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NAME
     exec, execl, execv  -  execute a file

SYNOPSIS
     (exec = 11.)
     sys exec; name; args
     name: <...\0>
     args: arg0; arg1; ...; 0
     arg0: <...\0>
     arg1: <...\0>
        ...

     execl(name, arg0, arg1, ..., argn, 0)
     char *name, *arg0, *arg1, ..., *argn;

     execv(name, argv)
     char *name;
     char *argv[ ];

DESCRIPTION
     Exec overlays the calling process with the named file, then
     transfers to the beginning of the core image of the file.
     There can be no return from the file; the calling core image
     is lost.

     Files remain open across exec calls.  Ignored signals remain
     ignored across exec, but signals that are caught are reset
     to their default values.

     Each user has a real user ID and group ID and an effective
     user ID and group ID.  The real ID identifies the person
     using the system; the effective ID determines his access
     privileges.  Exec changes the effective user and group ID to
     the owner of the executed file if the file has the ``set-
     user-ID'' or ``set-group-ID'' modes.  The real user ID is
     not affected.

     The form of this call differs somewhat depending on whether
     it is called from assembly language or C; see below for the
     C version.

     The first argument to exec is a pointer to the name of the
     file to be executed.  The second is the address of a null-
     terminated list of pointers to arguments to be passed to the
     file.  Conventionally, the first argument is the name of the
     file.  Each pointer addresses a string terminated by a null
     byte.

     Once the called file starts execution, the arguments are
     available as follows.  The stack pointer points to a word
     containing the number of arguments.  Just above this number
     is a list of pointers to the argument strings.  The argu-
     ments are placed as high as possible in core.

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       sp→   nargs
             arg0
             ...
             argn
             -1

      arg0:  <arg0\0>
             ...
      argn:  <argn\0>

     From C, two interfaces are available.  execl is useful when
     a known file with known arguments is being called; the argu-
     ments to execl are the character strings constituting the
     file and the arguments; as in the basic call, the first
     argument is conventionally the same as the file name (or its
     last component).  A 0 argument must end the argument list.

     The execv version is useful when the number of arguments is
     unknown in advance; the arguments to execv are the name of
     the file to be executed and a vector of strings containing
     the arguments.  The last argument string must be followed by
     a 0 pointer.

     When a C program is executed, it is called as follows:

             main(argc, argv)
             int argc;
             char **argv;

     where argc is the argument count and argv is an array of
     character pointers to the arguments themselves.  As indi-
     cated, argc is conventionally at least one and the first
     member of the array points to a string containing the name
     of the file.

     Argv is not directly usable in another execv, since
     argv[argc] is -1 and not 0.

SEE ALSO
     fork (II)

DIAGNOSTICS
     If the file cannot be found, if it is not executable, if it
     does not have a valid header (407, 410, or 411 octal as
     first word), if maximum memory is exceeded, or if the argu-
     ments require more than 512 bytes a return from exec consti-
     tutes the diagnostic; the error bit (c-bit) is set.  Even
     for the super-user, at least one of the execute-permission
     bits must be set for a file to be executed.  From C the
     returned value is -1.

BUGS
     Only 512 characters of arguments are allowed.

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