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32 .\" @(#)execve.2 8.5 (Berkeley) 6/1/94
33 .\" $FreeBSD: src/lib/libc/sys/execve.2,v 1.16.2.10 2001/12/22 01:21:30 jwd Exp $
34 .\" $DragonFly: src/lib/libc/sys/execve.2,v 1.3 2007/12/21 22:14:04 swildner Exp $
35 .\"
36 .Dd December 21, 2007
37 .Dt EXECVE 2
38 .Os
39 .Sh NAME
40 .Nm execve
41 .Nd execute a file
42 .Sh LIBRARY
43 .Lb libc
44 .Sh SYNOPSIS
45 .In unistd.h
46 .Ft int
47 .Fn execve "const char *path" "char *const argv[]" "char *const envp[]"
48 .Sh DESCRIPTION
49 .Fn Execve
50 transforms the calling process into a new process.
51 The new process is constructed from an ordinary file,
52 whose name is pointed to by
53 .Fa path ,
54 called the
55 .Em new process file .
56 This file is either an executable object file,
57 or a file of data for an interpreter.
58 An executable object file consists of an identifying header,
59 followed by pages of data representing the initial program (text)
60 and initialized data pages. Additional pages may be specified
61 by the header to be initialized with zero data; see
62 .Xr elf 5
63 and
64 .Xr a.out 5 .
65 .Pp
66 An interpreter file begins with a line of the form:
67 .Pp
68 .Bd -ragged -offset indent -compact
69 .Sy \&#!
70 .Em interpreter
71 .Bq Em arg
72 .Ed
73 .Pp
74 When an interpreter file is
75 .Sy execve Ap d ,
76 the system actually
77 .Sy execve Ap s
78 the specified
79 .Em interpreter .
80 If the optional
81 .Em arg
82 is specified, it becomes the first argument to the
83 .Em interpreter ,
84 and the name of the originally
85 .Sy execve Ap d
86 file becomes the second argument;
87 otherwise, the name of the originally
88 .Sy execve Ap d
89 file becomes the first argument. The original arguments are shifted over to
90 become the subsequent arguments.
91 The zeroth argument is set to the specified
92 .Em interpreter .
93 (See
94 .Xr script 7
95 for a detailed discussion of interpreter file execution.)
96 .Pp
97 The argument
98 .Fa argv
99 is a pointer to a null-terminated array of
100 character pointers to null-terminated character strings.
101 These strings construct the argument list to be made available to the new
102 process. At least one argument must be present in
103 the array; by custom, the first element should be
104 the name of the executed program (for example, the last component of
105 .Fa path ) .
106 .Pp
107 The argument
108 .Fa envp
109 is also a pointer to a null-terminated array of
110 character pointers to null-terminated strings.
111 A pointer to this array is normally stored in the global variable
112 .Va environ .
113 These strings pass information to the
114 new process that is not directly an argument to the command (see
115 .Xr environ 7 ) .
116 .Pp
117 File descriptors open in the calling process image remain open in
118 the new process image, except for those for which the close-on-exec
119 flag is set (see
120 .Xr close 2
121 and
122 .Xr fcntl 2 ) .
123 Descriptors that remain open are unaffected by
124 .Fn execve .
125 .Pp
126 Signals set to be ignored in the calling process are set to be ignored in
127 the
128 new process.
129 Signals which are set to be caught in the calling process image
130 are set to default action in the new process image.
131 Blocked signals remain blocked regardless of changes to the signal action.
132 The signal stack is reset to be undefined (see
133 .Xr sigaction 2
134 for more information).
135 .Pp
136 If the set-user-ID mode bit of the new process image file is set
137 (see
138 .Xr chmod 2 ) ,
139 the effective user ID of the new process image is set to the owner ID
140 of the new process image file.
141 If the set-group-ID mode bit of the new process image file is set,
142 the effective group ID of the new process image is set to the group ID
143 of the new process image file.
144 (The effective group ID is the first element of the group list.)
145 The real user ID, real group ID and
146 other group IDs of the new process image remain the same as the calling
147 process image.
148 After any set-user-ID and set-group-ID processing,
149 the effective user ID is recorded as the saved set-user-ID,
150 and the effective group ID is recorded as the saved set-group-ID.
151 These values may be used in changing the effective IDs later (see
152 .Xr setuid 2 ) .
153 .Pp
154 The set-ID bits are not honored if the respective file system has the
155 .Ar nosuid
156 option enabled or if the new process file is an interpreter file. Syscall
157 tracing is disabled if effective IDs are changed.
158 .Pp
159 The new process also inherits the following attributes from
160 the calling process:
161 .Pp
162 .Bl -column parent_process_ID -offset indent -compact
163 .It process ID Ta see Xr getpid 2
164 .It parent process ID Ta see Xr getppid 2
165 .It process group ID Ta see Xr getpgrp 2
166 .It access groups Ta see Xr getgroups 2
167 .It working directory Ta see Xr chdir 2
168 .It root directory Ta see Xr chroot 2
169 .It control terminal Ta see Xr termios 4
170 .It resource usages Ta see Xr getrusage 2
171 .It interval timers Ta see Xr getitimer 2
172 .It resource limits Ta see Xr getrlimit 2
173 .It file mode mask Ta see Xr umask 2
174 .It signal mask Ta see Xr sigvec 2 ,
175 .Xr sigsetmask 2
176 .El
177 .Pp
178 When a program is executed as a result of an
179 .Fn execve
180 call, it is entered as follows:
181 .Bd -literal -offset indent
182 main(argc, argv, envp)
183 int argc;
184 char **argv, **envp;
185 .Ed
186 .Pp
187 where
188 .Fa argc
189 is the number of elements in
190 .Fa argv
191 (the ``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 ENOTDIR
213 A component of the path prefix is not a directory.
214 .It Bq Er ENAMETOOLONG
215 A component of a pathname exceeded 255 characters,
216 or an entire path name exceeded 1023 characters.
217 .It Bq Er ENAMETOOLONG
218 When invoking an interpreted script, the interpreter name
219 exceeds
220 .Dv MAXSHELLCMDLEN
221 characters.
222 .It Bq Er ENOENT
223 The new process file does not exist.
224 .It Bq Er ELOOP
225 Too many symbolic links were encountered in translating the pathname.
226 .It Bq Er EACCES
227 Search permission is denied for a component of the path prefix.
228 .It Bq Er EACCES
229 The new process file is not an ordinary file.
230 .It Bq Er EACCES
231 The new process file mode denies execute permission.
232 .It Bq Er ENOEXEC
233 The new process file has the appropriate access
234 permission, but has an invalid magic number in its header.
235 .It Bq Er ETXTBSY
236 The new process file is a pure procedure (shared text)
237 file that is currently open for writing or reading by some process.
238 .It Bq Er ENOMEM
239 The new process requires more virtual memory than
240 is allowed by the imposed maximum
241 .Pq Xr getrlimit 2 .
242 .It Bq Er E2BIG
243 The number of bytes in the new process' argument list
244 is larger than the system-imposed limit.
245 This limit is specified by the
246 .Xr sysctl 3
247 MIB variable
248 .Dv KERN_ARGMAX .
249 .It Bq Er EFAULT
250 The new process file is not as long as indicated by
251 the size values in its header.
252 .It Bq Er EFAULT
253 .Fa Path ,
254 .Fa argv ,
255 or
256 .Fa envp
257 point
258 to an illegal address.
259 .It Bq Er EIO
260 An I/O error occurred while reading from the file system.
261 .El
262 .Sh CAVEAT
263 If a program is
264 .Em setuid
265 to a non-super-user, but is executed when
266 the real
267 .Em uid
268 is ``root'', then the program has some of the powers
269 of a super-user as well.
270 .Sh SEE ALSO
271 .Xr ktrace 1 ,
272 .Xr fork 2 ,
273 .Xr _exit 2 ,
274 .Xr execl 3 ,
275 .Xr exit 3 ,
276 .Xr sysctl 3 ,
277 .Xr a.out 5 ,
278 .Xr elf 5 ,
279 .Xr environ 7 ,
280 .Xr script 7 ,
281 .Xr mount 8
282 .Sh HISTORY
283 The
284 .Fn execve
285 function call appeared in
286 .Bx 4.2 .