@deftypefun void rl_function_dumper (int readable)
Print the readline function names and the key sequences currently
bound to them to @code{rl_outstream}.  If @var{readable} is non-zero,
the list is formatted in such a way that it can be made part of an
@code{inputrc} file and re-read.
@end deftypefun

@deftypefun void rl_list_funmap_names (void)
Print the names of all bindable Readline functions to @code{rl_outstream}.
@end deftypefun

@deftypefun {const char **} rl_funmap_names (void)
Return a NULL terminated array of known function names.  The array is
sorted.  The array itself is allocated, but not the strings inside.  You
should @code{free()} the array when you are done, but not the pointers.
@end deftypefun

@deftypefun int rl_add_funmap_entry (const char *name, rl_command_func_t *function)
Add @var{name} to the list of bindable Readline command names, and make
@var{function} the function to be called when @var{name} is invoked.
@end deftypefun

@node Allowing Undoing
@subsection Allowing Undoing

Supporting the undo command is a painless thing, and makes your
functions much more useful.  It is certainly easy to try
something if you know you can undo it.

If your function simply inserts text once, or deletes text once, and
uses @code{rl_insert_text()} or @code{rl_delete_text()} to do it, then
undoing is already done for you automatically.

If you do multiple insertions or multiple deletions, or any combination
of these operations, you should group them together into one operation.
This is done with @code{rl_begin_undo_group()} and
@code{rl_end_undo_group()}.

The types of events that can be undone are:

@smallexample
enum undo_code @{ UNDO_DELETE, UNDO_INSERT, UNDO_BEGIN, UNDO_END @}; 
@end smallexample

Notice that @code{UNDO_DELETE} means to insert some text, and
@code{UNDO_INSERT} means to delete some text.  That is, the undo code
tells what to undo, not how to undo it.  @code{UNDO_BEGIN} and
@code{UNDO_END} are tags added by @code{rl_begin_undo_group()} and
@code{rl_end_undo_group()}.

@deftypefun int rl_begin_undo_group (void)
Begins saving undo information in a group construct.  The undo
information usually comes from calls to @code{rl_insert_text()} and
@code{rl_delete_text()}, but could be the result of calls to
@code{rl_add_undo()}.
@end deftypefun

@deftypefun int rl_end_undo_group (void)
Closes the current undo group started with @code{rl_begin_undo_group
()}.  There should be one call to @code{rl_end_undo_group()}
for each call to @code{rl_begin_undo_group()}.
@end deftypefun

@deftypefun void rl_add_undo (enum undo_code what, int start, int end, char *text)
Remember how to undo an event (according to @var{what}).  The affected
text runs from @var{start} to @var{end}, and encompasses @var{text}.
@end deftypefun

@deftypefun void rl_free_undo_list (void)
Free the existing undo list.
@end deftypefun

@deftypefun int rl_do_undo (void)
Undo the first thing on the undo list.  Returns @code{0} if there was
nothing to undo, non-zero if something was undone.
@end deftypefun

Finally, if you neither insert nor delete text, but directly modify the
existing text (e.g., change its case), call @code{rl_modifying()}
once, just before you modify the text.  You must supply the indices of
the text range that you are going to modify.

@deftypefun int rl_modifying (int start, int end)
Tell Readline to save the text between @var{start} and @var{end} as a
single undo unit.  It is assumed that you will subsequently modify
that text.
@end deftypefun

@node Redisplay
@subsection Redisplay

@deftypefun void rl_redisplay (void)
Change what's displayed on the screen to reflect the current contents
of @code{rl_line_buffer}.
@end deftypefun

@deftypefun int rl_forced_update_display (void)
Force the line to be updated and redisplayed, whether or not
Readline thinks the screen display is correct.
@end deftypefun

@deftypefun int rl_on_new_line (void)
Tell the update functions that we have moved onto a new (empty) line,
usually after ouputting a newline.
@end deftypefun

@deftypefun int rl_on_new_line_with_prompt (void)
Tell the update functions that we have moved onto a new line, with
@var{rl_prompt} already displayed.
This could be used by applications that want to output the prompt string
themselves, but still need Readline to know the prompt string length for
redisplay.
It should be used after setting @var{rl_already_prompted}.
@end deftypefun

@deftypefun int rl_reset_line_state (void)
Reset the display state to a clean state and redisplay the current line
starting on a new line.
@end deftypefun

@deftypefun int rl_crlf (void)
Move the cursor to the start of the next screen line.
@end deftypefun

@deftypefun int rl_show_char (int c)
Display character @var{c} on @code{rl_outstream}.
If Readline has not been set to display meta characters directly, this
will convert meta characters to a meta-prefixed key sequence.
This is intended for use by applications which wish to do their own
redisplay.
@end deftypefun

@deftypefun int rl_message (const char *, @dots{})
The arguments are a format string as would be supplied to @code{printf},
possibly containing conversion specifications such as @samp{%d}, and
any additional arguments necessary to satisfy the conversion specifications.
The resulting string is displayed in the @dfn{echo area}.  The echo area
is also used to display numeric arguments and search strings.
@end deftypefun

@deftypefun int rl_clear_message (void)
Clear the message in the echo area.
@end deftypefun

@deftypefun void rl_save_prompt (void)
Save the local Readline prompt display state in preparation for
displaying a new message in the message area with @code{rl_message()}.
@end deftypefun

@deftypefun void rl_restore_prompt (void)
Restore the local Readline prompt display state saved by the most
recent call to @code{rl_save_prompt}.
@end deftypefun

@deftypefun int rl_expand_prompt (char *prompt)
Expand any special character sequences in @var{prompt} and set up the
local Readline prompt redisplay variables.
This function is called by @code{readline()}.  It may also be called to
expand the primary prompt if the @code{rl_on_new_line_with_prompt()}
function or @code{rl_already_prompted} variable is used.
It returns the number of visible characters on the last line of the
(possibly multi-line) prompt.
Applications may indicate that the prompt contains characters that take
up no physical screen space when displayed by bracketing a sequence of
such characters with the special markers @code{RL_PROMPT_START_IGNORE}
and @code{RL_PROMPT_END_IGNORE} (declared in @file{readline.h}.  This may
be used to embed terminal-specific escape sequences in prompts.
@end deftypefun

@deftypefun int rl_set_prompt (const char *prompt)
Make Readline use @var{prompt} for subsequent redisplay.  This calls
@code{rl_expand_prompt()} to expand the prompt and sets @code{rl_prompt}
to the result.
@end deftypefun

@node Modifying Text
@subsection Modifying Text

@deftypefun int rl_insert_text (const char *text)
Insert @var{text} into the line at the current cursor position.
Returns the number of characters inserted.
@end deftypefun

@deftypefun int rl_delete_text (int start, int end)
Delete the text between @var{start} and @var{end} in the current line.
Returns the number of characters deleted.
@end deftypefun

@deftypefun {char *} rl_copy_text (int start, int end)
Return a copy of the text between @var{start} and @var{end} in
the current line.
@end deftypefun

@deftypefun int rl_kill_text (int start, int end)
Copy the text between @var{start} and @var{end} in the current line
to the kill ring, appending or prepending to the last kill if the
last command was a kill command.  The text is deleted.
If @var{start} is less than @var{end},
the text is appended, otherwise prepended.  If the last command was
not a kill, a new kill ring slot is used.
@end deftypefun

@deftypefun int rl_push_macro_input (char *macro)
Cause @var{macro} to be inserted into the line, as if it had been invoked
by a key bound to a macro.  Not especially useful; use
@code{rl_insert_text()} instead.
@end deftypefun

@node Character Input
@subsection Character Input

@deftypefun int rl_read_key (void)
Return the next character available from Readline's current input stream.
This handles input inserted into
the input stream via @var{rl_pending_input} (@pxref{Readline Variables})
and @code{rl_stuff_char()}, macros, and characters read from the keyboard.
While waiting for input, this function will call any function assigned to
the @code{rl_event_hook} variable.
@end deftypefun

@deftypefun int rl_getc (FILE *stream)
Return the next character available from @var{stream}, which is assumed to
be the keyboard.
@end deftypefun

@deftypefun int rl_stuff_char (int c)
Insert @var{c} into the Readline input stream.  It will be "read"
before Readline attempts to read characters from the terminal with
@code{rl_read_key()}.  Up to 512 characters may be pushed back.
@code{rl_stuff_char} returns 1 if the character was successfully inserted;
0 otherwise.
@end deftypefun

@deftypefun int rl_execute_next (int c)
Make @var{c} be the next command to be executed when @code{rl_read_key()}
is called.  This sets @var{rl_pending_input}.
@end deftypefun

@deftypefun int rl_clear_pending_input (void)
Unset @var{rl_pending_input}, effectively negating the effect of any
previous call to @code{rl_execute_next()}.  This works only if the
pending input has not already been read with @code{rl_read_key()}.
@end deftypefun

@deftypefun int rl_set_keyboard_input_timeout (int u)
While waiting for keyboard input in @code{rl_read_key()}, Readline will
wait for @var{u} microseconds for input before calling any function
assigned to @code{rl_event_hook}.  The default waiting period is
one-tenth of a second.  Returns the old timeout value.
@end deftypefun

@node Terminal Management
@subsection Terminal Management

@deftypefun void rl_prep_terminal (int meta_flag)
Modify the terminal settings for Readline's use, so @code{readline()}
can read a single character at a time from the keyboard.
The @var{meta_flag} argument should be non-zero if Readline should
read eight-bit input.
@end deftypefun

@deftypefun void rl_deprep_terminal (void)
Undo the effects of @code{rl_prep_terminal()}, leaving the terminal in
the state in which it was before the most recent call to
@code{rl_prep_terminal()}.
@end deftypefun

@deftypefun void rl_tty_set_default_bindings (Keymap kmap)
Read the operating system's terminal editing characters (as would be
displayed by @code{stty}) to their Readline equivalents.
The bindings are performed in @var{kmap}.
@end deftypefun

@deftypefun void rl_tty_unset_default_bindings (Keymap kmap)
Reset the bindings manipulated by @code{rl_tty_set_default_bindings} so
that the terminal editing characters are bound to @code{rl_insert}.
The bindings are performed in @var{kmap}.
@end deftypefun

@deftypefun int rl_reset_terminal (const char *terminal_name)
Reinitialize Readline's idea of the terminal settings using
@var{terminal_name} as the terminal type (e.g., @code{vt100}).
If @var{terminal_name} is @code{NULL}, the value of the @code{TERM}
environment variable is used.
@end deftypefun

@node Utility Functions
@subsection Utility Functions

@deftypefun void rl_replace_line (const char *text, int clear_undo)
Replace the contents of @code{rl_line_buffer} with @var{text}.
The point and mark are preserved, if possible.
If @var{clear_undo} is non-zero, the undo list associated with the
current line is cleared.
@end deftypefun

@deftypefun int rl_extend_line_buffer (int len)
Ensure that @code{rl_line_buffer} has enough space to hold @var{len}
characters, possibly reallocating it if necessary.
@end deftypefun

@deftypefun int rl_initialize (void)
Initialize or re-initialize Readline's internal state.
It's not strictly necessary to call this; @code{readline()} calls it before
reading any input.
@end deftypefun

@deftypefun int rl_ding (void)
Ring the terminal bell, obeying the setting of @code{bell-style}.
@end deftypefun

@deftypefun int rl_alphabetic (int c)
Return 1 if @var{c} is an alphabetic character.
@end deftypefun

@deftypefun void rl_display_match_list (char **matches, int len, int max)
A convenience function for displaying a list of strings in
columnar format on Readline's output stream.  @code{matches} is the list
of strings, in argv format, such as a list of completion matches.
@code{len} is the number of strings in @code{matches}, and @code{max}
is the length of the longest string in @code{matches}.  This function uses
the setting of @code{print-completions-horizontally} to select how the
matches are displayed (@pxref{Readline Init File Syntax}).
@end deftypefun

The following are implemented as macros, defined in @code{chardefs.h}.
Applications should refrain from using them.

@deftypefun int _rl_uppercase_p (int c)
Return 1 if @var{c} is an uppercase alphabetic character.
@end deftypefun

@deftypefun int _rl_lowercase_p (int c)
Return 1 if @var{c} is a lowercase alphabetic character.
@end deftypefun

@deftypefun int _rl_digit_p (int c)
Return 1 if @var{c} is a numeric character.
@end deftypefun

@deftypefun int _rl_to_upper (int c)
If @var{c} is a lowercase alphabetic character, return the corresponding
uppercase character.
@end deftypefun

@deftypefun int _rl_to_lower (int c)
If @var{c} is an uppercase alphabetic character, return the corresponding
lowercase character.
@end deftypefun

@deftypefun int _rl_digit_value (int c)
If @var{c} is a number, return the value it represents.
@end deftypefun

@node Miscellaneous Functions
@subsection Miscellaneous Functions

@deftypefun int rl_macro_bind (const char *keyseq, const char *macro, Keymap map)
Bind the key sequence @var{keyseq} to invoke the macro @var{macro}.
The binding is performed in @var{map}.  When @var{keyseq} is invoked, the
@var{macro} will be inserted into the line.  This function is deprecated;
use @code{rl_generic_bind()} instead.
@end deftypefun

@deftypefun void rl_macro_dumper (int readable)
Print the key sequences bound to macros and their values, using
the current keymap, to @code{rl_outstream}.
If @var{readable} is non-zero, the list is formatted in such a way
that it can be made part of an @code{inputrc} file and re-read.
@end deftypefun

@deftypefun int rl_variable_bind (const char *variable, const char *value)
Make the Readline variable @var{variable} have @var{value}.
This behaves as if the readline command
@samp{set @var{variable} @var{value}} had been executed in an @code{inputrc}
file (@pxref{Readline Init File Syntax}).
@end deftypefun

@deftypefun void rl_variable_dumper (int readable)
Print the readline variable names and their current values
to @code{rl_outstream}.
If @var{readable} is non-zero, the list is formatted in such a way
that it can be made part of an @code{inputrc} file and re-read.
@end deftypefun

@deftypefun int rl_set_paren_blink_timeout (int u)
Set the time interval (in microseconds) that Readline waits when showing
a balancing character when @code{blink-matching-paren} has been enabled.
@end deftypefun

@deftypefun {char *} rl_get_termcap (const char *cap)
Retrieve the string value of the termcap capability @var{cap}.
Readline fetches the termcap entry for the current terminal name and
uses those capabilities to move around the screen line and perform other
terminal-specific operations, like erasing a line.  Readline does not
use all of a terminal's capabilities, and this function will return
values for only those capabilities Readline uses.
@end deftypefun

@node Alternate Interface
@subsection Alternate Interface

An alternate interface is available to plain @code{readline()}.  Some
applications need to interleave keyboard I/O with file, device, or
window system I/O, typically by using a main loop to @code{select()}
on various file descriptors.  To accomodate this need, readline can
also be invoked as a `callback' function from an event loop.  There
are functions available to make this easy.

@deftypefun void rl_callback_handler_install (const char *prompt, rl_vcpfunc_t *lhandler)
Set up the terminal for readline I/O and display the initial
expanded value of @var{prompt}.  Save the value of @var{lhandler} to
use as a function to call when a complete line of input has been entered.
The function takes the text of the line as an argument.
@end deftypefun

@deftypefun void rl_callback_read_char (void)
Whenever an application determines that keyboard input is available, it
should call @code{rl_callback_read_char()}, which will read the next
character from the current input source.
If that character completes the line, @code{rl_callback_read_char} will
invoke the @var{lhandler} function saved by @code{rl_callback_handler_install}
to process the line.
Before calling the @var{lhandler} function, the terminal settings are
reset to the values they had before calling
@code{rl_callback_handler_install}.
If the @var{lhandler} function returns,
the terminal settings are modified for Readline's use again.
@code{EOF} is  indicated by calling @var{lhandler} with a
@code{NULL} line.
@end deftypefun

@deftypefun void rl_callback_handler_remove (void)
Restore the terminal to its initial state and remove the line handler.
This may be called from within a callback as well as independently.
If the @var{lhandler} installed by @code{rl_callback_handler_install}
does not exit the program, either this function or the function referred
to by the value of @code{rl_deprep_term_function} should be called before
the program exits to reset the terminal settings.
@end deftypefun

@node A Readline Example
@subsection A Readline Example

Here is a function which changes lowercase characters to their uppercase
equivalents, and uppercase characters to lowercase.  If
this function was bound to @samp{M-c}, then typing @samp{M-c} would
change the case of the character under point.  Typing @samp{M-1 0 M-c}
would change the case of the following 10 characters, leaving the cursor on
the last character changed.

@example
/* Invert the case of the COUNT following characters. */
int
invert_case_line (count, key)
     int count, key;
@{
  register int start, end, i;

  start = rl_point;

  if (rl_point >= rl_end)
    return (0);

  if (count < 0)
    @{
      direction = -1;
      count = -count;
    @}
  else
    direction = 1;
      
  /* Find the end of the range to modify. */
  end = start + (count * direction);

  /* Force it to be within range. */
  if (end > rl_end)
    end = rl_end;
  else if (end < 0)
    end = 0;

  if (start == end)
    return (0);

  if (start > end)
    @{
      int temp = start;
      start = end;
      end = temp;
    @}

  /* Tell readline that we are modifying the line,
     so it will save the undo information. */
  rl_modifying (start, end);

  for (i = start; i != end; i++)
    @{
      if (_rl_uppercase_p (rl_line_buffer[i]))
        rl_line_buffer[i] = _rl_to_lower (rl_line_buffer[i]);
      else if (_rl_lowercase_p (rl_line_buffer[i]))
        rl_line_buffer[i] = _rl_to_upper (rl_line_buffer[i]);
    @}
  /* Move point to on top of the last character changed. */
  rl_point = (direction == 1) ? end - 1 : start;
  return (0);
@}
@end example

@node Readline Signal Handling
@section Readline Signal Handling

Signals are asynchronous events sent to a process by the Unix kernel,
sometimes on behalf of another process.  They are intended to indicate
exceptional events, like a user pressing the interrupt key on his terminal,
or a network connection being broken.  There is a class of signals that can
be sent to the process currently reading input from the keyboard.  Since
Readline changes the terminal attributes when it is called, it needs to
perform special processing when such a signal is received in order to
restore the terminal to a sane state, or provide application writers with
functions to do so manually. 

Readline contains an internal signal handler that is installed for a
number of signals (@code{SIGINT}, @code{SIGQUIT}, @code{SIGTERM},
@code{SIGALRM}, @code{SIGTSTP}, @code{SIGTTIN}, and @code{SIGTTOU}).
When one of these signals is received, the signal handler
will reset the terminal attributes to those that were in effect before
@code{readline()} was called, reset the signal handling to what it was
before @code{readline()} was called, and resend the signal to the calling
application.
If and when the calling application's signal handler returns, Readline
will reinitialize the terminal and continue to accept input.
When a @code{SIGINT} is received, the Readline signal handler performs
some additional work, which will cause any partially-entered line to be
aborted (see the description of @code{rl_free_line_state()} below).

There is an additional Readline signal handler, for @code{SIGWINCH}, which
the kernel sends to a process whenever the terminal's size changes (for
example, if a user resizes an @code{xterm}).  The Readline @code{SIGWINCH}
handler updates Readline's internal screen size information, and then calls
any @code{SIGWINCH} signal handler the calling application has installed. 
Readline calls the application's @code{SIGWINCH} signal handler without
resetting the terminal to its original state.  If the application's signal
handler does more than update its idea of the terminal size and return (for
example, a @code{longjmp} back to a main processing loop), it @emph{must}
call @code{rl_cleanup_after_signal()} (described below), to restore the
terminal state. 

Readline provides two variables that allow application writers to
control whether or not it will catch certain signals and act on them
when they are received.  It is important that applications change the
values of these variables only when calling @code{readline()}, not in
a signal handler, so Readline's internal signal state is not corrupted.

@deftypevar int rl_catch_signals
If this variable is non-zero, Readline will install signal handlers for
@code{SIGINT}, @code{SIGQUIT}, @code{SIGTERM}, @code{SIGALRM},
@code{SIGTSTP}, @code{SIGTTIN}, and @code{SIGTTOU}.

The default value of @code{rl_catch_signals} is 1.
@end deftypevar

@deftypevar int rl_catch_sigwinch
If this variable is non-zero, Readline will install a signal handler for
@code{SIGWINCH}.

The default value of @code{rl_catch_sigwinch} is 1.
@end deftypevar

If an application does not wish to have Readline catch any signals, or
to handle signals other than those Readline catches (@code{SIGHUP},
for example), 
Readline provides convenience functions to do the necessary terminal
and internal state cleanup upon receipt of a signal.

@deftypefun void rl_cleanup_after_signal (void)
This function will reset the state of the terminal to what it was before
@code{readline()} was called, and remove the Readline signal handlers for
all signals, depending on the values of @code{rl_catch_signals} and
@code{rl_catch_sigwinch}.
@end deftypefun

@deftypefun void rl_free_line_state (void)
This will free any partial state associated with the current input line
(undo information, any partial history entry, any partially-entered
keyboard macro, and any partially-entered numeric argument).  This
should be called before @code{rl_cleanup_after_signal()}.  The
Readline signal handler for @code{SIGINT} calls this to abort the
current input line.
@end deftypefun

@deftypefun void rl_reset_after_signal (void)
This will reinitialize the terminal and reinstall any Readline signal
handlers, depending on the values of @code{rl_catch_signals} and
@code{rl_catch_sigwinch}.
@end deftypefun

If an application does not wish Readline to catch @code{SIGWINCH}, it may
call @code{rl_resize_terminal()} or @code{rl_set_screen_size()} to force
Readline to update its idea of the terminal size when a @code{SIGWINCH}
is received.

@deftypefun void rl_resize_terminal (void)
Update Readline's internal screen size by reading values from the kernel.
@end deftypefun

@deftypefun void rl_set_screen_size (int rows, int cols)
Set Readline's idea of the terminal size to @var{rows} rows and
@var{cols} columns.
@end deftypefun

If an application does not want to install a @code{SIGWINCH} handler, but
is still interested in the screen dimensions, Readline's idea of the screen
size may be queried.

@deftypefun void rl_get_screen_size (int *rows, int *cols)
Return Readline's idea of the terminal's size in the
variables pointed to by the arguments.
@end deftypefun

The following functions install and remove Readline's signal handlers.

@deftypefun int rl_set_signals (void)
Install Readline's signal handler for @code{SIGINT}, @code{SIGQUIT},
@code{SIGTERM}, @code{SIGALRM}, @code{SIGTSTP}, @code{SIGTTIN},
@code{SIGTTOU}, and @code{SIGWINCH}, depending on the values of
@code{rl_catch_signals} and @code{rl_catch_sigwinch}.
@end deftypefun

@deftypefun int rl_clear_signals (void)
Remove all of the Readline signal handlers installed by
@code{rl_set_signals()}.
@end deftypefun

@node Custom Completers
@section Custom Completers
@cindex application-specific completion functions

Typically, a program that reads commands from the user has a way of
disambiguating commands and data.  If your program is one of these, then
it can provide completion for commands, data, or both.
The following sections describe how your program and Readline
cooperate to provide this service.

@menu
* How Completing Works::        The logic used to do completion.
* Completion Functions::        Functions provided by Readline.
* Completion Variables::        Variables which control completion.
* A Short Completion Example::  An example of writing completer subroutines.
@end menu

@node How Completing Works
@subsection How Completing Works

In order to complete some text, the full list of possible completions
must be available.  That is, it is not possible to accurately
expand a partial word without knowing all of the possible words
which make sense in that context.  The Readline library provides
the user interface to completion, and two of the most common
completion functions:  filename and username.  For completing other types
of text, you must write your own completion function.  This section
describes exactly what such functions must do, and provides an example.

There are three major functions used to perform completion:

@enumerate
@item
The user-interface function @code{rl_complete()}.  This function is
called with the same arguments as other bindable Readline functions:
@var{count} and @var{invoking_key}.
It isolates the word to be completed and calls
@code{rl_completion_matches()} to generate a list of possible completions.
It then either lists the possible completions, inserts the possible
completions, or actually performs the
completion, depending on which behavior is desired.

@item
The internal function @code{rl_completion_matches()} uses an
application-supplied @dfn{generator} function to generate the list of
possible matches, and then returns the array of these matches.
The caller should place the address of its generator function in
@code{rl_completion_entry_function}.

@item
The generator function is called repeatedly from
@code{rl_completion_matches()}, returning a string each time.  The
arguments to the generator function are @var{text} and @var{state}.
@var{text} is the partial word to be completed.  @var{state} is zero the
first time the function is called, allowing the generator to perform
any necessary initialization, and a positive non-zero integer for
each subsequent call.  The generator function returns
@code{(char *)NULL} to inform @code{rl_completion_matches()} that there are
no more possibilities left.  Usually the generator function computes the
list of possible completions when @var{state} is zero, and returns them
one at a time on subsequent calls.  Each string the generator function
returns as a match must be allocated with @code{malloc()}; Readline
frees the strings when it has finished with them.
Such a generator function is referred to as an
@dfn{application-specific completion function}.

@end enumerate

@deftypefun int rl_complete (int ignore, int invoking_key)
Complete the word at or before point.  You have supplied the function
that does the initial simple matching selection algorithm (see
@code{rl_completion_matches()}).  The default is to do filename completion.
@end deftypefun

@deftypevar {rl_compentry_func_t *} rl_completion_entry_function
This is a pointer to the generator function for
@code{rl_completion_matches()}.
If the value of @code{rl_completion_entry_function} is
@code{NULL} then the default filename generator
function, @code{rl_filename_completion_function()}, is used.
An @dfn{application-specific completion function} is a function whose
address is assigned to @code{rl_completion_entry_function} and whose
return values are used to  generate possible completions.
@end deftypevar

@node Completion Functions
@subsection Completion Functions

Here is the complete list of callable completion functions present in
Readline.

@deftypefun int rl_complete_internal (int what_to_do)
Complete the word at or before point.  @var{what_to_do} says what to do
with the completion.  A value of @samp{?} means list the possible
completions.  @samp{TAB} means do standard completion.  @samp{*} means
insert all of the possible completions.  @samp{!} means to display
all of the possible completions, if there is more than one, as well as
performing partial completion.  @samp{@@} is similar to @samp{!}, but
possible completions are not listed if the possible completions share
a common prefix.
@end deftypefun

@deftypefun int rl_complete (int ignore, int invoking_key)
Complete the word at or before point.  You have supplied the function
that does the initial simple matching selection algorithm (see
@code{rl_completion_matches()} and @code{rl_completion_entry_function}).
The default is to do filename
completion.  This calls @code{rl_complete_internal()} with an
argument depending on @var{invoking_key}.
@end deftypefun

@deftypefun int rl_possible_completions (int count, int invoking_key)
List the possible completions.  See description of @code{rl_complete
()}.  This calls @code{rl_complete_internal()} with an argument of
@samp{?}.
@end deftypefun

@deftypefun int rl_insert_completions (int count, int invoking_key)
Insert the list of possible completions into the line, deleting the
partially-completed word.  See description of @code{rl_complete()}.
This calls @code{rl_complete_internal()} with an argument of @samp{*}.
@end deftypefun

@deftypefun int rl_completion_mode (rl_command_func_t *cfunc)
Returns the apppriate value to pass to @code{rl_complete_internal()}
depending on whether @var{cfunc} was called twice in succession and
the values of the @code{show-all-if-ambiguous} and
@code{show-all-if-unmodified} variables.
Application-specific completion functions may use this function to present
the same interface as @code{rl_complete()}.
@end deftypefun

@deftypefun {char **} rl_completion_matches (const char *text, rl_compentry_func_t *entry_func)
Returns an array of strings which is a list of completions for
@var{text}.  If there are no completions, returns @code{NULL}.
The first entry in the returned array is the substitution for @var{text}.
The remaining entries are the possible completions.  The array is
terminated with a @code{NULL} pointer.

@var{entry_func} is a function of two args, and returns a
@code{char *}.  The first argument is @var{text}.  The second is a
state argument; it is zero on the first call, and non-zero on subsequent
calls.  @var{entry_func} returns a @code{NULL}  pointer to the caller
when there are no more matches.
@end deftypefun

@deftypefun {char *} rl_filename_completion_function (const char *text, int state)
A generator function for filename completion in the general case.
@var{text} is a partial filename.
The Bash source is a useful reference for writing application-specific
completion functions (the Bash completion functions call this and other
Readline functions).
@end deftypefun

@deftypefun {char *} rl_username_completion_function (const char *text, int state)
A completion generator for usernames.  @var{text} contains a partial
username preceded by a random character (usually @samp{~}).  As with all
completion generators, @var{state} is zero on the first call and non-zero
for subsequent calls.
@end deftypefun

@node Completion Variables
@subsection Completion Variables

@deftypevar {rl_compentry_func_t *} rl_completion_entry_function
A pointer to the generator function for @code{rl_completion_matches()}.
@code{NULL} means to use @code{rl_filename_completion_function()},
the default filename completer.
@end deftypevar

@deftypevar {rl_completion_func_t *} rl_attempted_completion_function
A pointer to an alternative function to create matches.
The function is called with @var{text}, @var{start}, and @var{end}.
@var{start} and @var{end} are indices in @code{rl_line_buffer} defining
the boundaries of @var{text}, which is a character string.
If this function exists and returns @code{NULL}, or if this variable is
set to @code{NULL}, then @code{rl_complete()} will call the value of
@code{rl_completion_entry_function} to generate matches, otherwise the
array of strings returned will be used.
If this function sets the @code{rl_attempted_completion_over}
variable to a non-zero value, Readline will not perform its default
completion even if this function returns no matches.
@end deftypevar

@deftypevar {rl_quote_func_t *} rl_filename_quoting_function
A pointer to a function that will quote a filename in an
application-specific fashion.  This is called if filename completion is being
attempted and one of the characters in @code{rl_filename_quote_characters}
appears in a completed filename.  The function is called with
@var{text}, @var{match_type}, and @var{quote_pointer}.  The @var{text}
is the filename to be quoted.  The @var{match_type} is either
@code{SINGLE_MATCH}, if there is only one completion match, or
@code{MULT_MATCH}.  Some functions use this to decide whether or not to
insert a closing quote character.  The @var{quote_pointer} is a pointer
to any opening quote character the user typed.  Some functions choose
to reset this character.
@end deftypevar

@deftypevar {rl_dequote_func_t *} rl_filename_dequoting_function
A pointer to a function that will remove application-specific quoting
characters from a filename before completion is attempted, so those
characters do not interfere with matching the text against names in
the filesystem.  It is called with @var{text}, the text of the word
to be dequoted, and @var{quote_char}, which is the quoting character 
that delimits the filename (usually @samp{'} or @samp{"}).  If
@var{quote_char} is zero, the filename was not in an embedded string.
@end deftypevar

@deftypevar {rl_linebuf_func_t *} rl_char_is_quoted_p
A pointer to a function to call that determines whether or not a specific
character in the line buffer is quoted, according to whatever quoting
mechanism the program calling Readline uses.  The function is called with
two arguments: @var{text}, the text of the line, and @var{index}, the
index of the character in the line.  It is used to decide whether a
character found in @code{rl_completer_word_break_characters} should be
used to break words for the completer.
@end deftypevar

@deftypevar {rl_compignore_func_t *} rl_ignore_some_completions_function
This function, if defined, is called by the completer when real filename
completion is done, after all the matching names have been generated.
It is passed a @code{NULL} terminated array of matches.
The first element (@code{matches[0]}) is the
maximal substring common to all matches. This function can
re-arrange the list of matches as required, but each element deleted
from the array must be freed.
@end deftypevar

@deftypevar {rl_icppfunc_t *} rl_directory_completion_hook
This function, if defined, is allowed to modify the directory portion
of filenames Readline completes.  It is called with the address of a
string (the current directory name) as an argument, and may modify that string.
If the string is replaced with a new string, the old value should be freed.
Any modified directory name should have a trailing slash.
The modified value will be displayed as part of the completion, replacing
the directory portion of the pathname the user typed.
It returns an integer that should be non-zero if the function modifies
its directory argument.
It could be used to expand symbolic links or shell variables in pathnames.
@end deftypevar

@deftypevar {rl_compdisp_func_t *} rl_completion_display_matches_hook
If non-zero, then this is the address of a function to call when
completing a word would normally display the list of possible matches.
This function is called in lieu of Readline displaying the list.
It takes three arguments:
(@code{char **}@var{matches}, @code{int} @var{num_matches}, @code{int} @var{max_length})
where @var{matches} is the array of matching strings,
@var{num_matches} is the number of strings in that array, and
@var{max_length} is the length of the longest string in that array.
Readline provides a convenience function, @code{rl_display_match_list},
that takes care of doing the display to Readline's output stream.  That
function may be called from this hook.
@end deftypevar

@deftypevar {const char *} rl_basic_word_break_characters
The basic list of characters that signal a break between words for the
completer routine.  The default value of this variable is the characters
which break words for completion in Bash:
@code{" \t\n\"\\'`@@$><=;|&@{("}.
@end deftypevar

@deftypevar {const char *} rl_basic_quote_characters
A list of quote characters which can cause a word break.
@end deftypevar

@deftypevar {const char *} rl_completer_word_break_characters
The list of characters that signal a break between words for
@code{rl_complete_internal()}.  The default list is the value of
@code{rl_basic_word_break_characters}.
@end deftypevar

@deftypevar {rl_cpvfunc_t *} rl_completion_word_break_hook
If non-zero, this is the address of a function to call when Readline is
deciding where to separate words for word completion.  It should return
a character string like @code{rl_completer_word_break_characters} to be
used to perform the current completion.  The function may choose to set
@code{rl_completer_word_break_characters} itself.  If the function
returns @code{NULL}, @code{rl_completer_word_break_characters} is used.
@end deftypevar

@deftypevar {const char *} rl_completer_quote_characters
A list of characters which can be used to quote a substring of the line.
Completion occurs on the entire substring, and within the substring
@code{rl_completer_word_break_characters} are treated as any other character,
unless they also appear within this list.
@end deftypevar

@deftypevar {const char *} rl_filename_quote_characters
A list of characters that cause a filename to be quoted by the completer
when they appear in a completed filename.  The default is the null string.
@end deftypevar

@deftypevar {const char *} rl_special_prefixes
The list of characters that are word break characters, but should be
left in @var{text} when it is passed to the completion function.
Programs can use this to help determine what kind of completing to do.
For instance, Bash sets this variable to "$@@" so that it can complete
shell variables and hostnames.
@end deftypevar

@deftypevar int rl_completion_query_items
Up to this many items will be displayed in response to a
possible-completions call.  After that, we ask the user if she is sure
she wants to see them all.  The default value is 100.
@end deftypevar

@deftypevar {int} rl_completion_append_character
When a single completion alternative matches at the end of the command
line, this character is appended to the inserted completion text.  The
default is a space character (@samp{ }).  Setting this to the null
character (@samp{\0}) prevents anything being appended automatically.
This can be changed in application-specific completion functions to
provide the ``most sensible word separator character'' according to
an application-specific command line syntax specification.
@end deftypevar

@deftypevar int rl_completion_suppress_append
If non-zero, @var{rl_completion_append_character} is not appended to
matches at the end of the command line, as described above.
It is set to 0 before any application-specific completion function
is called, and may only be changed within such a function.
@end deftypevar

@deftypevar int rl_completion_quote_character
When Readline is completing quoted text, as delimited by one of the
characters in @var{rl_completer_quote_characters}, it sets this variable
to the quoting character found.
This is set before any application-specific completion function is called.
@end deftypevar

@deftypevar int rl_completion_suppress_quote
If non-zero, Readline does not append a matching quote character when
performing completion on a quoted string.
It is set to 0 before any application-specific completion function
is called, and may only be changed within such a function.
@end deftypevar

@deftypevar int rl_completion_found_quote
When Readline is completing quoted text, it sets this variable
to a non-zero value if the word being completed contains or is delimited
by any quoting characters, including backslashes.
This is set before any application-specific completion function is called.
@end deftypevar

@deftypevar int rl_completion_mark_symlink_dirs
If non-zero, a slash will be appended to completed filenames that are
symbolic links to directory names, subject to the value of the
user-settable @var{mark-directories} variable.
This variable exists so that application-specific completion functions
can override the user's global preference (set via the
@var{mark-symlinked-directories} Readline variable) if appropriate.
This variable is set to the user's preference before any
application-specific completion function is called, so unless that
function modifies the value, the user's preferences are honored.
@end deftypevar

@deftypevar int rl_ignore_completion_duplicates
If non-zero, then duplicates in the matches are removed.
The default is 1.
@end deftypevar

@deftypevar int rl_filename_completion_desired
Non-zero means that the results of the matches are to be treated as
filenames.  This is @emph{always} zero when completion is attempted,
and can only be changed
within an application-specific completion function.  If it is set to a
non-zero value by such a function, directory names have a slash appended
and Readline attempts to quote completed filenames if they contain any
characters in @code{rl_filename_quote_characters} and
@code{rl_filename_quoting_desired} is set to a non-zero value.
@end deftypevar

@deftypevar int rl_filename_quoting_desired
Non-zero means that the results of the matches are to be quoted using
double quotes (or an application-specific quoting mechanism) if the
completed filename contains any characters in
@code{rl_filename_quote_chars}.  This is @emph{always} non-zero
when completion is attempted, and can only be changed within an
application-specific completion function.
The quoting is effected via a call to the function pointed to
by @code{rl_filename_quoting_function}.
@end deftypevar

@deftypevar int rl_attempted_completion_over
If an application-specific completion function assigned to
@code{rl_attempted_completion_function} sets this variable to a non-zero
value, Readline will not perform its default filename completion even
if the application's completion function returns no matches.
It should be set only by an application's completion function.
@end deftypevar

@deftypevar int rl_completion_type
Set to a character describing the type of completion Readline is currently
attempting; see the description of @code{rl_complete_internal()}
(@pxref{Completion Functions}) for the list of characters.
This is set to the appropriate value before any application-specific
completion function is called, allowing such functions to present
the same interface as @code{rl_complete()}.
@end deftypevar

@deftypevar int rl_inhibit_completion
If this variable is non-zero, completion is inhibited.  The completion
character will be inserted as any other bound to @code{self-insert}.
@end deftypevar

@node A Short Completion Example
@subsection A Short Completion Example

Here is a small application demonstrating the use of the GNU Readline
library.  It is called @code{fileman}, and the source code resides in
@file{examples/fileman.c}.  This sample application provides
completion of command names, line editing features, and access to the
history list.

@page
@smallexample
/* fileman.c -- A tiny application which demonstrates how to use the
   GNU Readline library.  This application interactively allows users
   to manipulate files and their modes. */

#include <stdio.h>
#include <sys/types.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/errno.h>

#include <readline/readline.h>
#include <readline/history.h>

extern char *xmalloc ();

/* The names of functions that actually do the manipulation. */
int com_list __P((char *));
int com_view __P((char *));
int com_rename __P((char *));
int com_stat __P((char *));
int com_pwd __P((char *));
int com_delete __P((char *));
int com_help __P((char *));
int com_cd __P((char *));
int com_quit __P((char *));

/* A structure which contains information on the commands this program
   can understand. */

typedef struct @{
  char *name;                   /* User printable name of the function. */
  rl_icpfunc_t *func;           /* Function to call to do the job. */
  char *doc;                    /* Documentation for this function.  */
@} COMMAND;

COMMAND commands[] = @{
  @{ "cd", com_cd, "Change to directory DIR" @},
  @{ "delete", com_delete, "Delete FILE" @},
  @{ "help", com_help, "Display this text" @},
  @{ "?", com_help, "Synonym for `help'" @},
  @{ "list", com_list, "List files in DIR" @},
  @{ "ls", com_list, "Synonym for `list'" @},
  @{ "pwd", com_pwd, "Print the current working directory" @},
  @{ "quit", com_quit, "Quit using Fileman" @},
  @{ "rename", com_rename, "Rename FILE to NEWNAME" @},
  @{ "stat", com_stat, "Print out statistics on FILE" @},
  @{ "view", com_view, "View the contents of FILE" @},
  @{ (char *)NULL, (rl_icpfunc_t *)NULL, (char *)NULL @}
@};

/* Forward declarations. */
char *stripwhite ();
COMMAND *find_command ();

/* The name of this program, as taken from argv[0]. */
char *progname;

/* When non-zero, this means the user is done using this program. */
int done;

char *
dupstr (s)
     int s;
@{
  char *r;

  r = xmalloc (strlen (s) + 1);
  strcpy (r, s);
  return (r);
@}

main (argc, argv)
     int argc;
     char **argv;
@{
  char *line, *s;

  progname = argv[0];

  initialize_readline ();       /* Bind our completer. */

  /* Loop reading and executing lines until the user quits. */
  for ( ; done == 0; )
    @{
      line = readline ("FileMan: ");

      if (!line)
        break;

      /* Remove leading and trailing whitespace from the line.
         Then, if there is anything left, add it to the history list
         and execute it. */
      s = stripwhite (line);

      if (*s)
        @{
          add_history (s);
          execute_line (s);
        @}

      free (line);
    @}
  exit (0);
@}

/* Execute a command line. */
int
execute_line (line)
     char *line;
@{
  register int i;
  COMMAND *command;
  char *word;

  /* Isolate the command word. */
  i = 0;
  while (line[i] && whitespace (line[i]))
    i++;
  word = line + i;

  while (line[i] && !whitespace (line[i]))
    i++;

  if (line[i])
    line[i++] = '\0';

  command = find_command (word);

  if (!command)
    @{
      fprintf (stderr, "%s: No such command for FileMan.\n", word);
      return (-1);
    @}

  /* Get argument to command, if any. */
  while (whitespace (line[i]))
    i++;

  word = line + i;

  /* Call the function. */
  return ((*(command->func)) (word));
@}

/* Look up NAME as the name of a command, and return a pointer to that
   command.  Return a NULL pointer if NAME isn't a command name. */
COMMAND *
find_command (name)
     char *name;
@{
  register int i;

  for (i = 0; commands[i].name; i++)
    if (strcmp (name, commands[i].name) == 0)
      return (&commands[i]);

  return ((COMMAND *)NULL);
@}

/* Strip whitespace from the start and end of STRING.  Return a pointer
   into STRING. */
char *
stripwhite (string)
     char *string;
@{
  register char *s, *t;

  for (s = string; whitespace (*s); s++)
    ;
    
  if (*s == 0)
    return (s);

  t = s + strlen (s) - 1;
  while (t > s && whitespace (*t))
    t--;
  *++t = '\0';

  return s;
@}

/* **************************************************************** */
/*                                                                  */
/*                  Interface to Readline Completion                */
/*                                                                  */
/* **************************************************************** */

char *command_generator __P((const char *, int));
char **fileman_completion __P((const char *, int, int));

/* Tell the GNU Readline library how to complete.  We want to try to
   complete on command names if this is the first word in the line, or
   on filenames if not. */
initialize_readline ()
@{
  /* Allow conditional parsing of the ~/.inputrc file. */
  rl_readline_name = "FileMan";

  /* Tell the completer that we want a crack first. */
  rl_attempted_completion_function = fileman_completion;
@}

/* Attempt to complete on the contents of TEXT.  START and END
   bound the region of rl_line_buffer that contains the word to
   complete.  TEXT is the word to complete.  We can use the entire
   contents of rl_line_buffer in case we want to do some simple
   parsing.  Returnthe array of matches, or NULL if there aren't any. */
char **
fileman_completion (text, start, end)
     const char *text;
     int start, end;
@{
  char **matches;

  matches = (char **)NULL;

  /* If this word is at the start of the line, then it is a command
     to complete.  Otherwise it is the name of a file in the current
     directory. */
  if (start == 0)
    matches = rl_completion_matches (text, command_generator);

  return (matches);
@}

/* Generator function for command completion.  STATE lets us
   know whether to start from scratch; without any state
   (i.e. STATE == 0), then we start at the top of the list. */
char *
command_generator (text, state)
     const char *text;
     int state;
@{
  static int list_index, len;
  char *name;

  /* If this is a new word to complete, initialize now.  This
     includes saving the length of TEXT for efficiency, and
     initializing the index variable to 0. */
  if (!state)
    @{
      list_index = 0;
      len = strlen (text);
    @}

  /* Return the next name which partially matches from the
     command list. */
  while (name = commands[list_index].name)
    @{
      list_index++;

      if (strncmp (name, text, len) == 0)
        return (dupstr(name));
    @}

  /* If no names matched, then return NULL. */
  return ((char *)NULL);
@}

/* **************************************************************** */
/*                                                                  */
/*                       FileMan Commands                           */
/*                                                                  */
/* **************************************************************** */

/* String to pass to system ().  This is for the LIST, VIEW and RENAME
   commands. */
static char syscom[1024];

/* List the file(s) named in arg. */
com_list (arg)
     char *arg;
@{
  if (!arg)
    arg = "";

  sprintf (syscom, "ls -FClg %s", arg);
  return (system (syscom));
@}

com_view (arg)
     char *arg;
@{
  if (!valid_argument ("view", arg))
    return 1;

  sprintf (syscom, "more %s", arg);
  return (system (syscom));
@}

com_rename (arg)
     char *arg;
@{
  too_dangerous ("rename");
  return (1);
@}

com_stat (arg)
     char *arg;
@{
  struct stat finfo;

  if (!valid_argument ("stat", arg))
    return (1);

  if (stat (arg, &finfo) == -1)
    @{
      perror (arg);
      return (1);
    @}

  printf ("Statistics for `%s':\n", arg);

  printf ("%s has %d link%s, and is %d byte%s in length.\n", arg,
          finfo.st_nlink,
          (finfo.st_nlink == 1) ? "" : "s",
          finfo.st_size,
          (finfo.st_size == 1) ? "" : "s");
  printf ("Inode Last Change at: %s", ctime (&finfo.st_ctime));
  printf ("      Last access at: %s", ctime (&finfo.st_atime));
  printf ("    Last modified at: %s", ctime (&finfo.st_mtime));
  return (0);
@}

com_delete (arg)
     char *arg;
@{
  too_dangerous ("delete");
  return (1);
@}

/* Print out help for ARG, or for all of the commands if ARG is
   not present. */
com_help (arg)
     char *arg;
@{
  register int i;
  int printed = 0;

  for (i = 0; commands[i].name; i++)
    @{
      if (!*arg || (strcmp (arg, commands[i].name) == 0))
        @{
          printf ("%s\t\t%s.\n", commands[i].name, commands[i].doc);
          printed++;
        @}
    @}

  if (!printed)
    @{
      printf ("No commands match `%s'.  Possibilties are:\n", arg);

      for (i = 0; commands[i].name; i++)
        @{
          /* Print in six columns. */
          if (printed == 6)
            @{
              printed = 0;
              printf ("\n");
            @}

          printf ("%s\t", commands[i].name);
          printed++;
        @}

      if (printed)
        printf ("\n");
    @}
  return (0);
@}

/* Change to the directory ARG. */
com_cd (arg)
     char *arg;
@{
  if (chdir (arg) == -1)
    @{
      perror (arg);
      return 1;
    @}

  com_pwd ("");
  return (0);
@}

/* Print out the current working directory. */
com_pwd (ignore)
     char *ignore;
@{
  char dir[1024], *s;

  s = getcwd (dir, sizeof(dir) - 1);
  if (s == 0)
    @{
      printf ("Error getting pwd: %s\n", dir);
      return 1;
    @}

  printf ("Current directory is %s\n", dir);
  return 0;
@}

/* The user wishes to quit using this program.  Just set DONE
   non-zero. */
com_quit (arg)
     char *arg;
@{
  done = 1;
  return (0);
@}

/* Function which tells you that you can't do this. */
too_dangerous (caller)
     char *caller;
@{
  fprintf (stderr,
           "%s: Too dangerous for me to distribute.\n"
           caller);
  fprintf (stderr, "Write it yourself.\n");
@}

/* Return non-zero if ARG is a valid argument for CALLER,
   else print an error message and return zero. */
int
valid_argument (caller, arg)
     char *caller, *arg;
@{
  if (!arg || !*arg)
    @{
      fprintf (stderr, "%s: Argument required.\n", caller);
      return (0);
    @}

  return (1);
@}
@end smallexample