Messages in this thread |  | | | Subject | Re: light weight user level semaphores | | Date | Fri, 20 Apr 2001 20:36:42 +0200 | | From | Olaf Titz <> |
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> Optimization use in select: If all "interesting" file id's are known > to be below "n", then only the first "n" bits in a FD_ISSET need to > be examined. As soon as the bits are scattered, it takes MUCH longer > to check for activity....
That's an optimization, not a correctness issue.
> for (i = 0; i <= MAX_LOG && i < argc; i++) { > sprintf(str,"/tmp/%s",pnames[i]); > mkfifo(str,0600); /* assume it exists */ > inlogfd[i] = open(str,O_RDONLY | O_NDELAY); > FD_SET(inlogfd[i],&in_files); > }
This works regardless of what the open() returns. What does not work is using MAX_LOG (assuming it is constant) later in the following form:
> while (select(MAX_LOG,&active,NULL,NULL,NULL) >= 0) {
I see no way around computing the maximum of the inlogfd[i] values +1. (Which can of course be done just after the opens above. Note that the last opened fd _is_ guaranteed to get the highest number; FD_SET is one of the library routines where you can be pretty confident they don't open fds...)
Btw. there are two problems even assuming you do get contiguous fds: - an off by one error in the case of argc > MAX_LOG, the first argument of select() is maximum fd _plus one_ - from an optimization POV it is highly advisable to take only the real maximum anyway. Olaf - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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