How does a program get its path name?

[Russell Kent]

in article <10106 at ulysses.homer.nj.att.com>, cjc at ulysses.homer.nj.att.com (Chris Calabrese[rs]) says:
> 
> In article <2933 at sdsu.UUCP>, turtle at sdsu.UUCP writes:
>> 
>> When a program executes, the full path to the executable file is kept in
>> the zero-th argument.  If you have a declaration of main which looks like this:
>> 	main(argc,argv)
>> 	int	argc;
>> 	char	*argv[];
>> 
>> then argv[0] is a pointer to the full path.
>> Is this what you were looking for?
> 
> This is true only if the program was not found in the
> current working directory, in which case argv[0] will contain
> only the name of the program, not the full path.  Of course,
> you can test to see if argv[0][0] != '/' and getcwd()
> to find the full path name.
> 
> 	Christopher Calabrese

Unfortunately, neither of these statements is entirely correct.  That
argv[0] contains any portion of the pathname of the process's executable
file is _merely_ convention on the part of _most_ shells (sh, rsh, csh, ksh,
and tcsh).  It is perfectly ok to put your mother's maiden name in argv[0]
of a process you are about to exec, although this is obviously not very
useful.

The convention that _most_ shells follows is that argv[0] contains the
same text as the first blank-separated field of the command line after
any aliasing takes place.  This means that:

    Command			Argv[0]
    cc				cc
    /bin/cc			/bin/cc
    alias rm /bin/rm -i		<internal command>
    rm jack			/bin/rm
    \rm jill			rm

The astute read may counter: "But if I do a /bin/cp with no parameters,
the computer comes back with "Usage: cp f1 f2" not "Usage: /bin/cp f1 f2".
This is because some programs intentionally use only the last portion of
the path given in argv[0] (using "argv0 = strrbrk (argv[0], '/');").

This convention is actually implemented in the shell through the use of
exec*p();  if you use execl() you can bypass it.
-- 
Russell Kent                    Phone: +1 214 995 3501
Texas Instruments               UUCP address:
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