Date: Wed, 2 Jul 2003 11:10:02 -0700 From: "Justin C. Walker" <justin@mac.com> To: freebsd-hackers@freebsd.org Subject: Re: Can a pass-by-reference var be assigned to a local var? Message-ID: <66D4CD3C-ACB8-11D7-A6BB-00306544D642@mac.com> In-Reply-To: <1560.216.120.158.65.1057168456.squirrel@www.mundomateo.com>
next in thread | previous in thread | raw e-mail | index | archive | help
I think you're missing something.
This is "structure assignment" in action. The variable 'sbp' is a
*statb, and the assignment assigns a structure, sb2, to *sbp, which is
itself a structure, not a pointer.
Regards,
Justin
On Wednesday, July 2, 2003, at 10:54 AM, Matthew Hagerty wrote:
> Greetings,
>
> I was looking over the code for the tail command and found something
> that
> seems wrong. Below is the abbreviated code that highlights my concern.
> Basically, sb is defined in main() and passed to forward() (and
> reverse())
> by reference. Then in forward() sb2 is defined, and finally sb is set
> to
> sb2.
>
> Two things come to mind that don't seem right: 1. What happens to sb
> when
> forward() returns? It was pointing to sb2 which was a local var of
> forward()... and 2. The assignment *sbp = sb2 happens in a loop and
> could
> be executed more than once, which seems to make the comparisons just
> prior
> to the assignment usless 2 or more loop iterations.
>
> Am I missing something? It seems to me that instead of *sbp = sb2, it
> should be done with memcpy() or something? Any insight would be
> greatly
> appreciated.
>
> Thanks,
> Matthew
>
>
> In tail.c:
>
> int
> main(argc, argv)
> int argc;
> char *argv[];
> {
> struct stat sb;
> .
> .
> if ((fp = fopen(fname, "r")) == NULL ||
> fstat(fileno(fp), &sb)) {
> .
> .
> if (rflag)
> reverse(fp, style, off, &sb);
> else
> forward(fp, style, off, &sb);
> .
> .
> }
>
> Then in forward.c:
>
> void
> forward(fp, style, off, sbp)
> FILE *fp;
> enum STYLE style;
> off_t off;
> struct stat *sbp;
> {
> int ch, n, kq = -1;
> int action = USE_SLEEP;
> struct kevent ev[2];
> struct stat sb2;
> struct timespec ts;
> .
> .
> if (Fflag && fileno(fp) != STDIN_FILENO) {
> while (stat(fname, &sb2) != 0)
> /* file was rotated, wait until it reappears */
> (void)sleep(1);
> if (sb2.st_ino != sbp->st_ino ||
> sb2.st_dev != sbp->st_dev ||
> sb2.st_rdev != sbp->st_rdev ||
> sb2.st_nlink == 0) {
> fp = freopen(fname, "r", fp);
> if (fp == NULL) {
> ierr();
> return;
> } else {
> *sbp = sb2;
> action = ADD_EVENTS;
> }
> }
> }
> .
> .
> }
>
> _______________________________________________
> freebsd-hackers@freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to
> "freebsd-hackers-unsubscribe@freebsd.org"
>
>
--
/~\ The ASCII Justin C. Walker, Curmudgeon-at-Large
\ / Ribbon Campaign
X Help cure HTML Email
/ \
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?66D4CD3C-ACB8-11D7-A6BB-00306544D642>