bsd/man/man2/execve.2

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  34 .\"     @(#)execve.2    8.3 (Berkeley) 1/24/94
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  36 .Dd January 24, 1994
  37 .Dt EXECVE 2
  38 .Os BSD 4
  39 .Sh NAME
  40 .Nm execve
  41 .Nd execute a file
  42 .Sh SYNOPSIS
  43 .Fd #include <unistd.h>
  44 .Ft int
  45 .Fn execve "const char *path" "char *const argv[]" "char *const envp[]"
  46 .Sh DESCRIPTION
  47 .Fn Execve
  48 transforms the calling process into a new process.
  49 The new process is constructed from an ordinary file,
  50 whose name is pointed to by
  51 .Fa path ,
  52 called the
  53 .Em new process file .
  54 This file is either an executable object file,
  55 or a file of data for an interpreter.
  56 An executable object file consists of an identifying header,
  57 followed by pages of data representing the initial program (text)
  58 and initialized data pages.  Additional pages may be specified
  59 by the header to be initialized with zero data;  see
  60 .Xr a.out 5 .
  61 .Pp
  62 An interpreter file begins with a line of the form:
  63 .Pp
  64 .Bd -filled -offset indent -compact
  65 .Sy \&#!
  66 .Em interpreter
  67 .Bq Em arg
  68 .Ed
  69 .Pp
  70 When an interpreter file is
  71 .Fn execve Ap d ,
  72 the system
  73 .Fn execve Ap s
  74 runs the specified
  75 .Em interpreter .
  76 If the optional
  77 .Em arg
  78 is specified, it becomes the first argument to the
  79 .Em interpreter ,
  80 and the name of the originally
  81 .Fn execve Ap d
  82 file becomes the second argument;
  83 otherwise, the name of the originally
  84 .Fn execve Ap d
  85 file becomes the first argument.  The original arguments are shifted over to
  86 become the subsequent arguments.  The zeroth argument, normally the name of the
  87 .Fn execve Ap d
  88 file, is left unchanged.
  89 .Pp
  90 The argument
  91 .Fa argv
  92 is a pointer to a null-terminated array of
  93 character pointers to null-terminated character strings.
  94 These strings construct the argument list to be made available to the new
  95 process.  At least one argument must be present in
  96 the array; by custom, the first element should be
  97 the name of the executed program (for example, the last component of
  98 .Fa path ) .
  99 .Pp
 100 The argument
 101 .Fa envp
 102 is also a pointer to a null-terminated array of
 103 character pointers to null-terminated strings.
 104 A pointer to this array is normally stored in the global variable
 105 .Va environ.
 106 These strings pass information to the
 107 new process that is not directly an argument to the command (see
 108 .Xr environ 7 ) .
 109 .Pp
 110 File descriptors open in the calling process image remain open in
 111 the new process image, except for those for which the close-on-exec
 112 flag is set (see
 113 .Xr close 2
 114 and
 115 .Xr fcntl 2 ) .
 116 Descriptors that remain open are unaffected by
 117 .Fn execve .
 118 .Pp
 119 Signals set to be ignored in the calling process are set to be ignored in
 120 the
 121 new process. Signals which are set to be caught in the calling process image
 122 are set to default action in the new process image.
 123 Blocked signals remain blocked regardless of changes to the signal action.
 124 The signal stack is reset to be undefined (see
 125 .Xr sigaction 2
 126 for more information).
 127 .Pp
 128 If the set-user-ID mode bit of the new process image file is set
 129 (see
 130 .Xr chmod 2 ) ,
 131 the effective user ID of the new process image is set to the owner ID
 132 of the new process image file.
 133 If the set-group-ID mode bit of the new process image file is set,
 134 the effective group ID of the new process image is set to the group ID
 135 of the new process image file.
 136 (The effective group ID is the first element of the group list.)
 137 The real user ID, real group ID and
 138 other group IDs of the new process image remain the same as the calling
 139 process image.
 140 After any set-user-ID and set-group-ID processing,
 141 the effective user ID is recorded as the saved set-user-ID,
 142 and the effective group ID is recorded as the saved set-group-ID.
 143 These values may be used in changing the effective IDs later (see
 144 .Xr setuid 2 ) .
 145 .Pp
 146 The new process also inherits the following attributes from
 147 the calling process:
 148 .Pp
 149 .Bl -column parent_process_ID -offset indent -compact
 150 .It process ID Ta see Xr getpid 2
 151 .It parent process ID Ta see Xr getppid 2
 152 .It process group ID Ta see Xr getpgrp 2
 153 .It access groups Ta see Xr getgroups 2
 154 .It working directory Ta see Xr chdir 2
 155 .It root directory Ta see Xr chroot 2
 156 .It control terminal Ta see Xr termios 4
 157 .It resource usages Ta see Xr getrusage 2
 158 .It interval timers Ta see Xr getitimer 2
 159 .It resource limits Ta see Xr getrlimit 2
 160 .It file mode mask Ta see Xr umask 2
 161 .It signal mask Ta see Xr sigaction 2 ,
 162 .Xr sigsetmask 2
 163 .El
 164 .Pp
 165 When a program is executed as a result of an
 166 .Fn execve
 167 call, it is entered as follows:
 168 .Bd -literal -offset indent
 169 main(argc, argv, envp)
 170 int argc;
 171 char **argv, **envp;
 172 .Ed
 173 .Pp
 174 where
 175 .Fa argc
 176 is the number of elements in
 177 .Fa argv
 178 (the ``arg count'')
 179 and
 180 .Fa argv
 181 points to the array of character pointers
 182 to the arguments themselves.
 183 .Sh RETURN VALUES
 184 As the
 185 .Fn execve
 186 function overlays the current process image
 187 with a new process image the successful call
 188 has no process to return to.
 189 If
 190 .Fn execve
 191 does return to the calling process an error has occurred; the
 192 return value will be -1 and the global variable
 193 .Va errno
 194 is set to indicate the error.
 195 .Sh ERRORS
 196 .Fn Execve
 197 will fail and return to the calling process if:
 198 .Bl -tag -width Er
 199 .It Bq Er ENOTDIR
 200 A component of the path prefix is not a directory.
 201 .It Bq Er ENAMETOOLONG
 202 A component of a pathname exceeded
 203 .Dv {NAME_MAX}
 204 characters, or an entire path name exceeded
 205 .Dv {PATH_MAX}
 206 characters.
 207 .It Bq Er ENOENT
 208 The new process file does not exist.
 209 .It Bq Er ELOOP
 210 Too many symbolic links were encountered in translating the pathname.
 211 .It Bq Er EACCES
 212 Search permission is denied for a component of the path prefix.
 213 .It Bq Er EACCES
 214 The new process file is not an ordinary file.
 215 .It Bq Er EACCES
 216 The new process file mode denies execute permission.
 217 .It Bq Er EACCES
 218 The new process file is on a filesystem mounted with execution
 219 disabled
 220 .Pf ( Dv MNT_NOEXEC
 221 in
 222 .Ao Pa sys/mount.h Ac ) .
 223 .It Bq Er ENOEXEC
 224 The new process file has the appropriate access
 225 permission, but has an invalid magic number in its header.
 226 .It Bq Er ETXTBSY
 227 The new process file is a pure procedure (shared text)
 228 file that is currently open for writing or reading by some process.
 229 .It Bq Er ENOMEM
 230 The new process requires more virtual memory than
 231 is allowed by the imposed maximum
 232 .Pq Xr getrlimit 2 .
 233 .It Bq Er E2BIG
 234 The number of bytes in the new process's argument list
 235 is larger than the system-imposed limit.
 236 This limit is specified by the
 237 .Xr sysctl 3
 238 MIB variable
 239 .Dv KERN_ARGMAX .
 240 .It Bq Er EFAULT
 241 The new process file is not as long as indicated by
 242 the size values in its header.
 243 .It Bq Er EFAULT
 244 .Fa Path ,
 245 .Fa argv ,
 246 or
 247 .Fa envp
 248 point
 249 to an illegal address.
 250 .It Bq Er EIO
 251 An I/O error occurred while reading from the file system.
 252 .El
 253 .Sh CAVEAT
 254 If a program is
 255 .Em setuid
 256 to a non-super-user, but is executed when
 257 the real
 258 .Em uid
 259 is ``root'', then the program has some of the powers
 260 of a super-user as well.
 261 .Sh SEE ALSO
 262 .Xr exit 2 ,
 263 .Xr fork 2 ,
 264 .Xr execl 3 ,
 265 .Xr environ 7
 266 .Sh HISTORY
 267 The
 268 .Fn execve
 269 function call appeared in
 270 .Bx 4.2 .