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30 .\" @(#)execve.2 8.5 (Berkeley) 6/1/94
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32 .Dd February 24, 2008
33 .Dt EXECVE 2
34 .Os
35 .Sh NAME
36 .Nm execve
37 .Nd execute a file
38 .Sh LIBRARY
39 .Lb libc
40 .Sh SYNOPSIS
41 .In unistd.h
42 .Ft int
43 .Fn execve "const char *path" "char *const argv[]" "char *const envp[]"
44 .Sh DESCRIPTION
45 .Fn execve
46 transforms the calling process into a new process.
47 The new process is constructed from an ordinary file,
48 whose name is pointed to by
49 .Fa path ,
50 called the
51 .Em new process file .
52 This file is either an executable object file,
53 or a file of data for an interpreter.
54 An executable object file consists of an identifying header,
55 followed by pages of data representing the initial program (text)
56 and initialized data pages.
57 Additional pages may be specified
58 by the header to be initialized with zero data; see
59 .Xr a.out 5 .
60 .Pp
61 An interpreter file begins with a line of the form:
62 .Pp
63 .Bd -filled -offset indent -compact
64 .Sy \&#!
65 .Em interpreter
66 .Bq Em arg
67 .Ed
68 .Pp
69 When an interpreter file is
70 .Fn execve Ns d
71 the system runs the specified
72 .Em interpreter .
73 If the optional
74 .Em arg
75 is specified, it becomes the first argument to the
76 .Em interpreter ,
77 and the name of the originally
78 .Fn execve Ns d
79 file becomes the second argument;
80 otherwise, the name of the originally
81 .Fn execve Ns d
82 file becomes the first argument.
83 The original arguments are shifted over to become the subsequent arguments.
84 The zeroth argument, normally the name of the
85 .Fn execve Ns d
86 file, is left unchanged.
87 The interpreter named by
88 .Em interpreter
89 must not itself be an interpreter file.
90 (See
91 .Xr script 7
92 for a detailed discussion of interpreter file execution.)
93 .Pp
94 The argument
95 .Fa argv
96 is a pointer to a null-terminated array of
97 character pointers to null-terminated character strings.
98 These strings construct the argument list to be made available to the new
99 process.
100 By custom, the first element should be
101 the name of the executed program (for example, the last component of
102 .Fa path ) .
103 .Pp
104 The argument
105 .Fa envp
106 is also a pointer to a null-terminated array of
107 character pointers to null-terminated strings.
108 A pointer to this array is normally stored in the global variable
109 .Va environ .
110 These strings pass information to the
111 new process that is not directly an argument to the command (see
112 .Xr environ 7 ) .
113 .Pp
114 File descriptors open in the calling process image remain open in
115 the new process image, except for those for which the close-on-exec
116 flag is set (see
117 .Xr close 2
118 and
119 .Xr fcntl 2 ) .
120 Descriptors that remain open are unaffected by
121 .Fn execve .
122 .Pp
123 In the case of a new setuid or setgid executable being executed, if
124 file descriptors 0, 1, or 2 (representing stdin, stdout, and stderr)
125 are currently unallocated, these descriptors will be opened to point to
126 some system file like
127 .Pa /dev/null .
128 The intent is to ensure these descriptors are not unallocated, since
129 many libraries make assumptions about the use of these 3 file descriptors.
130 .Pp
131 Signals set to be ignored in the calling process are set to be ignored in
132 the new process.
133 Signals which are set to be caught in the calling process image
134 are set to default action in the new process image.
135 Blocked signals remain blocked regardless of changes to the signal action.
136 The signal stack is reset to be undefined (see
137 .Xr sigaction 2
138 for more information).
139 .Pp
140 If the set-user-ID mode bit of the new process image file is set
141 (see
142 .Xr chmod 2 ) ,
143 the effective user ID of the new process image is set to the owner ID
144 of the new process image file.
145 If the set-group-ID mode bit of the new process image file is set,
146 the effective group ID of the new process image is set to the group ID
147 of the new process image file.
148 (The effective group ID is the first element of the group list.)
149 The real user ID, real group ID and
150 other group IDs of the new process image remain the same as the calling
151 process image.
152 After any set-user-ID and set-group-ID processing,
153 the effective user ID is recorded as the saved set-user-ID,
154 and the effective group ID is recorded as the saved set-group-ID.
155 These values may be used in changing the effective IDs later (see
156 .Xr setuid 2 ) .
157 .Pp
158 The new process also inherits the following attributes from
159 the calling process:
160 .Pp
161 .Bl -column parent_process_ID -offset indent -compact
162 .It process ID Ta see Xr getpid 2
163 .It parent process ID Ta see Xr getppid 2
164 .It process group ID Ta see Xr getpgrp 2
165 .It access groups Ta see Xr getgroups 2
166 .It working directory Ta see Xr chdir 2
167 .It root directory Ta see Xr chroot 2
168 .It control terminal Ta see Xr termios 4
169 .It resource usages Ta see Xr getrusage 2
170 .It interval timers Ta see Xr getitimer 2
171 .It resource limits Ta see Xr getrlimit 2
172 .It file mode mask Ta see Xr umask 2
173 .It signal mask Ta see Xr sigaction 2 ,
174 .Xr sigprocmask 2
175 .El
176 .Pp
177 When a program is executed as a result of an
178 .Fn execve
179 call, it is entered as follows:
180 .Bd -literal -offset indent
181 main(argc, argv, envp)
182 int argc;
183 char **argv, **envp;
184 .Ed
185 .Pp
186 where
187 .Fa argc
188 is the number of elements in
189 .Fa argv
190 (the
191 .Dq arg count )
192 and
193 .Fa argv
194 points to the array of character pointers
195 to the arguments themselves.
196 .Sh RETURN VALUES
197 As the
198 .Fn execve
199 function overlays the current process image
200 with a new process image the successful call
201 has no process to return to.
202 If
203 .Fn execve
204 does return to the calling process an error has occurred; the
205 return value will be \-1 and the global variable
206 .Va errno
207 is set to indicate the error.
208 .Sh ERRORS
209 .Fn execve
210 will fail and return to the calling process if:
211 .Bl -tag -width Er
212 .It Bq Er E2BIG
213 The number of bytes in the new process's argument list
214 is larger than the system-imposed limit.
215 The limit in the system as released is 262144 bytes
216 .Dv ( NCARGS
217 in
218 .Ao Pa sys/param.h Ac ) .
219 .It Bq Er EACCES
220 Search permission is denied for a component of the path prefix,
221 the new process file is not an ordinary file,
222 its file mode denies execute permission, or
223 it is on a filesystem mounted with execution
224 disabled
225 .Dv ( MNT_NOEXEC
226 in
227 .Ao Pa sys/mount.h Ac ) .
228 .It Bq Er EAGAIN
229 A
230 .Xr setuid 7
231 process has exceeded the current resource limit for the number of
232 processes it is allowed to run concurrently.
233 .It Bq Er EFAULT
234 The new process file is not as long as indicated by
235 the size values in its header; or
236 .Fa path ,
237 .Fa argv ,
238 or
239 .Fa envp
240 point to an illegal address.
241 .It Bq Er EIO
242 An I/O error occurred while reading from the file system.
243 .It Bq Er ELOOP
244 Too many symbolic links were encountered in translating the pathname.
245 .It Bq Er ENAMETOOLONG
246 A component of a pathname exceeded
247 .Brq Dv NAME_MAX
248 characters, or an entire path name exceeded
249 .Brq Dv PATH_MAX
250 characters.
251 .It Bq Er ENOENT
252 The new process file does not exist, or
253 the new process file is a script starting with
254 .Li #!
255 and the script interpreter does not exist.
256 .It Bq Er ENOEXEC
257 The new process file has the appropriate access
258 permission, but has an invalid magic number in its header.
259 .It Bq Er ENOMEM
260 The new process requires more virtual memory than
261 is allowed by the imposed maximum
262 .Pq Xr getrlimit 2 .
263 .It Bq Er ENOTDIR
264 A component of the path prefix is not a directory.
265 .It Bq Er ETXTBSY
266 The new process file is a pure procedure (shared text)
267 file that is currently open for writing or reading by some process.
268 .El
269 .Sh SEE ALSO
270 .Xr _exit 2 ,
271 .Xr fork 2 ,
272 .Xr execl 3 ,
273 .Xr environ 7 ,
274 .Xr script 7
275 .Sh STANDARDS
276 The
277 .Fn execve
278 function conforms to
279 .St -p1003.1-90 .
280 .Sh HISTORY
281 The
282 .Fn execve
283 function call first appeared in
284 .At v7 .
285 .Sh BUGS
286 If a program is
287 .Em setuid
288 to a non-super-user, but is executed when
289 the real
290 .Em uid
291 is
292 .Dq root ,
293 then the program has some of the powers of a super-user as well.