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[dragonfly.git] / lib / libc / gen / exec.3
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1.\" Copyright (c) 1991, 1993
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32.\" @(#)exec.3 8.3 (Berkeley) 1/24/94
33.\" $FreeBSD: src/lib/libc/gen/exec.3,v 1.7.2.8 2003/03/15 15:11:05 trhodes Exp $
44cb301e 34.\" $DragonFly: src/lib/libc/gen/exec.3,v 1.4 2006/05/26 19:39:36 swildner Exp $
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35.\"
36.Dd January 24, 1994
37.Dt EXEC 3
38.Os
39.Sh NAME
40.Nm execl ,
41.Nm execlp ,
42.Nm execle ,
43.Nm exect ,
44.Nm execv ,
45.Nm execvp
46.Nd execute a file
47.Sh LIBRARY
48.Lb libc
49.Sh SYNOPSIS
50.In unistd.h
51.Vt extern char **environ ;
52.Ft int
53.Fn execl "const char *path" "const char *arg" ...
54.Ft int
55.Fn execlp "const char *file" "const char *arg" ...
56.Ft int
57.Fn execle "const char *path" "const char *arg" ...
58.Ft int
59.Fn exect "const char *path" "char *const argv[]" "char *const envp[]"
60.Ft int
61.Fn execv "const char *path" "char *const argv[]"
62.Ft int
63.Fn execvp "const char *file" "char *const argv[]"
64.Sh DESCRIPTION
65The
66.Nm exec
67family of functions replaces the current process image with a
68new process image.
69The functions described in this manual page are front-ends for the function
70.Xr execve 2 .
71(See the manual page for
72.Xr execve 2
73for detailed information about the replacement of the current process.)
74.Pp
75The initial argument for these functions is the pathname of a file which
76is to be executed.
77.Pp
78The
79.Fa "const char *arg"
80and subsequent ellipses in the
81.Fn execl ,
82.Fn execlp ,
83and
84.Fn execle
85functions can be thought of as
86.Em arg0 ,
87.Em arg1 ,
88\&...,
89.Em argn .
90Together they describe a list of one or more pointers to null-terminated
91strings that represent the argument list available to the executed program.
92The first argument, by convention, should point to the file name associated
93with the file being executed.
94The list of arguments
95.Em must
96be terminated by a
97.Dv NULL
98pointer.
99.Pp
100The
101.Fn exect ,
102.Fn execv ,
103and
104.Fn execvp
105functions provide an array of pointers to null-terminated strings that
106represent the argument list available to the new program.
107The first argument, by convention, should point to the file name associated
108with the file being executed.
109The array of pointers
110.Sy must
111be terminated by a
112.Dv NULL
113pointer.
114.Pp
115The
116.Fn execle
117and
118.Fn exect
119functions also specify the environment of the executed process by following
120the
121.Dv NULL
122pointer that terminates the list of arguments in the argument list
123or the pointer to the argv array with an additional argument.
124This additional argument is an array of pointers to null-terminated strings
125and
126.Em must
127be terminated by a
128.Dv NULL
129pointer.
130The other functions take the environment for the new process image from the
131external variable
132.Va environ
133in the current process.
134.Pp
135Some of these functions have special semantics.
136.Pp
137The functions
138.Fn execlp
139and
140.Fn execvp
141will duplicate the actions of the shell in searching for an executable file
142if the specified file name does not contain a slash
143.Dq Li /
144character.
145The search path is the path specified in the environment by
146.Dq Ev PATH
147variable.
148If this variable isn't specified,
149the default path is set according to the
150.Dv _PATH_DEFPATH
151definition in
44cb301e 152.In paths.h ,
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153which is set to
154.Dq Ev /usr/bin:/bin .
155In addition, certain errors are treated specially.
156.Pp
157If an error is ambiguous (for simplicity, we shall consider all
158errors except
159.Er ENOEXEC
160as being ambiguous here, although only the critical error
161.Er EACCES
162is really ambiguous),
163then these functions will act as if they stat the file to determine
164whether the file exists and has suitable execute permissions.
165If it does, they will return immediately with the global variable
166.Va errno
167restored to the value set by
168.Fn execve .
169Otherwise, the search will be continued.
170If the search completes without performing a successful
171.Fn execve
172or terminating due to an error,
173these functions will return with the global variable
174.Va errno
175set to
176.Er EACCES
177or
178.Er ENOENT
179according to whether at least one file with suitable execute permissions
180was found.
181.Pp
182If the header of a file isn't recognized (the attempted
183.Fn execve
184returned
185.Er ENOEXEC ) ,
186these functions will execute the shell with the path of
187the file as its first argument.
188(If this attempt fails, no further searching is done.)
189.Pp
190The function
191.Fn exect
192executes a file with the program tracing facilities enabled (see
193.Xr ptrace 2 ) .
194.Sh RETURN VALUES
195If any of the
196.Fn exec
197functions returns, an error will have occurred.
198The return value is \-1, and the global variable
199.Va errno
200will be set to indicate the error.
201.Sh FILES
202.Bl -tag -width /bin/sh -compact
203.It Pa /bin/sh
204The shell.
205.El
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206.Sh COMPATIBILITY
207Historically, the default path for the
208.Fn execlp
209and
210.Fn execvp
211functions was
212.Dq Pa :/bin:/usr/bin .
213This was changed to place the current directory last to enhance system
214security.
215.Pp
216The behavior of
217.Fn execlp
218and
219.Fn execvp
220when errors occur while attempting to execute the file is not quite historic
221practice, and has not traditionally been documented and is not specified
222by the
223.Tn POSIX
224standard.
225.Pp
226Traditionally, the functions
227.Fn execlp
228and
229.Fn execvp
230ignored all errors except for the ones described above and
231.Er ETXTBSY ,
232upon which they retried after sleeping for several seconds, and
233.Er ENOMEM
234and
235.Er E2BIG ,
236upon which they returned.
237They now return for
238.Er ETXTBSY ,
239and determine existence and executability more carefully.
240In particular,
241.Er EACCES
242for inaccessible directories in the path prefix is no longer
243confused with
244.Er EACCES
245for files with unsuitable execute permissions.
246In
247.Bx 4.4 ,
248they returned upon all errors except
249.Er EACCES ,
250.Er ENOENT ,
251.Er ENOEXEC
252and
253.Er ETXTBSY .
254This was inferior to the traditional error handling,
255since it breaks the ignoring of errors for path prefixes
256and only improves the handling of the unusual ambiguous error
257.Er EFAULT
258and the unusual error
259.Er EIO .
260The behaviour was changed to match the behaviour of
261.Xr sh 1 .
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262.Sh ERRORS
263The
264.Fn execl ,
265.Fn execle ,
266.Fn execlp
267and
268.Fn execvp
269functions
270may fail and set
271.Va errno
272for any of the errors specified for the library functions
273.Xr execve 2
274and
275.Xr malloc 3 .
276.Pp
277The
278.Fn exect
279and
280.Fn execv
281functions
282may fail and set
283.Va errno
284for any of the errors specified for the library function
285.Xr execve 2 .
286.Sh SEE ALSO
287.Xr sh 1 ,
288.Xr execve 2 ,
289.Xr fork 2 ,
290.Xr ktrace 2 ,
291.Xr ptrace 2 ,
292.Xr environ 7
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293.Sh STANDARDS
294The
295.Fn execl ,
296.Fn execv ,
297.Fn execle ,
298.Fn execlp
299and
300.Fn execvp
301functions
302conform to
303.St -p1003.1-88 .