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EXECVE(2)		  FreeBSD System Calls Manual		     EXECVE(2)

NAME
     execve, fexecve --	execute	a file

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

     int
     fexecve(int fd, char *const argv[], char *const envp[]);

DESCRIPTION
     The execve() system call 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.  The fexecve()
     system call is equivalent to execve() except that the file	to be executed
     is	determined by the file descriptor fd instead of	a path.	 This file is
     either an executable object file, or a file of data for an	interpreter.
     An	executable object file consists	of an identifying header, followed by
     pages of data representing	the initial program (text) and initialized
     data pages.  Additional pages may be specified by the header to be	ini-
     tialized with zero	data; see elf(5) and a.out(5).

     An	interpreter file begins	with a line of the form:

	   #! interpreter [arg]

     When an interpreter file is execve'd, the system actually execve's	the
     specified interpreter.  If	the optional arg is specified, it becomes the
     first argument 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	argu-
     ments are shifted over to become the subsequent arguments.	 The zeroth
     argument is set to	the specified interpreter.

     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.	 At least one
     argument must be present in the array; by custom, the first element
     should be the name	of the executed	program	(for example, the last compo-
     nent 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().  If any of the standard descriptors (0, 1, and/or 2) are
     closed at the time	execve() is called, and	the process will gain privi-
     lege as a result of set-id	semantics, those descriptors will be re-opened
     automatically.  No	programs, whether privileged or	not, should assume
     that these	descriptors will remain	closed across a	call to	execve().

     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
     action.  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
     remain 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 set-ID	bits are not honored if	the respective file system has the
     nosuid option enabled or if the new process file is an interpreter	file.
     Syscall tracing is	disabled if effective IDs are changed.

     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 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() system 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.

     The fexecve() ignores the file offset of fd.  Since execute permission is
     checked by	fexecve(), the file descriptor fd need not have	been opened
     with the O_EXEC flag.  However, if	the file to be executed	denies read
     permission	for the	process	preparing to do	the exec, the only way to pro-
     vide the fd to fexecve() is to use	the O_EXEC flag	when opening fd.  Note
     that the file to be executed can not be open for writing.

RETURN VALUES
     As	the execve() system call 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.

ERRORS
     The execve() system call will fail	and return to the calling process if:

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

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

     [ENAMETOOLONG]	When invoking an interpreted script, the interpreter
			name exceeds MAXSHELLCMDLEN characters.

     [ENOENT]		The new	process	file does not exist.

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

     [ENOEXEC]		The new	process	file has the appropriate access	per-
			mission, 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	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'	argument list
			is larger than the system-imposed limit.  This limit
			is specified by	the sysctl(3) MIB variable
			KERN_ARGMAX.

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

     [EFAULT]		The path, argv,	or envp	arguments point	to an illegal
			address.

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

     In	addition, the fexecve()	will fail and return to	the calling process
     if:

     [EBADF]		The fd argument	is not a valid file descriptor open
			for executing.

CAVEATS
     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	well.

     When executing an interpreted program through fexecve(), kernel supplies
     /dev/fd/n as a second argument to the interpreter,	where n	is the file
     descriptor	passed in the fd argument to fexecve().	 For this construction
     to	work correctly,	the fdescfs(5) filesystem shall	be mounted on /dev/fd.

SEE ALSO
     ktrace(1),	_exit(2), fork(2), open(2), execl(3), exit(3), sysctl(3),
     a.out(5), elf(5), fdescfs(5), environ(7), mount(8)

STANDARDS
     The execve() system call conforms to IEEE Std 1003.1-2001 (``POSIX.1''),
     with the exception	of reopening descriptors 0, 1, and/or 2	in certain
     circumstances.  A future update of	the Standard is	expected to require
     this behavior, and	it may become the default for non-privileged processes
     as	well.  The support for executing interpreted programs is an extension.
     The fexecve() system call conforms	to The Open Group Extended API Set 2
     specification.

HISTORY
     The execve() system call appeared in 4.2BSD.  The fexecve() system	call
     appeared in FreeBSD 8.0.

FreeBSD	10.1			April 10, 2008			  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | CAVEATS | SEE ALSO | STANDARDS | HISTORY

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