130 lines
3.1 KiB
C
130 lines
3.1 KiB
C
/* This file is part of the sample code and exercises
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* used by the class "Advanced Programming in the UNIX
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* Environment" taught by Jan Schaumann
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* <jschauma@netmeister.org> at Stevens Institute of
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* Technology.
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*
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* This file is in the public domain.
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*
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* You don't have to, but if you feel like
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* acknowledging where you got this code, you may
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* reference me by name, email address, or point
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* people to the course website:
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* https://stevens.netmeister.org/631/
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*/
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/*
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* This trivial program attempts to determine how many
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* open files a process can have. It illustrates the
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* use of pre-processor directives and sysconf(3) to
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* identify a resource that can be changed at system
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* run time.
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*
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* This program also asks getconf(1) and explicitly
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* asks getrlimit(2). Look at the source code for
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* getconf(1) and sysconf(3) to show how they are
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* implemented, as well as manual page and
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* implementation of getdtablesize(3).
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*
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* Run this once, then use 'ulimit -n' to change the
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* limit and run again.
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*
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* If you (as root) set 'ulimit -n unlimited', what
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* value will you get? Why?
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*
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* What happens if you set this limit to, say, 0?
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* Why?
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*/
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#include <sys/resource.h>
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#include <sys/stat.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <limits.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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int
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countOpenFiles(int num) {
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struct stat stats;
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int count = 0;
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for (int i = 0; i < num; i++) {
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if (fstat(i, &stats) == 0) {
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printf("Currently open: fd #%d (inode %ld)\n", i,
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stats.st_ino);
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count++;
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}
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}
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return count;
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}
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void
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openFiles(int num) {
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int count, fd;
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count = countOpenFiles(num);
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printf("Currently open files: %d\n", count);
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for (int i = count; i <= num ; i++) {
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if ((fd = open("/dev/null", O_RDONLY)) < 0) {
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if (errno == EMFILE) {
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printf("Opened %d additional files, then failed: %s (%d)\n", i - count, strerror(errno), errno);
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break;
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} else {
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fprintf(stderr, "Unable to open '/dev/null' on fd#%d: %s (errno %d)\n",
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i, strerror(errno), errno);
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break;
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}
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}
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}
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}
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int
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main() {
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int openmax;
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struct rlimit rlp;
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#ifdef OPEN_MAX
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printf("OPEN_MAX is defined as %d.\n", OPEN_MAX);
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#else
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printf("OPEN_MAX is not defined on this platform.\n");
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#endif
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printf("'getconf OPEN_MAX' says: ");
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(void)fflush(stdout);
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(void)system("getconf OPEN_MAX");
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/* see also: getdtablesize(3), notably the HISTORY section */
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errno = 0;
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if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) {
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if (errno == 0) {
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fprintf(stderr, "sysconf(3) considers _SC_OPEN_MAX unsupported?\n");
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} else {
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fprintf(stderr, "sysconf(3) error for _SC_OPEN_MAX: %s\n",
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strerror(errno));
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}
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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printf("sysconf(3) says this process can open %d files.\n", openmax);
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if (getrlimit(RLIMIT_NOFILE, &rlp) != 0) {
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fprintf(stderr, "Unable to get per process rlimit: %s\n", strerror(errno));
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exit(EXIT_FAILURE);
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}
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openmax = (int)rlp.rlim_cur;
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printf("getrlimit(2) says this process can open %d files.\n", openmax);
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printf("Which one is it?\n\n");
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openFiles(openmax);
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return EXIT_SUCCESS;
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}
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