print time periodically

ptime.c

#define _DEFAULT_SOURCE

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

int main(int argc, char *argv[]) {
  char outstr[200];
  int usec = 100;
  if (argc > 1) {
    usec = atoi(argv[1]);
  }

  while (usec < 0 || usleep(usec) == 0) {
    time_t t = time(NULL);
    struct tm *tmp = localtime(&t);
    if (tmp == NULL) {
      perror("localtime");
      exit(EXIT_FAILURE);
    }

    if (strftime(outstr, sizeof(outstr), "%T", tmp)) {
      fprintf(stdout, "%s\n", outstr);
    }
  }

  exit(EXIT_FAILURE);
}