This is timers/itimerspec_from_str.c (Listing 23-6, page 502), an example from the book, The Linux Programming Interface.

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

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/* itimerspec_from_str.c

   Implement our itimerspecFromStr() function.
#ifndef __APPLE__       /* Mac OS X doesn't define the 'itimerspec' structure
                           (or the POSIX timer functions (timer_*()) */
#include <string.h>
#include <stdlib.h>
#include "itimerspec_from_str.h"        /* Declares function defined here */
/* Convert a string of the following form to an itimerspec structure:
   Optional components that are omitted cause 0 to be assigned to the
   corresponding structure fields. */

itimerspecFromStr(char *str, struct itimerspec *tsp)
    char *dupstr ,*cptr, *sptr;

    dupstr = strdup(str);

    cptr = strchr(dupstr, ':');
    if (cptr != NULL)
        *cptr = '\0';

    sptr = strchr(dupstr, '/');
    if (sptr != NULL)
        *sptr = '\0';

    tsp->it_value.tv_sec = atoi(dupstr);
    tsp->it_value.tv_nsec = (sptr != NULL) ? atoi(sptr + 1) : 0;

    if (cptr == NULL) {
        tsp->it_interval.tv_sec = 0;
        tsp->it_interval.tv_nsec = 0;
    } else {
        sptr = strchr(cptr + 1, '/');
        if (sptr != NULL)
            *sptr = '\0';
        tsp->it_interval.tv_sec = atoi(cptr + 1);
        tsp->it_interval.tv_nsec = (sptr != NULL) ? atoi(sptr + 1) : 0;


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