EXECVE(2)                 NetBSD System Calls Manual                 EXECVE(2)

     execve -- execute a file

     Standard C Library (libc, -lc)

     #include <unistd.h>

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

     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 repre-
     senting the initial program (text) and initialized data pages.  Addi-
     tional pages may be specified by the header to be initialized with zero
     data;  see a.out(5).

     An interpreter file begins with a line of the form:

           #! interpreter [arg]

     When an interpreter file is execve()d the system runs 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,
     normally the name of the execve()d file, is left unchanged.  The inter-
     preter named by interpreter must not itself be an interpreter file.  (See
     script(7) for a detailed discussion of interpreter file execution.)

     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.  By custom,
     the first element should be the name of the executed program (for exam-
     ple, the last component 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().

     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 cur-
     rently 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 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 sigaction(2), sigprocmask(2)

     When a program is executed as a result of an execve() 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.

     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.

     execve() will fail and return to the calling process if:

     [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

     [EACCES]           Search permission is denied for a component of the
                        path prefix, the new process file is not an ordinary
                        file, its file mode denies execute permission, or it
                        is on a filesystem mounted with execution disabled
                        (MNT_NOEXEC in <sys/mount.h>).

     [EAGAIN]           A setuid(7) process has exceeded the current resource
                        limit for the number of processes it is allowed to run

     [EFAULT]           The new process file is not as long as indicated by
                        the size values in its header; or path, argv, or envp
                        point to an illegal address.

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

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

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-

     [ENOENT]           The new process file does not exist, or the new
                        process file is a script starting with #! and the
                        script interpreter does not exist.

     [ENOEXEC]          The new process file has the appropriate access per-
                        mission, but has an invalid magic number in its

     [ENOMEM]           The new process requires more virtual memory than is
                        allowed by the imposed maximum (getrlimit(2)).

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

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

     _exit(2), fork(2), execl(3), environ(7), script(7)

     The execve() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').

     The execve() function call first appeared in Version 7 AT&T UNIX.

     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.

NetBSD 6.0                     February 24, 2008                    NetBSD 6.0

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