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

     execve, fexecve — execute a file

     Standard C Library (libc, -lc)

     #include <unistd.h>

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

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

     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 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 sigvec(2), sigsetmask(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.

     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.

     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

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

     [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

     [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

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

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

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

     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.

     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)

     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

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

BSD				April 10, 2008				   BSD

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