pthread testing code

output

$ ./whyc
starting...
jobname.2019-12-27_17.23.33_01 (seq = 05, lseq = 01)
jobname.2019-12-27_17.23.33_05 (seq = 07, lseq = 05)
jobname.2019-12-27_17.23.33_03 (seq = 07, lseq = 03)
jobname.2019-12-27_17.23.33_08 (seq = 11, lseq = 08)
jobname.2019-12-27_17.23.33_06 (seq = 11, lseq = 06)
jobname.2019-12-27_17.23.33_02 (seq = 11, lseq = 02)
jobname.2019-12-27_17.23.33_07 (seq = 12, lseq = 07)
jobname.2019-12-27_17.23.33_04 (seq = 13, lseq = 04)
jobname.2019-12-27_17.23.33_09 (seq = 13, lseq = 09)
jobname.2019-12-27_17.23.33_10 (seq = 14, lseq = 10)
jobname.2019-12-27_17.23.33_14 (seq = 14, lseq = 14)
jobname.2019-12-27_17.23.33_12 (seq = 14, lseq = 12)
jobname.2019-12-27_17.23.33_13 (seq = 14, lseq = 13)
jobname.2019-12-27_17.23.33_15 (seq = 15, lseq = 15)
jobname.2019-12-27_17.23.33_11 (seq = 16, lseq = 11)
jobname.2019-12-27_17.23.33_17 (seq = 17, lseq = 17)
jobname.2019-12-27_17.23.33_16 (seq = 17, lseq = 16)
jobname.2019-12-27_17.23.33_18 (seq = 18, lseq = 18)
jobname.2019-12-27_17.23.33_19 (seq = 19, lseq = 19)
threads dispatched. waiting...
jobname.2019-12-27_17.23.33_20 (seq = 20, lseq = 20)
done

why.c

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


#define THREADCOUNT 20

/* compile with "gcc -o whyc why.c -lpthread" */

void *maybeunique(void* arg) {


        static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
        static time_t last_start_time = 0;
        static int seq = 0;
        int lseq;
        time_t now = time(NULL);
        struct tm *info;
        char dt[50];
        time_t rawtime;

        pthread_mutex_lock(&mutex);
        seq++;
        if (seq > 59) {
                seq = 0;
                while (now == last_start_time) {
                        usleep(500000);
                        now = time(NULL);
                }
        }
        last_start_time = now;
        lseq = seq; //assign static value to thread-local variable
        pthread_mutex_unlock(&mutex);

        time(&rawtime);
        info = localtime(&rawtime);
        strftime(dt, sizeof(dt), "%Y-%m-%d_%H.%M.%S", info);

        fprintf(stdout, "jobname.%s_%02d (seq = %02d, lseq = %02d)\n", dt, lseq, seq, lseq);
        fflush(stdout);
        return (void*) NULL;
}

int main(int argc, char** argv) {

        int r;
        pthread_t threads[THREADCOUNT];

        fprintf(stdout, "starting...\n");
        fflush(stdout);
        for (int i = 0; i < THREADCOUNT; i++) {
                r = pthread_create(&threads[i], NULL, &maybeunique, NULL);
                if (r) {
                        fprintf(stderr, "failed to create thread %d, errno is %d\n", i, r);
                }
        }
        fprintf(stdout, "threads dispatched. waiting...\n");
        fflush(stdout);
        for (int i = 0; i < 5; i++) {
                r = pthread_join(threads[i], NULL);
                if (r) {
                        fprintf(stderr, "failed to wait on thread %d, errno is %d\n", i, r);
                }
        }

        fprintf(stdout, "done\n");
        fflush(stdout);
        exit(0);
}