EXEC (II)                    3/15/72                    EXEC (II)

NAME            exec  --  execute a file

SYNOPSIS        sys exec; name; args  / exec = 11.


          name: <...\0>


          args: arg1; arg2; ...; 0

          arg1: <...\0>


DESCRIPTION     exec overlays the calling process with the named

                file, then transfers to the beginning of the core

                image of the file.  The first argument to exec is

                a pointer to the name of the file to be executed.

                The second is the address of a list of pointers

                to arguments to be passed to the file.  Conven-

                tionally, the first argument is the name of the

                file.  Each pointer addresses a string terminated

                by a null byte.

                There can be no return from the file; the calling

                core image is lost.

                The program break is set from the executed file;

                see the format of a.out.

                Once the called file starts execution, the argu-

                ments 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.

                  sp->  nargs




                 arg1:  <arg1\0>


                 argn:  <argn\0>

                The arguments are placed as high as possible in

                core: just below 57000(8).

                Files remain open across exec calls.  However,

                the illegal instruction, emt, quit, and interrupt

                trap specifications are reset to the standard

                values.  (See ilgins, cemt, quit, intr.)

                Each user has a real user ID and an effective

                user ID (The real ID identifies the person using

                the system; the effective ID determines his ac-

                cess privileges.)  exec changes the effective

                user ID to the owner of the executed file if the

                file has the "set-user-ID" mode.  The real user

                ID is not affected.

SEE ALSO        fork(II)

DIAGNOSTICS     If the file cannot be read or if it is not exe-

                cutable, a return from exec constitutes the diag-

                nostic.  The error bit (c-bit) is set.

BUGS            Very high core and very low core are used by exec

                to construct the argument list for the new core

                image.  If the original copies of the arguments

                reside in these places, problems can result.