EXEC(II) 8/5/73 EXEC(II)
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
arguments are placed as high as possible in core.
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
arguments 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
indicated, 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
arguments require more than 512 bytes a return from exec
constitutes 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.