man3/exec.3

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  34 .\"     @(#)exec.3      6.4 (Berkeley) 4/19/91
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  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  2015-04-19 "GNU" "Linux Programmer's Manual"
  42 .SH NAME
  43 execl, execlp, execle, execv, execvp, execvpe \- execute a file
  44 .SH SYNOPSIS
  45 .B #include <unistd.h>
  46 .sp
  47 .B extern char **environ;
  48 .sp
  49 .BI "int execl(const char *" path ", const char *" arg ", ..."
  50 .br
  51 .B  "                /* (char  *) NULL */);"
  52 .br
  53 .BI "int execlp(const char *" file ", const char *" arg ", ..."
  54 .br
  55 .B  "                /* (char  *) NULL */);"
  56 .br
  57 .BI "int execle(const char *" path ", const char *" arg ", ..."
  58 .br
  59 .BI "                /*, (char *) NULL, char * const " envp "[] */);"
  60 .br
  61 .BI "int execv(const char *" path ", char *const " argv "[]);"
  62 .br
  63 .BI "int execvp(const char *" file ", char *const " argv "[]);"
  64 .br
  65 .BI "int execvpe(const char *" file ", char *const " argv "[],"
  66 .br
  67 .BI "                char *const " envp "[]);"
  68 .sp
  69 .in -4n
  70 Feature Test Macro Requirements for glibc (see
  71 .BR feature_test_macros (7)):
  72 .in
  73 .sp
  74 .BR execvpe ():
  75 _GNU_SOURCE
  76 .SH DESCRIPTION
  77 The
  78 .BR exec ()
  79 family of functions replaces the current process image with a new process
  80 image.
  81 The functions described in this manual page are front-ends for
  82 .BR execve (2).
  83 (See the manual page for
  84 .BR execve (2)
  85 for further details about the replacement of the current process image.)
  86 .PP
  87 The initial argument for these functions is the name of a file that is
  88 to be executed.
  89 .PP
  90 The
  91 .I "const char\ *arg"
  92 and subsequent ellipses in the
  93 .BR execl (),
  94 .BR execlp (),
  95 and
  96 .BR execle ()
  97 functions can be thought of as
  98 .IR arg0 ,
  99 .IR arg1 ,
 100 \&...,
 101 .IR argn .
 102 Together they describe a list of one or more pointers to null-terminated
 103 strings that represent the argument list available to the executed program.
 104 The first argument, by convention, should point to the filename associated
 105 with the file being executed.
 106 The list of arguments
 107 .I must
 108 be terminated by a null pointer,
 109 and, since these are variadic functions, this pointer must be cast
 110 .IR "(char\ *) NULL" .
 111 .PP
 112 The
 113 .BR execv (),
 114 .BR execvp (),
 115 and
 116 .BR execvpe ()
 117 functions provide an array of pointers to null-terminated strings that
 118 represent the argument list available to the new program.
 119 The first argument, by convention, should point to the filename
 120 associated with the file being executed.
 121 The array of pointers
 122 .I must
 123 be terminated by a null pointer.
 124 .PP
 125 The
 126 .BR execle ()
 127 and
 128 .BR execvpe ()
 129 functions allow the caller to specify the environment of the
 130 executed program via the argument
 131 .IR envp .
 132 The
 133 .I envp
 134 argument is an array of pointers to null-terminated strings and
 135 .I must
 136 be terminated by a null pointer.
 137 The other functions take the environment for the new process
 138 image from the external variable
 139 .I environ
 140 in the calling process.
 141 .SS Special semantics for execlp() and execvp()
 142 .PP
 143 The
 144 .BR execlp (),
 145 .BR execvp (),
 146 and
 147 .BR execvpe ()
 148 functions duplicate the actions of the shell in
 149 searching for an executable file
 150 if the specified filename does not contain a slash (/) character.
 151 The file is sought in the colon-separated list of directory pathnames
 152 specified in the
 153 .B PATH
 154 environment variable.
 155 If this variable isn't defined, the path list defaults to
 156 the current directory followed by the list of directories returned by
 157 .IR confstr(_CS_PATH) .
 158 (This
 159 .BR confstr (3)
 160 call typically returns the value "/bin:/usr/bin".)
 161
 162 If the specified filename includes a slash character, then
 163 .B PATH
 164 is ignored, and the file at the specified pathname is executed.
 165
 166 In addition, certain errors are treated specially.
 167
 168 If permission is denied for a file (the attempted
 169 .BR execve (2)
 170 failed with the error
 171 .BR EACCES ),
 172 these functions will continue searching the rest of the search path.
 173 If no other file is found, however,
 174 they will return with
 175 .I errno
 176 set to
 177 .BR EACCES .
 178
 179 If the header of a file isn't recognized (the attempted
 180 .BR execve (2)
 181 failed with the error
 182 .BR ENOEXEC ),
 183 these functions will execute the shell
 184 .RI ( /bin/sh )
 185 with the path of the file as its first argument.
 186 (If this attempt fails, no further searching is done.)
 187 .SH RETURN VALUE
 188 The
 189 .BR exec ()
 190 functions return only if an error has occurred.
 191 The return value is \-1, and
 192 .I errno
 193 is set to indicate the error.
 194 .SH ERRORS
 195 All of these functions may fail and set
 196 .I errno
 197 for any of the errors specified for
 198 .BR execve (2).
 199 .SH VERSIONS
 200 The
 201 .BR execvpe ()
 202 function first appeared in glibc 2.11.
 203 .SH ATTRIBUTES
 204 For an explanation of the terms used in this section, see
 205 .BR attributes (7).
 206 .TS
 207 allbox;
 208 lbw29 lb lb
 209 l l l.
 210 Interface       Attribute       Value
 211 T{
 212 .BR execl (),
 213 .BR execle (),
 214 .BR execv ()
 215 T}      Thread safety   MT-Safe
 216 T{
 217 .BR execlp (),
 218 .BR execvp (),
 219 .BR execvpe ()
 220 T}      Thread safety   MT-Safe env
 221 .TE
 222 .SH CONFORMING TO
 223 POSIX.1-2001, POSIX.1-2008.
 224
 225 The
 226 .BR execvpe ()
 227 function is a GNU extension.
 228 .SH NOTES
 229 On some other systems, the default path (used when the environment
 230 does not contain the variable \fBPATH\fR) has the current working
 231 directory listed after
 232 .I /bin
 233 and
 234 .IR /usr/bin ,
 235 as an anti-Trojan-horse measure.
 236 Linux uses here the
 237 traditional "current directory first" default path.
 238 .PP
 239 The behavior of
 240 .BR execlp ()
 241 and
 242 .BR execvp ()
 243 when errors occur while attempting to execute the file is historic
 244 practice, but has not traditionally been documented and is not specified by
 245 the POSIX standard.
 246 BSD (and possibly other systems) do an automatic
 247 sleep and retry if
 248 .B ETXTBSY
 249 is encountered.
 250 Linux treats it as a hard
 251 error and returns immediately.
 252 .PP
 253 Traditionally, the functions
 254 .BR execlp ()
 255 and
 256 .BR execvp ()
 257 ignored all errors except for the ones described above and
 258 .B ENOMEM
 259 and
 260 .BR E2BIG ,
 261 upon which they returned.
 262 They now return if any error other than the ones
 263 described above occurs.
 264 .SH SEE ALSO
 265 .BR sh (1),
 266 .BR execve (2),
 267 .BR execveat (2),
 268 .BR fork (2),
 269 .BR ptrace (2),
 270 .BR fexecve (3),
 271 .BR environ (7)