man3/exec.3

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  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  2019-03-06 "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
  84 .I "const char\ *arg"
  85 and subsequent ellipses in the
  86 .BR execl (),
  87 .BR execlp (),
  88 and
  89 .BR execle ()
  90 functions can be thought of as
  91 .IR arg0 ,
  92 .IR arg1 ,
  93 \&...,
  94 .IR argn .
  95 Together they describe a list of one or more pointers to null-terminated
  96 strings that represent the argument list available to the executed program.
  97 The first argument, by convention, should point to the filename associated
  98 with the file being executed.
  99 The list of arguments
 100 .I must
 101 be terminated by a null pointer,
 102 and, since these are variadic functions, this pointer must be cast
 103 .IR "(char\ *) NULL" .
 104 .PP
 105 The
 106 .BR execv (),
 107 .BR execvp (),
 108 and
 109 .BR execvpe ()
 110 functions provide an array of pointers to null-terminated strings that
 111 represent the argument list available to the new program.
 112 The first argument, by convention, should point to the filename
 113 associated with the file being executed.
 114 The array of pointers
 115 .I must
 116 be terminated by a null pointer.
 117 .PP
 118 The
 119 .BR execle ()
 120 and
 121 .BR execvpe ()
 122 functions allow the caller to specify the environment of the
 123 executed program via the argument
 124 .IR envp .
 125 The
 126 .I envp
 127 argument is an array of pointers to null-terminated strings and
 128 .I must
 129 be terminated by a null pointer.
 130 The other functions take the environment for the new process
 131 image from the external variable
 132 .I environ
 133 in the calling process.
 134 .SS Special semantics for execlp(), execvp(), and execvpe()
 135 .PP
 136 The
 137 .BR execlp (),
 138 .BR execvp (),
 139 and
 140 .BR execvpe ()
 141 functions duplicate the actions of the shell in
 142 searching for an executable file
 143 if the specified filename does not contain a slash (/) character.
 144 The file is sought in the colon-separated list of directory pathnames
 145 specified in the
 146 .B PATH
 147 environment variable.
 148 If this variable isn't defined, the path list defaults to
 149 a list that includes the directories returned by
 150 .IR confstr(_CS_PATH)
 151 (which typically returns the value "/bin:/usr/bin")
 152 and possibly also the current working directory;
 153 see NOTES for further details.
 154 .PP
 155 If the specified filename includes a slash character, then
 156 .B PATH
 157 is ignored, and the file at the specified pathname is executed.
 158 .PP
 159 In addition, certain errors are treated specially.
 160 .PP
 161 If permission is denied for a file (the attempted
 162 .BR execve (2)
 163 failed with the error
 164 .BR EACCES ),
 165 these functions will continue searching the rest of the search path.
 166 If no other file is found, however,
 167 they will return with
 168 .I errno
 169 set to
 170 .BR EACCES .
 171 .PP
 172 If the header of a file isn't recognized (the attempted
 173 .BR execve (2)
 174 failed with the error
 175 .BR ENOEXEC ),
 176 these functions will execute the shell
 177 .RI ( /bin/sh )
 178 with the path of the file as its first argument.
 179 (If this attempt fails, no further searching is done.)
 180 .SH RETURN VALUE
 181 The
 182 .BR exec ()
 183 functions return only if an error has occurred.
 184 The return value is \-1, and
 185 .I errno
 186 is set to indicate the error.
 187 .SH ERRORS
 188 All of these functions may fail and set
 189 .I errno
 190 for any of the errors specified for
 191 .BR execve (2).
 192 .SH VERSIONS
 193 The
 194 .BR execvpe ()
 195 function first appeared in glibc 2.11.
 196 .SH ATTRIBUTES
 197 For an explanation of the terms used in this section, see
 198 .BR attributes (7).
 199 .TS
 200 allbox;
 201 lbw29 lb lb
 202 l l l.
 203 Interface       Attribute       Value
 204 T{
 205 .BR execl (),
 206 .BR execle (),
 207 .BR execv ()
 208 T}      Thread safety   MT-Safe
 209 T{
 210 .BR execlp (),
 211 .BR execvp (),
 212 .BR execvpe ()
 213 T}      Thread safety   MT-Safe env
 214 .TE
 215 .SH CONFORMING TO
 216 POSIX.1-2001, POSIX.1-2008.
 217 .PP
 218 The
 219 .BR execvpe ()
 220 function is a GNU extension.
 221 .SH NOTES
 222 The default search path (used when the environment
 223 does not contain the variable \fBPATH\fR)
 224 shows some variation across systems.
 225 It generally includes
 226 .I /bin
 227 and
 228 .IR /usr/bin
 229 (in that order) and may also include the current working directory.
 230 On some other systems, the current working is included after
 231 .I /bin
 232 and
 233 .IR /usr/bin ,
 234 as an anti-Trojan-horse measure.
 235 The glibc implementation long followed the traditional default where
 236 the current working directory is included at the start of the search path.
 237 However, some code refactoring during the development of glibc 2.24
 238 .\" glibc commit 1eb8930608705702d5746e5491bab4e4429fcb83
 239 caused the current working directory to be dropped altogether
 240 from the default search path.
 241 This accidental behavior change is considered mildly beneficial,
 242 and won't be reverted.
 243 .PP
 244 The behavior of
 245 .BR execlp ()
 246 and
 247 .BR execvp ()
 248 when errors occur while attempting to execute the file is historic
 249 practice, but has not traditionally been documented and is not specified by
 250 the POSIX standard.
 251 BSD (and possibly other systems) do an automatic
 252 sleep and retry if
 253 .B ETXTBSY
 254 is encountered.
 255 Linux treats it as a hard
 256 error and returns immediately.
 257 .PP
 258 Traditionally, the functions
 259 .BR execlp ()
 260 and
 261 .BR execvp ()
 262 ignored all errors except for the ones described above and
 263 .B ENOMEM
 264 and
 265 .BR E2BIG ,
 266 upon which they returned.
 267 They now return if any error other than the ones
 268 described above occurs.
 269 .SH BUGS
 270 Before glibc 2.24,
 271 .BR execl ()
 272 and
 273 .BR execle ()
 274 employed
 275 .BR realloc (3)
 276 internally and were consequently not async-signal-safe,
 277 in violation of the requirements of POSIX.1.
 278 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=19534
 279 This was fixed in glibc 2.24.
 280 .\"
 281 .SS Architecture-specific details
 282 On sparc and sparc64,
 283 .BR execv ()
 284 is provided as a system call by the kernel
 285 (with the prototype shown above)
 286 for compatibility with SunOS.
 287 This function is
 288 .I not
 289 employed by the
 290 .BR execv ()
 291 wrapper function on those architectures.
 292 .SH SEE ALSO
 293 .BR sh (1),
 294 .BR execve (2),
 295 .BR execveat (2),
 296 .BR fork (2),
 297 .BR ptrace (2),
 298 .BR fexecve (3),
 299 .BR system (3),
 300 .BR environ (7)