EXECVE(2)	    UNIX Programmer's Manual		EXECVE(2)


NAME
     execve - execute a file

SYNOPSIS
     execve(name, argv, envp)
     char *name, *argv[], *envp[];

DESCRIPTION
     Execve transforms the calling process into a new process.
     The new process is constructed from an ordinary file called
     the new process file.  This file is either an executable
     object file, or a file of data for an interpreter.  An exe-
     cutable object file consists of an identifying header, fol-
     lowed by pages of data representing the initial program
     (text) and initialized data pages.  Additional pages may be
     specified by the header to be initialized with zero data.
     See a.out(5).

     An interpreter file begins with a line of the form ``#!
     interpreter''.  When an interpreter file is execve'd, the
     system execve's the specified interpreter, giving it the
     name of the originally exec'd file as an argument and shift-
     ing over the rest of the original arguments.

     There can be no return from a successful execve because the
     calling core image is lost.  This is the mechanism whereby
     different process images become active.

     The argument argv is a null-terminated array of character
     pointers to null-terminated character strings.  These
     strings constitute the argument list to be made available to
     the new process.  By convention, at least one argument must
     be present in this array, and the first element of this
     array should be the name of the executed program (i.e., the
     last component of name).

     The argument envp is also a null-terminated array of charac-
     ter pointers to null-terminated strings.  These strings pass
     information to the new process that is not directly an argu-
     ment to the command (see environ(7)).

     Descriptors open in the calling process remain open in the
     new process, except for those for which the close-on-exec
     flag is set (see close(2)).  Descriptors that remain open
     are unaffected by execve.

     Ignored signals remain ignored across an execve, but signals
     that are caught are reset to their default values.  Blocked
     signals remain blocked regardless of changes to the signal
     action.  The signal stack is reset to be undefined (see
     sigvec(2) for more information).


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EXECVE(2)	    UNIX Programmer's Manual		EXECVE(2)


     Each process has real user and group IDs and an effective
     user and group IDs.  The real ID identifies the person using
     the system; the effective ID determines his access
     privileges.  Execve changes the effective user and group ID
     to the owner of the executed file if the file has the "set-
     user-ID" or "set-group-ID" modes.	The real user ID is not
     affected.

     The new process also inherits the following attributes from
     the calling process:

	  process ID	      see getpid(2)
	  parent process ID   see getppid(2)
	  process group ID    see getpgrp(2)
	  access groups       see getgroups(2)
	  working directory   see chdir(2)
	  root directory      see chroot(2)
	  control terminal    see tty(4)
	  resource usages     see getrusage(2)
	  interval timers     see getitimer(2)
	  resource limits     see getrlimit(2)
	  file mode mask      see umask(2)
	  signal mask	      see sigvec(2), sigmask(2)

     When the executed program begins, it is called as follows:

	  main(argc, argv, envp)
	  int argc;
	  char **argv, **envp;

     where argc is the number of elements in argv (the ``arg
     count'') and argv is the array of character pointers to the
     arguments themselves.

     Envp is a pointer to an array of strings that constitute the
     environment of the process.  A pointer to this array is also
     stored in the global variable ``environ''.  Each string con-
     sists of a name, an "=", and a null-terminated value.  The
     array of pointers is terminated by a null pointer.  The
     shell sh(1) passes an environment entry for each global
     shell variable defined when the program is called.  See
     environ(7) for some conventionally used names.

RETURN VALUE
     If execve returns to the calling process an error has
     occurred; the return value will be -1 and the global vari-
     able errno will contain an error code.

ERRORS
     Execve will fail and return to the calling process if one or
     more of the following are true:


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EXECVE(2)	    UNIX Programmer's Manual		EXECVE(2)


     [ENOTDIR]	    A component of the path prefix is not a
		    directory.

     [EINVAL]	    The pathname contains a character with the
		    high-order bit set.

     [ENAMETOOLONG] A component of a pathname exceeded 255 char-
		    acters, or an entire path name exceeded 1023
		    characters.

     [ENOENT]	    The new process file does not exist.

     [ELOOP]	    Too many symbolic links were encountered in
		    translating the pathname.

     [EACCES]	    Search permission is denied for a component
		    of the path prefix.

     [EACCES]	    The new process file is not an ordinary file.

     [EACCES]	    The new process file mode denies execute per-
		    mission.

     [ENOEXEC]	    The new process file has the appropriate
		    access permission, but has an invalid magic
		    number in its header.

     [ETXTBSY]	    The new process file is a pure procedure
		    (shared text) file that is currently open for
		    writing or reading by some process.

     [ENOMEM]	    The new process requires more virtual memory
		    than is allowed by the imposed maximum
		    (getrlimit(2)).

     [E2BIG]	    The number of bytes in the new process's
		    argument list is larger than the system-
		    imposed limit.  The limit in the system as
		    released is 20480 bytes (NCARGS in
		    <sys/param.h>.

     [EFAULT]	    The new process file is not as long as indi-
		    cated by the size values in its header.

     [EFAULT]	    Path, argv, or envp point to an illegal
		    address.

     [EIO]	    An I/O error occurred while reading from the
		    file system.

CAVEATS
     If a program is setuid to a non-super-user, but is executed


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     when the real uid is ``root'', then the program has some of
     the powers of a super-user as well.

SEE ALSO
     exit(2), fork(2), execl(3), environ(7)


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