gen/exec.3

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  28 .\"     @(#)exec.3      8.3 (Berkeley) 1/24/94
  29 .\" $FreeBSD: src/lib/libc/gen/exec.3,v 1.28 2008/06/23 05:22:06 ed Exp $
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  31 .Dd January 24, 1994
  32 .Dt EXEC 3
  33 .Os
  34 .Sh NAME
  35 .Nm execl ,
  36 .Nm execle ,
  37 .Nm execlp ,
  38 .Nm execv ,
  39 .Nm execvp ,
  40 .Nm execvP
  41 .Nd execute a file
  42 .Sh LIBRARY
  43 .Lb libc
  44 .Sh SYNOPSIS
  45 .In unistd.h
  46 .Vt extern char **environ ;
  47 .Ft int
  48 .Fo execl
  49 .Fa "const char *path"
  50 .Fa "const char *arg0"
  51 .Fa ... /* "(char *)0" */
  52 .Fc
  53 .Ft int
  54 .Fo execle
  55 .Fa "const char *path"
  56 .Fa "const char *arg0"
  57 .Fa ...
  58 .Fa /*
  59 .Bk -words
  60 .Fa "(char *)0" "char *const envp[]" */
  61 .Ek
  62 .Fc
  63 .Ft int
  64 .Fo execlp
  65 .Fa "const char *file"
  66 .Fa "const char *arg0"
  67 .Fa ... /* "(char *)0" */
  68 .Fc
  69 .Ft int
  70 .Fo execv
  71 .Fa "const char *path"
  72 .Fa "char *const argv[]"
  73 .Fc
  74 .Ft int
  75 .Fo execvp
  76 .Fa "const char *file"
  77 .Fa "char *const argv[]"
  78 .Fc
  79 .Ft int
  80 .Fo execvP
  81 .Fa "const char *file"
  82 .Fa "const char *search_path"
  83 .Fa "char *const argv[]"
  84 .Fc
  85 .Sh DESCRIPTION
  86 The
  87 .Nm exec
  88 family of functions replaces the current process image with a
  89 new process image.
  90 The functions described in this manual page are front-ends for the function
  91 .Xr execve 2 .
  92 (See the manual page for
  93 .Xr execve 2
  94 for detailed information about the replacement of the current process.)
  95 .Pp
  96 The initial argument for these functions is the pathname of a file which
  97 is to be executed.
  98 .Pp
  99 The
 100 .Fa "const char *arg0"
 101 and subsequent ellipses in the
 102 .Fn execl ,
 103 .Fn execlp ,
 104 and
 105 .Fn execle
 106 functions can be thought of as
 107 .Em arg0 ,
 108 .Em arg1 ,
 109 \&...,
 110 .Em argn .
 111 Together they describe a list of one or more pointers to null-terminated
 112 strings that represent the argument list available to the executed program.
 113 The first argument, by convention, should point to the file name associated
 114 with the file being executed.
 115 The list of arguments
 116 .Em must
 117 be terminated by a
 118 .Dv NULL
 119 pointer.
 120 .Pp
 121 The
 122 .Fn execv ,
 123 .Fn execvp ,
 124 and
 125 .Fn execvP
 126 functions provide an array of pointers to null-terminated strings that
 127 represent the argument list available to the new program.
 128 The first argument, by convention, should point to the file name associated
 129 with the file being executed.
 130 The array of pointers
 131 .Sy must
 132 be terminated by a
 133 .Dv NULL
 134 pointer.
 135 .Pp
 136 The
 137 .Fn execle
 138 function also specifies the environment of the executed process
 139 by following the
 140 .Dv NULL
 141 pointer that terminates the list of arguments in the argument list
 142 or the pointer to the argv array with an additional argument.
 143 This additional argument is an array of pointers to null-terminated strings
 144 and
 145 .Em must
 146 be terminated by a
 147 .Dv NULL
 148 pointer.
 149 The other functions take the environment for the new process image from the
 150 external variable
 151 .Va environ
 152 in the current process.
 153 .Pp
 154 Some of these functions have special semantics.
 155 .Pp
 156 The functions
 157 .Fn execlp ,
 158 .Fn execvp ,
 159 and
 160 .Fn execvP
 161 will duplicate the actions of the shell in searching for an executable file
 162 if the specified file name does not contain a slash
 163 .Dq Li /
 164 character.
 165 For
 166 .Fn execlp
 167 and
 168 .Fn execvp ,
 169 search path is the path specified in the environment by
 170 .Dq Ev PATH
 171 variable.
 172 If this variable is not specified,
 173 the default path is set according to the
 174 .Dv _PATH_DEFPATH
 175 definition in
 176 .In paths.h ,
 177 which is set to
 178 .Dq Ev /usr/bin:/bin .
 179 For
 180 .Fn execvP ,
 181 the search path is specified as an argument to the function.
 182 In addition, certain errors are treated specially.
 183 .Pp
 184 If an error is ambiguous (for simplicity, we shall consider all
 185 errors except
 186 .Er ENOEXEC
 187 as being ambiguous here, although only the critical error
 188 .Er EACCES
 189 is really ambiguous),
 190 then these functions will act as if they stat the file to determine
 191 whether the file exists and has suitable execute permissions.
 192 If it does, they will return immediately with the global variable
 193 .Va errno
 194 restored to the value set by
 195 .Fn execve .
 196 Otherwise, the search will be continued.
 197 If the search completes without performing a successful
 198 .Fn execve
 199 or terminating due to an error,
 200 these functions will return with the global variable
 201 .Va errno
 202 set to
 203 .Er EACCES
 204 or
 205 .Er ENOENT
 206 according to whether at least one file with suitable execute permissions
 207 was found.
 208 .Pp
 209 If the header of a file is not recognized (the attempted
 210 .Fn execve
 211 returned
 212 .Er ENOEXEC ) ,
 213 these functions will execute the shell with the path of
 214 the file as its first argument.
 215 (If this attempt fails, no further searching is done.)
 216 .Sh RETURN VALUES
 217 If any of the
 218 .Fn exec
 219 functions returns, an error will have occurred.
 220 The return value is \-1, and the global variable
 221 .Va errno
 222 will be set to indicate the error.
 223 .Sh FILES
 224 .Bl -tag -width /bin/sh -compact
 225 .It Pa /bin/sh
 226 The shell.
 227 .El
 228 .Sh COMPATIBILITY
 229 Historically, the default path for the
 230 .Fn execlp
 231 and
 232 .Fn execvp
 233 functions was
 234 .Dq Pa :/bin:/usr/bin .
 235 This was changed to place the current directory last to enhance system
 236 security.
 237 .Pp
 238 The behavior of
 239 .Fn execlp
 240 and
 241 .Fn execvp
 242 when errors occur while attempting to execute the file is not quite historic
 243 practice, and has not traditionally been documented and is not specified
 244 by the
 245 .Tn POSIX
 246 standard.
 247 .Pp
 248 Traditionally, the functions
 249 .Fn execlp
 250 and
 251 .Fn execvp
 252 ignored all errors except for the ones described above and
 253 .Er ETXTBSY ,
 254 upon which they retried after sleeping for several seconds, and
 255 .Er ENOMEM
 256 and
 257 .Er E2BIG ,
 258 upon which they returned.
 259 They now return for
 260 .Er ETXTBSY ,
 261 and determine existence and executability more carefully.
 262 In particular,
 263 .Er EACCES
 264 for inaccessible directories in the path prefix is no longer
 265 confused with
 266 .Er EACCES
 267 for files with unsuitable execute permissions.
 268 In
 269 .Bx 4.4 ,
 270 they returned upon all errors except
 271 .Er EACCES ,
 272 .Er ENOENT ,
 273 .Er ENOEXEC
 274 and
 275 .Er ETXTBSY .
 276 This was inferior to the traditional error handling,
 277 since it breaks the ignoring of errors for path prefixes
 278 and only improves the handling of the unusual ambiguous error
 279 .Er EFAULT
 280 and the unusual error
 281 .Er EIO .
 282 The behaviour was changed to match the behaviour of
 283 .Xr sh 1 .
 284 .Sh ERRORS
 285 The
 286 .Fn execl ,
 287 .Fn execle ,
 288 .Fn execlp ,
 289 .Fn execvp ,
 290 and
 291 .Fn execvP
 292 functions
 293 may fail and set
 294 .Va errno
 295 for any of the errors specified for the library functions
 296 .Xr execve 2
 297 and
 298 .Xr malloc 3 .
 299 .Pp
 300 The
 301 .Fn execv
 302 function may fail and set
 303 .Va errno
 304 for any of the errors specified for the library function
 305 .Xr execve 2 .
 306 .Sh SEE ALSO
 307 .Xr sh 1 ,
 308 .Xr execve 2 ,
 309 .Xr fork 2 ,
 310 .Xr ptrace 2 ,
 311 .Xr environ 7
 312 .Sh STANDARDS
 313 The
 314 .Fn execl ,
 315 .Fn execv ,
 316 .Fn execle ,
 317 .Fn execlp ,
 318 and
 319 .Fn execvp
 320 functions
 321 conform to
 322 .St -p1003.1-88 .
 323 The
 324 .Fn execvP
 325 function first appeared in
 326 .Fx 5.2 .