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32 .\" @(#) dlopen.3 1.6 90/01/31 SMI
33 .\" $FreeBSD: src/lib/libc/gen/dlopen.3,v 1.8.2.10 2003/03/15 15:11:05 trhodes Exp $
34 .\" $DragonFly: src/lib/libcr/gen/Attic/dlopen.3,v 1.2 2003/06/17 04:26:42 dillon Exp $
36 .Dd September 24, 1989
44 .Nd programmatic interface to the dynamic linker
50 .Fn dlopen "const char *path" "int mode"
52 .Fn dlsym "void *handle" "const char *symbol"
56 .Fn dlclose "void *handle"
58 These functions provide a simple programmatic interface to the services of the
60 Operations are provided to add new shared objects to a
61 program's address space, to obtain the address bindings of symbols
63 objects, and to remove such objects when their use is no longer required.
68 provides access to the shared object in
70 returning a descriptor that can be used for later
71 references to the object in calls to
77 was not in the address space prior to the call to
79 it is placed in the address space.
80 When an object is first loaded into the address space in this way, its
83 if any, is called by the dynamic linker.
86 has already been placed in the address space in a previous call to
88 it is not added a second time, although a reference count of
93 A null pointer supplied for
95 is interpreted as a reference to the main
96 executable of the process.
100 controls the way in which external function references from the
101 loaded object are bound to their referents.
102 It must contain one of the following values, possibly ORed with
103 additional flags which will be described subsequently:
104 .Bl -tag -width RTLD_LAZYX
106 Each external function reference is resolved when the function is first
109 All external function references are bound immediately by
114 is normally preferred, for reasons of efficiency.
117 is useful to ensure that any undefined symbols are discovered during the
121 One of the following flags may be ORed into the
124 .Bl -tag -width RTLD_GLOBALX
126 Symbols from this shared object and its directed acyclic graph (DAG)
127 of needed objects will be available for resolving undefined references
128 from all other shared objects.
130 Symbols in this shared object and its DAG of needed objects will be
131 available for resolving undefined references only from other objects
133 This is the default, but it may be specified
134 explicitly with this flag.
136 When set, causes dynamic linker to exit after loading all objects
137 needed by this shared object and printing a summary which includes
138 the absolute pathnames of all objects, to standard output.
141 will return to the caller only in the case of error.
146 fails, it returns a null pointer, and sets an error condition which may
153 returns the address binding of the symbol described in the null-terminated
156 as it occurs in the shared object identified by
158 The symbols exported by objects added to the address space by
160 can be accessed only through calls to
162 Such symbols do not supersede any definition of those symbols already present
163 in the address space when the object is loaded, nor are they available to
164 satisfy normal dynamic linking references.
168 is called with the special
171 it is interpreted as a reference to the executable or shared object
174 Thus a shared object can reference its own symbols.
178 is called with the special
181 the search for the symbol follows the algorithm used for resolving
182 undefined symbols when objects are loaded.
183 The objects searched are
184 as follows, in the given order:
187 The referencing object itself (or the object from which the call to
189 is made), if that object was linked using the
194 All objects loaded at program start-up.
196 All objects loaded via
198 which are in needed-object DAGs that also contain the referencing object.
200 All objects loaded via
211 is called with the special
214 then the search for the symbol is limited to the shared objects
215 which were loaded after the one issuing the call to
217 Thus, if the function is called from the main program, all
218 the shared libraries are searched.
219 If it is called from a shared library, all subsequent shared
220 libraries are searched.
222 is useful for implementing wrappers around library functions.
223 For example, a wrapper function
229 .Li dlsym(RTLD_NEXT, \&"getpid\&") .
233 is called with the special
236 then the search for the symbol is limited to the shared object
239 and those shared objects which were loaded after it.
244 returns a null pointer if the symbol cannot be found, and sets an error
245 condition which may be queried with
251 returns a null-terminated character string describing the last error that
252 occurred during a call to
259 If no such error has occurred,
261 returns a null pointer.
264 the error indication is reset.
265 Thus in the case of two calls
268 where the second call follows the first immediately, the second call
269 will always return a null pointer.
274 deletes a reference to the shared object referenced by
276 If the reference count drops to 0, the object is removed from the
280 Just before removing a shared object in this way, the dynamic linker
283 function, if such a function is defined by the object.
286 is successful, it returns a value of 0.
287 Otherwise it returns -1, and sets an error condition that can be
291 The object-intrinsic functions
295 are called with no arguments, and are not expected to return values.
297 ELF executables need to be linked
302 for symbols defined in the executable to become visible to
305 In previous implementations, it was necessary to prepend an underscore
306 to all external symbols in order to gain symbol
307 compatibility with object code compiled from the C language.
309 still the case when using the (obsolete)
311 option to the C language compiler.
318 return a null pointer in the event of errors.
322 returns 0 on success, or -1 if an error occurred.
323 Whenever an error has been detected, a message detailing it can be
324 retrieved via a call to