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EXECVE(2)		    BSD	System Calls Manual		     EXECVE(2)

     execve -- execute a file

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     execve(const char *path, char *const argv[], char *const envp[]);

     execve() transforms the calling process into a new	process.  The new
     process is	constructed from an ordinary file, whose name is pointed to by
     path, called the new process file.	 This file is either an	executable ob-
     ject file,	or a file of data for an interpreter.  An executable object
     file consists of an identifying header, followed by pages of data repre-
     senting the initial program (text)	and initialized	data pages.  Addi-
     tional 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 [arg]

     When an interpreter file is execve()d the system runs the specified
     interpreter.  If the optional arg is specified, it	becomes	the first ar-
     gument to the interpreter,	and the	name of	the originally execve()d file
     becomes the second	argument; otherwise, the name of the originally
     execve()d file becomes the	first argument.	 The original arguments	are
     shifted over to become the	subsequent arguments.  The zeroth argument,
     normally the name of the execve()d	file, is left unchanged.  The inter-
     preter named by interpreter must not itself be an interpreter file.  (See
     script(7) for a detailed discussion of interpreter	file execution.)

     The argument argv is a pointer to a null-terminated array of character
     pointers to null-terminated character strings.  These strings construct
     the argument list to be made available to the new process.	 By custom,
     the first element should be the name of the executed program (for exam-
     ple, the last component of	path).

     The argument envp is also a pointer to a null-terminated array of charac-
     ter pointers to null-terminated strings.  A pointer to this array is nor-
     mally stored in the global	variable environ.  These strings pass informa-
     tion to the new process that is not directly an argument to the command
     (see environ(7)).

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

     In	the case of a new setuid or setgid executable being executed, if file
     descriptors 0, 1, or 2 (representing stdin, stdout, and stderr) are cur-
     rently unallocated, these descriptors will	be opened to point to some
     system file like /dev/null.  The intent is	to ensure these	descriptors
     are not unallocated, since	many libraries make assumptions	about the use
     of	these 3	file descriptors.

     Signals set to be ignored in the calling process are set to be ignored in
     the new process.  Signals which are set to	be caught in the calling
     process image are set to default action in	the new	process	image.
     Blocked signals remain blocked regardless of changes to the signal	ac-
     tion.  The	signal stack is	reset to be undefined (see sigaction(2)	for
     more information).

     If	the set-user-ID	mode bit of the	new process image file is set (see
     chmod(2)),	the effective user ID of the new process image is set to the
     owner ID of the new process image file.  If the set-group-ID mode bit of
     the new process image file	is set,	the effective group ID of the new
     process image is set to the group ID of the new process image file.  (The
     effective group ID	is the first element of	the group list.)  The real
     user ID, real group ID and	other group IDs	of the new process image re-
     main the same as the calling process image.  After	any set-user-ID	and
     set-group-ID processing, the effective user ID is recorded	as the saved
     set-user-ID, and the effective group ID is	recorded as the	saved set-
     group-ID.	These values may be used in changing the effective IDs later
     (see setuid(2)).

     The new process also inherits the following attributes from the calling

	   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 termios(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 sigaction(2), sigprocmask(2)

     When a program is executed	as a result of an execve() call, it is entered
     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
     points to the array of character pointers to the arguments	themselves.

     As	the execve() function overlays the current process image with a	new
     process image the successful call has no process to return	to.  If
     execve() does return to the calling process an error has occurred;	the
     return value will be -1 and the global variable errno is set to indicate
     the error.

     execve() will fail	and return to the calling process if:

     [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 262144 bytes (NCARGS in

     [EACCES]		Search permission is denied for	a component of the
			path prefix, the new process file is not an ordinary
			file, its file mode denies execute permission, or it
			is on a	filesystem mounted with	execution disabled
			(MNT_NOEXEC in <sys/mount.h>).

     [EAGAIN]		A setuid(7) process has	exceeded the current resource
			limit for the number of	processes it is	allowed	to run

     [EFAULT]		The new	process	file is	not as long as indicated by
			the size values	in its header; or path,	argv, or envp
			point to an illegal address.

     [EIO]		An I/O error occurred while reading from the file sys-

     [ELOOP]		Too many symbolic links	were encountered in translat-
			ing the	pathname.

     [ENAMETOOLONG]	A component of a pathname exceeded {NAME_MAX} charac-
			ters, or an entire path	name exceeded {PATH_MAX} char-

     [ENOENT]		The new	process	file does not exist, or	the new
			process	file is	a script starting with #! and the
			script interpreter does	not exist.

     [ENOEXEC]		The new	process	file has the appropriate access	per-
			mission, but has an invalid magic number in its

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

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

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

     _exit(2), fork(2),	execl(3), environ(7), script(7)

     The execve() function conforms to ISO/IEC 9945-1:1990 ("POSIX.1").

     The execve() function call	first appeared in Version 7 AT&T UNIX.

     If	a program is setuid to a non-super-user, but is	executed when the real
     uid is "root", then the program has some of the powers of a super-user as

BSD			       February	24, 2008			   BSD


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