bsd/man/man2/execve.2

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  34 .\"     @(#)execve.2    8.3 (Berkeley) 1/24/94
  35 .\"
  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 .Fo execve
  46 .Fa "const char *path"
  47 .Fa "char *const argv[]"
  48 .Fa "char *const envp[]"
  49 .Fc
  50 .Sh DESCRIPTION
  51 .Fn Execve
  52 transforms the calling process into a new process.
  53 The new process is constructed from an ordinary file,
  54 whose name is pointed to by
  55 .Fa path ,
  56 called the
  57 .Em new process file .
  58 This file is either an executable object file,
  59 or a file of data for an interpreter.
  60 An executable object file consists of an identifying header,
  61 followed by pages of data representing the initial program (text)
  62 and initialized data pages.  Additional pages may be specified
  63 by the header to be initialized with zero data;  see
  64 .Xr a.out 5 .
  65 .Pp
  66 An interpreter file begins with a line of the form:
  67 .Pp
  68 .Bd -filled -offset indent -compact
  69 .Sy \&#!
  70 .Em interpreter
  71 .Bq Em arg ...
  72 .Ed
  73 .Pp
  74 When an interpreter file is
  75 .Fn execve Ap d ,
  76 the system runs the specified
  77 .Em interpreter .
  78 If any optional
  79 .Em args
  80 are specified, they become the first (second, ...) argument to the
  81 .Em interpreter.
  82 The name of the originally
  83 .Fn execve Ap d
  84 file becomes the subsequent argument;
  85 otherwise, the name of the originally
  86 .Fn execve Ap d
  87 file is the first argument.
  88 The original arguments to the invocation of the interpreter
  89 are shifted over to become the final arguments.
  90 The zeroth argument, normally the name of the
  91 .Fn execve Ap d
  92 file, is left unchanged.
  93 .Pp
  94 The argument
  95 .Fa argv
  96 is a pointer to a null-terminated array of
  97 character pointers to null-terminated character strings.
  98 These strings construct the argument list to be made available to the new
  99 process.  At least one argument must be present in
 100 the array; by custom, the first element should be
 101 the name of the executed program (for example, the last component of
 102 .Fa path ) .
 103 .Pp
 104 The argument
 105 .Fa envp
 106 is also a pointer to a null-terminated array of
 107 character pointers to null-terminated strings.
 108 A pointer to this array is normally stored in the global variable
 109 .Va environ.
 110 These strings pass information to the
 111 new process that is not directly an argument to the command (see
 112 .Xr environ 7 ) .
 113 .Pp
 114 File descriptors open in the calling process image remain open in
 115 the new process image, except for those for which the close-on-exec
 116 flag is set (see
 117 .Xr close 2
 118 and
 119 .Xr fcntl 2 ) .
 120 Descriptors that remain open are unaffected by
 121 .Fn execve .
 122 .Pp
 123 Signals set to be ignored in the calling process are set to be ignored in
 124 the
 125 new process. Signals which are set to be caught in the calling process image
 126 are set to default action in the new process image.
 127 Blocked signals remain blocked regardless of changes to the signal action.
 128 The signal stack is reset to be undefined (see
 129 .Xr sigaction 2
 130 for more information).
 131 .Pp
 132 If the set-user-ID mode bit of the new process image file is set
 133 (see
 134 .Xr chmod 2 ) ,
 135 the effective user ID of the new process image is set to the owner ID
 136 of the new process image file.
 137 If the set-group-ID mode bit of the new process image file is set,
 138 the effective group ID of the new process image is set to the group ID
 139 of the new process image file.
 140 (The effective group ID is the first element of the group list.)
 141 The real user ID, real group ID and
 142 other group IDs of the new process image remain the same as the calling
 143 process image.
 144 After any set-user-ID and set-group-ID processing,
 145 the effective user ID is recorded as the saved set-user-ID,
 146 and the effective group ID is recorded as the saved set-group-ID.
 147 These values may be used in changing the effective IDs later (see
 148 .Xr setuid 2 ) .
 149 .Pp
 150 The new process also inherits the following attributes from
 151 the calling process:
 152 .Pp
 153 .Bl -column parent_process_ID -offset indent -compact
 154 .It process ID Ta see Xr getpid 2
 155 .It parent process ID Ta see Xr getppid 2
 156 .It process group ID Ta see Xr getpgrp 2
 157 .It access groups Ta see Xr getgroups 2
 158 .It working directory Ta see Xr chdir 2
 159 .It root directory Ta see Xr chroot 2
 160 .It control terminal Ta see Xr termios 4
 161 .It resource usages Ta see Xr getrusage 2
 162 .It interval timers Ta see Xr getitimer 2
 163 .It resource limits Ta see Xr getrlimit 2
 164 .It file mode mask Ta see Xr umask 2
 165 .It signal mask Ta see Xr sigaction 2 ,
 166 .Xr sigsetmask 2
 167 .El
 168 .Pp
 169 When a program is executed as a result of an
 170 .Fn execve
 171 call, it is entered as follows:
 172 .Bd -literal -offset indent
 173 main(argc, argv, envp)
 174 int argc;
 175 char **argv, **envp;
 176 .Ed
 177 .Pp
 178 where
 179 .Fa argc
 180 is the number of elements in
 181 .Fa argv
 182 (the ``arg count'')
 183 and
 184 .Fa argv
 185 points to the array of character pointers
 186 to the arguments themselves.
 187 .Sh RETURN VALUES
 188 As the
 189 .Fn execve
 190 function overlays the current process image  with a new process image,
 191 the successful call has no process to return to.
 192 If
 193 .Fn execve
 194 does return to the calling process, an error has occurred;
 195 the return value will be -1 and the global variable
 196 .Va errno
 197 is set to indicate the error.
 198 .Sh ERRORS
 199 .Fn Execve
 200 will fail and return to the calling process if:
 201 .Bl -tag -width Er
 202 .\" ==========
 203 .It Bq Er E2BIG
 204 The number of bytes in the new process's argument list
 205 is larger than the system-imposed limit.
 206 This limit is specified by the
 207 .Xr sysctl 3
 208 MIB variable
 209 .Dv KERN_ARGMAX .
 210 .\" ==========
 211 .It Bq Er EACCES
 212 Search permission is denied for a component of the path prefix.
 213 .\" ==========
 214 .It Bq Er EACCES
 215 The new process file is not an ordinary file.
 216 .\" ==========
 217 .It Bq Er EACCES
 218 The new process file mode denies execute permission.
 219 .\" ==========
 220 .It Bq Er EACCES
 221 The new process file is on a filesystem mounted
 222 with execution disabled
 223 .Pf ( Dv MNT_NOEXEC
 224 in
 225 .Ao Pa sys/mount.h Ac ) .
 226 .\" ==========
 227 .It Bq Er EFAULT
 228 The new process file is not as long as indicated by
 229 the size values in its header.
 230 .\" ==========
 231 .It Bq Er EFAULT
 232 .Fa Path ,
 233 .Fa argv ,
 234 or
 235 .Fa envp
 236 point
 237 to an illegal address.
 238 .\" ==========
 239 .It Bq Er EIO
 240 An I/O error occurred while reading from the file system.
 241 .\" ==========
 242 .It Bq Er ELOOP
 243 Too many symbolic links were encountered in translating the pathname.
 244 This is taken to be indicative of a looping symbolic link.
 245 .\" ==========
 246 .It Bq Er ENAMETOOLONG
 247 A component of a pathname exceeded
 248 .Dv {NAME_MAX}
 249 characters, or an entire path name exceeded
 250 .Dv {PATH_MAX}
 251 characters.
 252 .\" ==========
 253 .It Bq Er ENOENT
 254 The new process file does not exist.
 255 .\" ==========
 256 .It Bq Er ENOEXEC
 257 The new process file has the appropriate access
 258 permission, but has an unrecognized format
 259 (e.g., an invalid magic number in its header).
 260 .\" ==========
 261 .It Bq Er ENOMEM
 262 The new process requires more virtual memory than
 263 is allowed by the imposed maximum
 264 .Pq Xr getrlimit 2 .
 265 .\" ==========
 266 .It Bq Er ENOTDIR
 267 A component of the path prefix is not a directory.
 268 .\" ==========
 269 .It Bq Er ETXTBSY
 270 The new process file is a pure procedure (shared text)
 271 file that is currently open for writing or reading by some process.
 272 .El
 273 .Sh CAVEAT
 274 If a program is
 275 .Em setuid
 276 to a non-super-user, but is executed when
 277 the real
 278 .Em uid
 279 is ``root'', then the program has some of the powers
 280 of a super-user as well.
 281 .Sh SEE ALSO
 282 .Xr exit 2 ,
 283 .Xr fork 2 ,
 284 .Xr execl 3 ,
 285 .Xr sysctl 3 ,
 286 .Xr environ 7
 287 .Sh HISTORY
 288 The
 289 .Fn execve
 290 function call appeared in
 291 .Bx 4.2 .