On Sun, 30 Dec 2001 00:14:30 +0100, martin f krafft <[EMAIL PROTECTED]> wrote:
> also sprach Eric G. Miller <egm2@jps.net> [2001.12.29.2350 +0100]:
> > C always uses pass by value, C++ can do both. But, what does that
> > have to do with shell scripts and pipes?
>
> C can pass by reference, no?
No. C always passes by value. Passing a pointer always passes a copy of
the value. That's why you must do things like pass pointers to pointers,
if a procedure will change the thing being pointed at. For instance,
double strtod (const char *nptr, char **endptr);
then...
{
double dbl;
char *ss, *buff = "42.2 Forty-Two and Two-Tenths";
dbl = strtod (buff, &ss);
if (ss == buff) { /* failed conversion */ }
else { /* it worked */ }
}
If "endptr" weren't a pointer to a pointer, it wouldn't be possible to
set *endptr to the location in the character string after the number (if
any).
> think about it: i pass by value when i pipe into a program, and i pass
> by reference when i tell that same program which file to read. standard
> unix filter procedure.
>
> when passed as a pipe, the original data can't be modified.
> when passed as a file name, the data can be modified *and* there is
> nothing copied (ignoring what the program does)...
Somewhat analogous, but a pipe is a stream of bytes that is not exactly
analogous to passing by value, because if the program needs to move
around in that byte stream, it still must copy the data...
--
Eric G. Miller <egm2@jps.net>
