execve



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


NAME

     execve, exect - execute a file


SYNOPSIS

     #include <unistd.h>

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

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


DESCRIPTION

     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
     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 initialized with
     zero data;  see a.out(5) and elf(5).

     An interpreter file begins with a line of the form:

           #! interpreter [arg]

     When an interpreter file is passed to execve() the system instead calls
     execve() with the specified interpreter.  If the optional arg is
     specified, it becomes the first argument to the interpreter, and the
     original path becomes the second argument; otherwise, path becomes the
     first argument.  The original arguments are shifted over to become the
     subsequent arguments.  The zeroth argument, normally the name of the file
     being executed, is left unchanged.

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

     The argument envp is also a pointer to a null-terminated array of
     character pointers to NUL-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().  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 currently 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
     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-user-ID and set-group-ID bits have no effect if
     the new process image file is located on a file system mounted with the
     nosuid flag.  The process will be started without the new permissions.

     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)
           session ID         see getsid(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) (unless process image file is
                              setuid or setgid, in which case all timers are
                              disabled)
           resource limits    see getrlimit(2)
           file mode mask     see umask(2)
           signal mask        see sigaction(2), sigsetmask(3)

     When a program is executed as a result of an execve() call, it is entered
     as follows:

           main(int argc, char **argv, char **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 exect() function is equivalent to execve() with the additional
     property that it executes the file with the program tracing facilities
     enabled (see ptrace(2)).


RETURN VALUES

     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.


ERRORS

     execve() 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 {NAME_MAX}
                        characters, or an entire path name exceeded {PATH_MAX}
                        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.

     [EACCES]           The new process file is on a filesystem mounted with
                        execution disabled (MNT_NOEXEC in <sys/mount.h>).

     [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 262144 bytes (NCARGS in
                        <sys/param.h>).

     [EFAULT]           The new process file is not as long as indicated 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.

     [ENFILE]           During startup of an interpreter, the system file
                        table was found to be full.


SEE ALSO

     _exit(2), fork(2), execl(3), exit(3), a.out(5), elf(5), environ(7)


STANDARDS

     The execve() function is expected to conform to IEEE Std 1003.1-2008
     (``POSIX.1'').  The exect() function should not be used in portable
     applications.


HISTORY

     The predecessor of these functions, the former exec() system call, first
     appeared in Version 1 AT&T UNIX.  The execve() function first appeared in
     Version 7 AT&T UNIX.


CAVEATS

     If a program is setuid to a non-superuser, but is executed when the real
     uid is ``root'', then the program has some of the powers of a superuser
     as well.

OpenBSD 5.4                    November 17, 2011                   OpenBSD 5.4

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