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EXECVE(2)			     BSD 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 pro-
     gram (text) and initialized data pages.  Additional pages may be specified by the header to
     be initialized 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; oth-
     erwise, 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 is set
     to the specified interpreter.

     The argument argv is a pointer to a null-terminated array of character pointers to null-ter-
     minated 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 component of
     path).

     The argument envp is also a pointer to a null-terminated array of character pointers to
     null-terminated strings.  A pointer to this array is normally stored in the global variable
     environ.  These strings pass information 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 effec-
     tive 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 provide 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.

     [ENOEXEC]		When invoking an interpreted script, the length of the first line, inclu-
			sive of the #! prefix and terminating newline, exceeds MAXSHELLCMDLEN
			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 permission.

     [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 cur-
			rently 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 vari-
			able 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 system.

     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.

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-privi-
     leged 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.

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 sec-
     ond 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.

BSD					September 21, 2010				      BSD
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