This is daemons/t_syslog.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 37-1 (page 782).

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download daemons/t_syslog.c

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* t_syslog.c

   Demonstrate the use of syslog(3) to write arbitrary messages to
   the system log. Usage is as shown in usageError() below.
#include <syslog.h>
#include "tlpi_hdr.h"
static void
usageError(const char *progName)
    fprintf(stderr, "Usage: %s [-p] [-e] [-l level] \"message\"\n", progName);
    fprintf(stderr, "    -p   log PID\n");
    fprintf(stderr, "    -e   log to stderr also\n");
    fprintf(stderr, "    -l   level (g=EMERG; a=ALERT; c=CRIT; e=ERR\n");
    fprintf(stderr, "                w=WARNING; n=NOTICE; i=INFO; d=DEBUG)\n");
main(int argc, char *argv[])
    int options = 0;
    int level = LOG_INFO;
    int opt;

    while ((opt = getopt(argc, argv, "l:pe")) != -1) {
        switch (opt) {
        case 'l':
            switch (optarg[0]) {
            case 'a': level = LOG_ALERT;        break;
            case 'c': level = LOG_CRIT;         break;
            case 'e': level = LOG_ERR;          break;
            case 'w': level = LOG_WARNING;      break;
            case 'n': level = LOG_NOTICE;       break;
            case 'i': level = LOG_INFO;         break;
            case 'd': level = LOG_DEBUG;        break;
            default:  cmdLineErr("Bad facility: %c\n", optarg[0]);

        case 'p':
            options |= LOG_PID;

#if ! defined(__hpux) && ! defined(__sun)

        /* Not on HP-UX 11 or Solaris 8 */

        case 'e':
            options |= LOG_PERROR;

            fprintf(stderr, "Bad option\n");

    if (argc != optind + 1)

    openlog(argv[0], options, LOG_USER);
    syslog(LOG_USER | level, "%s", argv[optind]);



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