man3/exec.3

   1 .\" Copyright (c) 1991 The Regents of the University of California.
   2 .\" All rights reserved.
   3 .\"
   4 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
   5 .\" Redistribution and use in source and binary forms, with or without
   6 .\" modification, are permitted provided that the following conditions
   7 .\" are met:
   8 .\" 1. Redistributions of source code must retain the above copyright
   9 .\"    notice, this list of conditions and the following disclaimer.
  10 .\" 2. Redistributions in binary form must reproduce the above copyright
  11 .\"    notice, this list of conditions and the following disclaimer in the
  12 .\"    documentation and/or other materials provided with the distribution.
  13 .\" 3. All advertising materials mentioning features or use of this software
  14 .\"    must display the following acknowledgement:
  15 .\"     This product includes software developed by the University of
  16 .\"     California, Berkeley and its contributors.
  17 .\" 4. Neither the name of the University nor the names of its contributors
  18 .\"    may be used to endorse or promote products derived from this software
  19 .\"    without specific prior written permission.
  20 .\"
  21 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  22 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  25 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31 .\" SUCH DAMAGE.
  32 .\" %%%LICENSE_END
  33 .\"
  34 .\"     @(#)exec.3      6.4 (Berkeley) 4/19/91
  35 .\"
  36 .\" Converted for Linux, Mon Nov 29 11:12:48 1993, faith@cs.unc.edu
  37 .\" Updated more for Linux, Tue Jul 15 11:54:18 1997, pacman@cqc.com
  38 .\" Modified, 24 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  39 .\"     Added note on casting NULL
  40 .\"
  41 .TH EXEC 3  2019-08-02 "GNU" "Linux Programmer's Manual"
  42 .SH NAME
  43 execl, execlp, execle, execv, execvp, execvpe \- execute a file
  44 .SH SYNOPSIS
  45 .nf
  46 .B #include <unistd.h>
  47 .PP
  48 .B extern char **environ;
  49 .PP
  50 .BI "int execl(const char *" pathname ", const char *" arg ", ..."
  51 .B  "                /* (char  *) NULL */);"
  52 .BI "int execlp(const char *" file ", const char *" arg ", ..."
  53 .B  "                /* (char  *) NULL */);"
  54 .BI "int execle(const char *" pathname ", const char *" arg ", ..."
  55 .BI "                /*, (char *) NULL, char * const " envp "[] */);"
  56 .BI "int execv(const char *" pathname ", char *const " argv "[]);"
  57 .BI "int execvp(const char *" file ", char *const " argv "[]);"
  58 .BI "int execvpe(const char *" file ", char *const " argv "[],"
  59 .BI "                char *const " envp "[]);"
  60 .fi
  61 .PP
  62 .in -4n
  63 Feature Test Macro Requirements for glibc (see
  64 .BR feature_test_macros (7)):
  65 .in
  66 .PP
  67 .BR execvpe ():
  68 _GNU_SOURCE
  69 .SH DESCRIPTION
  70 The
  71 .BR exec ()
  72 family of functions replaces the current process image with a new process
  73 image.
  74 The functions described in this manual page are front-ends for
  75 .BR execve (2).
  76 (See the manual page for
  77 .BR execve (2)
  78 for further details about the replacement of the current process image.)
  79 .PP
  80 The initial argument for these functions is the name of a file that is
  81 to be executed.
  82 .PP
  83 The functions can be grouped based on the letters following the "exec" prefix.
  84 .\"
  85 .SS l - execl(), execlp(), execle()
  86 .PP
  87 The
  88 .I "const char\ *arg"
  89 and subsequent ellipses can be thought of as
  90 .IR arg0 ,
  91 .IR arg1 ,
  92 \&...,
  93 .IR argn .
  94 Together they describe a list of one or more pointers to null-terminated
  95 strings that represent the argument list available to the executed program.
  96 The first argument, by convention, should point to the filename associated
  97 with the file being executed.
  98 The list of arguments
  99 .I must
 100 be terminated by a null pointer,
 101 and, since these are variadic functions, this pointer must be cast
 102 .IR "(char\ *) NULL" .
 103 .PP
 104 By contrast with the 'l' functions, the 'v' functions (below) specify the
 105 command-line arguments of the executed program as a vector.
 106 .\"
 107 .SS v - execv(), execvp(), execvpe()
 108 .PP
 109 The
 110 .I "char\ *const argv[]"
 111 argument is an array of pointers to null-terminated strings that
 112 represent the argument list available to the new program.
 113 The first argument, by convention, should point to the filename
 114 associated with the file being executed.
 115 The array of pointers
 116 .I must
 117 be terminated by a null pointer.
 118 .SS e - execle(), execvpe()
 119 .PP
 120 The environment of the caller is specified via the argument
 121 .IR envp .
 122 The
 123 .I envp
 124 argument is an array of pointers to null-terminated strings and
 125 .I must
 126 be terminated by a null pointer.
 127 .PP
 128 All other
 129 .BR exec ()
 130 functions (which do not include 'e' in the suffix)
 131 take the environment for the new process
 132 image from the external variable
 133 .I environ
 134 in the calling process.
 135 .SS p - execlp(), execvp(), execvpe()
 136 .PP
 137 These functions duplicate the actions of the shell in
 138 searching for an executable file
 139 if the specified filename does not contain a slash (/) character.
 140 The file is sought in the colon-separated list of directory pathnames
 141 specified in the
 142 .B PATH
 143 environment variable.
 144 If this variable isn't defined, the path list defaults to
 145 a list that includes the directories returned by
 146 .IR confstr(_CS_PATH)
 147 (which typically returns the value "/bin:/usr/bin")
 148 and possibly also the current working directory;
 149 see NOTES for further details.
 150 .PP
 151 If the specified filename includes a slash character, then
 152 .B PATH
 153 is ignored, and the file at the specified pathname is executed.
 154 .PP
 155 In addition, certain errors are treated specially.
 156 .PP
 157 If permission is denied for a file (the attempted
 158 .BR execve (2)
 159 failed with the error
 160 .BR EACCES ),
 161 these functions will continue searching the rest of the search path.
 162 If no other file is found, however,
 163 they will return with
 164 .I errno
 165 set to
 166 .BR EACCES .
 167 .PP
 168 If the header of a file isn't recognized (the attempted
 169 .BR execve (2)
 170 failed with the error
 171 .BR ENOEXEC ),
 172 these functions will execute the shell
 173 .RI ( /bin/sh )
 174 with the path of the file as its first argument.
 175 (If this attempt fails, no further searching is done.)
 176 .PP
 177 All other
 178 .BR exec ()
 179 functions (which do not include 'p' in the suffix)
 180 take as their first argument a (relative or absolute) pathname
 181 that identifies the program to be executed.
 182 .SH RETURN VALUE
 183 The
 184 .BR exec ()
 185 functions return only if an error has occurred.
 186 The return value is \-1, and
 187 .I errno
 188 is set to indicate the error.
 189 .SH ERRORS
 190 All of these functions may fail and set
 191 .I errno
 192 for any of the errors specified for
 193 .BR execve (2).
 194 .SH VERSIONS
 195 The
 196 .BR execvpe ()
 197 function first appeared in glibc 2.11.
 198 .SH ATTRIBUTES
 199 For an explanation of the terms used in this section, see
 200 .BR attributes (7).
 201 .TS
 202 allbox;
 203 lbw29 lb lb
 204 l l l.
 205 Interface       Attribute       Value
 206 T{
 207 .BR execl (),
 208 .BR execle (),
 209 .BR execv ()
 210 T}      Thread safety   MT-Safe
 211 T{
 212 .BR execlp (),
 213 .BR execvp (),
 214 .BR execvpe ()
 215 T}      Thread safety   MT-Safe env
 216 .TE
 217 .SH CONFORMING TO
 218 POSIX.1-2001, POSIX.1-2008.
 219 .PP
 220 The
 221 .BR execvpe ()
 222 function is a GNU extension.
 223 .SH NOTES
 224 The default search path (used when the environment
 225 does not contain the variable \fBPATH\fR)
 226 shows some variation across systems.
 227 It generally includes
 228 .I /bin
 229 and
 230 .IR /usr/bin
 231 (in that order) and may also include the current working directory.
 232 On some other systems, the current working is included after
 233 .I /bin
 234 and
 235 .IR /usr/bin ,
 236 as an anti-Trojan-horse measure.
 237 The glibc implementation long followed the traditional default where
 238 the current working directory is included at the start of the search path.
 239 However, some code refactoring during the development of glibc 2.24
 240 .\" glibc commit 1eb8930608705702d5746e5491bab4e4429fcb83
 241 caused the current working directory to be dropped altogether
 242 from the default search path.
 243 This accidental behavior change is considered mildly beneficial,
 244 and won't be reverted.
 245 .PP
 246 The behavior of
 247 .BR execlp ()
 248 and
 249 .BR execvp ()
 250 when errors occur while attempting to execute the file is historic
 251 practice, but has not traditionally been documented and is not specified by
 252 the POSIX standard.
 253 BSD (and possibly other systems) do an automatic
 254 sleep and retry if
 255 .B ETXTBSY
 256 is encountered.
 257 Linux treats it as a hard
 258 error and returns immediately.
 259 .PP
 260 Traditionally, the functions
 261 .BR execlp ()
 262 and
 263 .BR execvp ()
 264 ignored all errors except for the ones described above and
 265 .B ENOMEM
 266 and
 267 .BR E2BIG ,
 268 upon which they returned.
 269 They now return if any error other than the ones
 270 described above occurs.
 271 .SH BUGS
 272 Before glibc 2.24,
 273 .BR execl ()
 274 and
 275 .BR execle ()
 276 employed
 277 .BR realloc (3)
 278 internally and were consequently not async-signal-safe,
 279 in violation of the requirements of POSIX.1.
 280 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=19534
 281 This was fixed in glibc 2.24.
 282 .\"
 283 .SS Architecture-specific details
 284 On sparc and sparc64,
 285 .BR execv ()
 286 is provided as a system call by the kernel
 287 (with the prototype shown above)
 288 for compatibility with SunOS.
 289 This function is
 290 .I not
 291 employed by the
 292 .BR execv ()
 293 wrapper function on those architectures.
 294 .SH SEE ALSO
 295 .BR sh (1),
 296 .BR execve (2),
 297 .BR execveat (2),
 298 .BR fork (2),
 299 .BR ptrace (2),
 300 .BR fexecve (3),
 301 .BR system (3),
 302 .BR environ (7)