gen/FreeBSD/exec.3

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  32 .\"     @(#)exec.3      8.3 (Berkeley) 1/24/94
  33 .\" $FreeBSD: src/lib/libc/gen/exec.3,v 1.23 2003/09/10 19:24:32 ru Exp $
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  35 .Dd January 24, 1994
  36 .Dt EXEC 3
  37 .Os
  38 .Sh NAME
  39 .Nm execl ,
  40 .Nm execlp ,
  41 .Nm execle ,
  42 .Nm exect ,
  43 .Nm execv ,
  44 .Nm execvp ,
  45 .Nm execvP
  46 .Nd execute a file
  47 .Sh LIBRARY
  48 .Lb libc
  49 .Sh SYNOPSIS
  50 .In unistd.h
  51 .Vt extern char **environ ;
  52 .Ft int
  53 .Fn execl "const char *path" "const char *arg" ... /* "(char *)0" */
  54 .Ft int
  55 .Fn execlp "const char *file" "const char *arg" ... /* "(char *)0" */
  56 .Ft int
  57 .Fo execle
  58 .Fa "const char *path" "const char *arg" ...
  59 .Fa /*
  60 .Bk -words
  61 .Fa "(char *)0" "char *const envp[]" */
  62 .Ek
  63 .Fc
  64 .Ft int
  65 .Fn exect "const char *path" "char *const argv[]" "char *const envp[]"
  66 .Ft int
  67 .Fn execv "const char *path" "char *const argv[]"
  68 .Ft int
  69 .Fn execvp "const char *file" "char *const argv[]"
  70 .Ft int
  71 .Fn execvP "const char *file" "const char *search_path" "char *const argv[]"
  72 .Sh DESCRIPTION
  73 The
  74 .Nm exec
  75 family of functions replaces the current process image with a
  76 new process image.
  77 The functions described in this manual page are front-ends for the function
  78 .Xr execve 2 .
  79 (See the manual page for
  80 .Xr execve 2
  81 for detailed information about the replacement of the current process.)
  82 .Pp
  83 The initial argument for these functions is the pathname of a file which
  84 is to be executed.
  85 .Pp
  86 The
  87 .Fa "const char *arg"
  88 and subsequent ellipses in the
  89 .Fn execl ,
  90 .Fn execlp ,
  91 and
  92 .Fn execle
  93 functions can be thought of as
  94 .Em arg0 ,
  95 .Em arg1 ,
  96 \&...,
  97 .Em argn .
  98 Together they describe a list of one or more pointers to null-terminated
  99 strings that represent the argument list available to the executed program.
 100 The first argument, by convention, should point to the file name associated
 101 with the file being executed.
 102 The list of arguments
 103 .Em must
 104 be terminated by a
 105 .Dv NULL
 106 pointer.
 107 .Pp
 108 The
 109 .Fn exect ,
 110 .Fn execv ,
 111 .Fn execvp ,
 112 and
 113 .Fn execvP
 114 functions provide an array of pointers to null-terminated strings that
 115 represent the argument list available to the new program.
 116 The first argument, by convention, should point to the file name associated
 117 with the file being executed.
 118 The array of pointers
 119 .Sy must
 120 be terminated by a
 121 .Dv NULL
 122 pointer.
 123 .Pp
 124 The
 125 .Fn execle
 126 and
 127 .Fn exect
 128 functions also specify the environment of the executed process by following
 129 the
 130 .Dv NULL
 131 pointer that terminates the list of arguments in the argument list
 132 or the pointer to the argv array with an additional argument.
 133 This additional argument is an array of pointers to null-terminated strings
 134 and
 135 .Em must
 136 be terminated by a
 137 .Dv NULL
 138 pointer.
 139 The other functions take the environment for the new process image from the
 140 external variable
 141 .Va environ
 142 in the current process.
 143 .Pp
 144 Some of these functions have special semantics.
 145 .Pp
 146 The functions
 147 .Fn execlp ,
 148 .Fn execvp ,
 149 and
 150 .Fn execvP
 151 will duplicate the actions of the shell in searching for an executable file
 152 if the specified file name does not contain a slash
 153 .Dq Li /
 154 character.
 155 For
 156 .Fn execlp
 157 and
 158 .Fn execvp ,
 159 search path is the path specified in the environment by
 160 .Dq Ev PATH
 161 variable.
 162 If this variable isn't specified,
 163 the default path is set according to the
 164 .Dv _PATH_DEFPATH
 165 definition in
 166 .In paths.h ,
 167 which is set to
 168 .Dq Ev /usr/bin:/bin .
 169 For
 170 .Fn execvP ,
 171 the search path is specified as an argument to the function.
 172 In addition, certain errors are treated specially.
 173 .Pp
 174 If an error is ambiguous (for simplicity, we shall consider all
 175 errors except
 176 .Er ENOEXEC
 177 as being ambiguous here, although only the critical error
 178 .Er EACCES
 179 is really ambiguous),
 180 then these functions will act as if they stat the file to determine
 181 whether the file exists and has suitable execute permissions.
 182 If it does, they will return immediately with the global variable
 183 .Va errno
 184 restored to the value set by
 185 .Fn execve .
 186 Otherwise, the search will be continued.
 187 If the search completes without performing a successful
 188 .Fn execve
 189 or terminating due to an error,
 190 these functions will return with the global variable
 191 .Va errno
 192 set to
 193 .Er EACCES
 194 or
 195 .Er ENOENT
 196 according to whether at least one file with suitable execute permissions
 197 was found.
 198 .Pp
 199 If the header of a file isn't recognized (the attempted
 200 .Fn execve
 201 returned
 202 .Er ENOEXEC ) ,
 203 these functions will execute the shell with the path of
 204 the file as its first argument.
 205 (If this attempt fails, no further searching is done.)
 206 .Pp
 207 The function
 208 .Fn exect
 209 executes a file with the program tracing facilities enabled (see
 210 .Xr ptrace 2 ) .
 211 .Sh RETURN VALUES
 212 If any of the
 213 .Fn exec
 214 functions returns, an error will have occurred.
 215 The return value is \-1, and the global variable
 216 .Va errno
 217 will be set to indicate the error.
 218 .Sh FILES
 219 .Bl -tag -width /bin/sh -compact
 220 .It Pa /bin/sh
 221 The shell.
 222 .El
 223 .Sh ERRORS
 224 The
 225 .Fn execl ,
 226 .Fn execle ,
 227 .Fn execlp ,
 228 .Fn execvp
 229 and
 230 .Fn execvP
 231 functions
 232 may fail and set
 233 .Va errno
 234 for any of the errors specified for the library functions
 235 .Xr execve 2
 236 and
 237 .Xr malloc 3 .
 238 .Pp
 239 The
 240 .Fn exect
 241 and
 242 .Fn execv
 243 functions
 244 may fail and set
 245 .Va errno
 246 for any of the errors specified for the library function
 247 .Xr execve 2 .
 248 .Sh SEE ALSO
 249 .Xr sh 1 ,
 250 .Xr execve 2 ,
 251 .Xr fork 2 ,
 252 .Xr ktrace 2 ,
 253 .Xr ptrace 2 ,
 254 .Xr environ 7
 255 .Sh COMPATIBILITY
 256 Historically, the default path for the
 257 .Fn execlp
 258 and
 259 .Fn execvp
 260 functions was
 261 .Dq Pa :/bin:/usr/bin .
 262 This was changed to place the current directory last to enhance system
 263 security.
 264 .Pp
 265 The behavior of
 266 .Fn execlp
 267 and
 268 .Fn execvp
 269 when errors occur while attempting to execute the file is not quite historic
 270 practice, and has not traditionally been documented and is not specified
 271 by the
 272 .Tn POSIX
 273 standard.
 274 .Pp
 275 Traditionally, the functions
 276 .Fn execlp
 277 and
 278 .Fn execvp
 279 ignored all errors except for the ones described above and
 280 .Er ETXTBSY ,
 281 upon which they retried after sleeping for several seconds, and
 282 .Er ENOMEM
 283 and
 284 .Er E2BIG ,
 285 upon which they returned.
 286 They now return for
 287 .Er ETXTBSY ,
 288 and determine existence and executability more carefully.
 289 In particular,
 290 .Er EACCES
 291 for inaccessible directories in the path prefix is no longer
 292 confused with
 293 .Er EACCES
 294 for files with unsuitable execute permissions.
 295 In
 296 .Bx 4.4 ,
 297 they returned upon all errors except
 298 .Er EACCES ,
 299 .Er ENOENT ,
 300 .Er ENOEXEC
 301 and
 302 .Er ETXTBSY .
 303 This was inferior to the traditional error handling,
 304 since it breaks the ignoring of errors for path prefixes
 305 and only improves the handling of the unusual ambiguous error
 306 .Er EFAULT
 307 and the unusual error
 308 .Er EIO .
 309 The behaviour was changed to match the behaviour of
 310 .Xr sh 1 .
 311 .Sh STANDARDS
 312 The
 313 .Fn execl ,
 314 .Fn execv ,
 315 .Fn execle ,
 316 .Fn execlp
 317 and
 318 .Fn execvp
 319 functions
 320 conform to
 321 .St -p1003.1-88 .
 322 The
 323 .Fn execvP
 324 function first appeared in
 325 .Fx 5.2 .