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 1.18 2002/12/19 09:40:21 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 .Nd execute a file
  46 .Sh LIBRARY
  47 .Lb libc
  48 .Sh SYNOPSIS
  49 .In unistd.h
  50 .Vt extern char **environ ;
  51 .Ft int
  52 .Fn execl "const char *path" "const char *arg" ...
  53 .Ft int
  54 .Fn execlp "const char *file" "const char *arg" ...
  55 .Ft int
  56 .Fn execle "const char *path" "const char *arg" ...
  57 .Ft int
  58 .Fn exect "const char *path" "char *const argv[]" "char *const envp[]"
  59 .Ft int
  60 .Fn execv "const char *path" "char *const argv[]"
  61 .Ft int
  62 .Fn execvp "const char *file" "char *const argv[]"
  63 .Sh DESCRIPTION
  64 The
  65 .Nm exec
  66 family of functions replaces the current process image with a
  67 new process image.
  68 The functions described in this manual page are front-ends for the function
  69 .Xr execve 2 .
  70 (See the manual page for
  71 .Xr execve 2
  72 for detailed information about the replacement of the current process.)
  73 .Pp
  74 The initial argument for these functions is the pathname of a file which
  75 is to be executed.
  76 .Pp
  77 The
  78 .Fa "const char *arg"
  79 and subsequent ellipses in the
  80 .Fn execl ,
  81 .Fn execlp ,
  82 and
  83 .Fn execle
  84 functions can be thought of as
  85 .Em arg0 ,
  86 .Em arg1 ,
  87 \&...,
  88 .Em argn .
  89 Together they describe a list of one or more pointers to null-terminated
  90 strings that represent the argument list available to the executed program.
  91 The first argument, by convention, should point to the file name associated
  92 with the file being executed.
  93 The list of arguments
  94 .Em must
  95 be terminated by a
  96 .Dv NULL
  97 pointer.
  98 .Pp
  99 The
 100 .Fn exect ,
 101 .Fn execv ,
 102 and
 103 .Fn execvp
 104 functions provide an array of pointers to null-terminated strings that
 105 represent the argument list available to the new program.
 106 The first argument, by convention, should point to the file name associated
 107 with the file being executed.
 108 The array of pointers
 109 .Sy must
 110 be terminated by a
 111 .Dv NULL
 112 pointer.
 113 .Pp
 114 The
 115 .Fn execle
 116 and
 117 .Fn exect
 118 functions also specify the environment of the executed process by following
 119 the
 120 .Dv NULL
 121 pointer that terminates the list of arguments in the argument list
 122 or the pointer to the argv array with an additional argument.
 123 This additional argument is an array of pointers to null-terminated strings
 124 and
 125 .Em must
 126 be terminated by a
 127 .Dv NULL
 128 pointer.
 129 The other functions take the environment for the new process image from the
 130 external variable
 131 .Va environ
 132 in the current process.
 133 .Pp
 134 Some of these functions have special semantics.
 135 .Pp
 136 The functions
 137 .Fn execlp
 138 and
 139 .Fn execvp
 140 will duplicate the actions of the shell in searching for an executable file
 141 if the specified file name does not contain a slash
 142 .Dq Li /
 143 character.
 144 The search path is the path specified in the environment by
 145 .Dq Ev PATH
 146 variable.
 147 If this variable isn't specified,
 148 the default path is set according to the
 149 .Dv _PATH_DEFPATH
 150 definition in
 151 .Aq paths.h ,
 152 which is set to
 153 .Dq Ev /usr/bin:/bin .
 154 In addition, certain errors are treated specially.
 155 .Pp
 156 If an error is ambiguous (for simplicity, we shall consider all
 157 errors except
 158 .Er ENOEXEC
 159 as being ambiguous here, although only the critical error
 160 .Er EACCES
 161 is really ambiguous),
 162 then these functions will act as if they stat the file to determine
 163 whether the file exists and has suitable execute permissions.
 164 If it does, they will return immediately with the global variable
 165 .Va errno
 166 restored to the value set by
 167 .Fn execve .
 168 Otherwise, the search will be continued.
 169 If the search completes without performing a successful
 170 .Fn execve
 171 or terminating due to an error,
 172 these functions will return with the global variable
 173 .Va errno
 174 set to
 175 .Er EACCES
 176 or
 177 .Er ENOENT
 178 according to whether at least one file with suitable execute permissions
 179 was found.
 180 .Pp
 181 If the header of a file isn't recognized (the attempted
 182 .Fn execve
 183 returned
 184 .Er ENOEXEC ) ,
 185 these functions will execute the shell with the path of
 186 the file as its first argument.
 187 (If this attempt fails, no further searching is done.)
 188 .Pp
 189 The function
 190 .Fn exect
 191 executes a file with the program tracing facilities enabled (see
 192 .Xr ptrace 2 ) .
 193 .Sh RETURN VALUES
 194 If any of the
 195 .Fn exec
 196 functions returns, an error will have occurred.
 197 The return value is \-1, and the global variable
 198 .Va errno
 199 will be set to indicate the error.
 200 .Sh FILES
 201 .Bl -tag -width /bin/sh -compact
 202 .It Pa /bin/sh
 203 The shell.
 204 .El
 205 .Sh ERRORS
 206 The
 207 .Fn execl ,
 208 .Fn execle ,
 209 .Fn execlp
 210 and
 211 .Fn execvp
 212 functions
 213 may fail and set
 214 .Va errno
 215 for any of the errors specified for the library functions
 216 .Xr execve 2
 217 and
 218 .Xr malloc 3 .
 219 .Pp
 220 The
 221 .Fn exect
 222 and
 223 .Fn execv
 224 functions
 225 may fail and set
 226 .Va errno
 227 for any of the errors specified for the library function
 228 .Xr execve 2 .
 229 .Sh SEE ALSO
 230 .Xr sh 1 ,
 231 .Xr execve 2 ,
 232 .Xr fork 2 ,
 233 .Xr ktrace 2 ,
 234 .Xr ptrace 2 ,
 235 .Xr environ 7
 236 .Sh COMPATIBILITY
 237 Historically, the default path for the
 238 .Fn execlp
 239 and
 240 .Fn execvp
 241 functions was
 242 .Dq Pa :/bin:/usr/bin .
 243 This was changed to place the current directory last to enhance system
 244 security.
 245 .Pp
 246 The behavior of
 247 .Fn execlp
 248 and
 249 .Fn execvp
 250 when errors occur while attempting to execute the file is not quite historic
 251 practice, and has not traditionally been documented and is not specified
 252 by the
 253 .Tn POSIX
 254 standard.
 255 .Pp
 256 Traditionally, the functions
 257 .Fn execlp
 258 and
 259 .Fn execvp
 260 ignored all errors except for the ones described above and
 261 .Er ETXTBSY ,
 262 upon which they retried after sleeping for several seconds, and
 263 .Er ENOMEM
 264 and
 265 .Er E2BIG ,
 266 upon which they returned.
 267 They now return for
 268 .Er ETXTBSY ,
 269 and determine existence and executability more carefully.
 270 In particular,
 271 .Er EACCES
 272 for inaccessible directories in the path prefix is no longer
 273 confused with
 274 .Er EACCES
 275 for files with unsuitable execute permissions.
 276 In
 277 .Bx 4.4 ,
 278 they returned upon all errors except
 279 .Er EACCES ,
 280 .Er ENOENT ,
 281 .Er ENOEXEC
 282 and
 283 .Er ETXTBSY .
 284 This was inferior to the traditional error handling,
 285 since it breaks the ignoring of errors for path prefixes
 286 and only improves the handling of the unusual ambiguous error
 287 .Er EFAULT
 288 and the unusual error
 289 .Er EIO .
 290 The behaviour was changed to match the behaviour of
 291 .Xr sh 1 .
 292 .Sh STANDARDS
 293 The
 294 .Fn execl ,
 295 .Fn execv ,
 296 .Fn execle ,
 297 .Fn execlp
 298 and
 299 .Fn execvp
 300 functions
 301 conform to
 302 .St -p1003.1-88 .