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NAME

     execve - execute a file

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     int
     execve(const char *path, 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.  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 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
     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-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 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
     privilege 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 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.

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
                        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 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’ 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
                        system.

CAVEAT

     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.

SEE ALSO

     ktrace(1), _exit(2), fork(2), execl(3), exit(3), sysctl(3), a.out(5),
     elf(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.

HISTORY

     The execve() system call appeared in 4.2BSD.