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    SubjectRe: light weight user level semaphores
    Olaf Titz <>:
    > > Ehh.. I will bet you $10 USD that if libc allocates the next file
    > > descriptor on the first "malloc()" in user space (in order to use the
    > > semaphores for mm protection), programs _will_ break.
    > Of course, but this is a result from sloppy coding. In general, open()
    > can just return anything and about the only case where you can even
    > think of ignoring its result is this:
    > close(0); close(1); close(2);
    > open("/dev/null", O_RDWR); dup(0); dup(0);
    > (which is even not clean for other reasons).
    > I can't imagine depending on the "fact" that the first fd I open is 3,
    > the next is 4, etc. And what if the routine in question is not
    > malloc() but e.g. getpwuid()? Both are just arbitrary library
    > functions, and one of them clearly does open file descriptors,
    > depending on their implementation.
    > What would the reason[1] be for wanting contiguous fd space anyway?
    > Olaf
    > [1] apart from not having understood how poll() works of course.

    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....

    It may not be the "best" way, but what I tend to do is:

    Umm - this is snipped from a multiplexed logger using FIFOs for
    and indeterminate amount of data from differet utilities sending
    text buffers (normally one line at a time but could be more).

    static void fd_init(argc,argv)
    int argc; /* number of parameters */
    char **argv; /* parameter list */
    int i,j; /* scratch counters */
    static char str[50];

    pnames = argv;
    FD_ZERO(&in_files); /* init all file descriptor sets */

    for (i = 0; i <= MAX_LOG && i < argc; i++) {
    mkfifo(str,0600); /* assume it exists */
    inlogfd[i] = open(str,O_RDONLY | O_NDELAY);
    used = i;

    Then I can scan for any activity by:

    do {
    while (select(MAX_LOG,&active,NULL,NULL,NULL) >= 0) {
    for(i = 0; i <= used; i++) {
    if (FD_ISSET(inlogfd[i],&active)) {
    while (n > 0) {
    r = (n > BUF_MAX - 1) ? BUF_MAX - 1: n;
    n -= r;
    active = in_files;
    } while (errno == EINTR);

    Jesse I Pollard, II

    Any opinions expressed are solely my own.
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