How-to use strftime with setlocale

15 August 2025

Recently I was puzzled when I was attempting to use libc.strftime in Odin, trying to understand how to make it behave respecting the system localization setting. You can see here the forum thread where I posted the question.

I was calling libc.setlocale(.LC_ALL, nil) expecting it to set the program locale according to the environment variables, and then running libc.strftime("%B") expecting it to print the current month according to my system internationalization settings (set to French for that).

It turns out I was misunderstanding the C API of the libc.setlocale function, which has different behavior if you give it a NULL value (nil in Odin) or an empty string (""). It’s all there explained in its manual page.

If you give it NULL, setlocale() will merely return the current locale, NOT modifying the current locale.
If you give it an empty string "", setlocale() will set the current locale respecting the system environment localization settings via the LC_* variables.

Once I changed my program to pass the empty-string like so libc.setlocale(.LC_ALL, ""), then libc.strftime("%B") behaved as expected.

In Python, we have a nice API through the locale and datetime modules that wrap around this stuff, I guess I had lost touch with the C way.

Here’s a C program that exemplifies how this work:

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

void print_locale_and_date(const char *locale_string, const char *date_format) {
  char outstr[200];
  time_t t;
  struct tm *tmp;

  t = time(NULL);
  tmp = localtime(&t);
  if (tmp == NULL) {
    perror("localtime");
    exit(EXIT_FAILURE);
  }

  char *cur_locale = setlocale(LC_ALL, locale_string);
  printf("Current locale is \"%s\"\n", cur_locale);

  if (strftime(outstr, sizeof(outstr), date_format, tmp) == 0) {
    fprintf(stderr, "strftime returned 0");
    exit(EXIT_FAILURE);
  }

  printf("strftime result was: \"%s\"\n\n", outstr);
}

int main(int argc, char *argv[]) {
  const char *date_format = "%d %B, %Y";

  // If given NULL, setlocale will just return the current locale
  print_locale_and_date(NULL, date_format);

  // But if given empty string, it will set the locale to the default locale
  // according to the environment LC_* variables (in my system, it is
  // configured to use French for LC_TIME)
  print_locale_and_date("", date_format);

  // US English locale
  print_locale_and_date("en_US.UTF-8", date_format);

  // If given NULL, setlocale will just return the current locale
  print_locale_and_date(NULL, date_format);

  // French locale
  print_locale_and_date("fr_FR.UTF-8", date_format);

  // Back to default locale
  print_locale_and_date("", date_format);

  exit(EXIT_SUCCESS);
}

Note that it has some locales hardcoded, if you’re not getting the output in French where expected language, run: sudo locale-gen fr_FR.UTF-8 to generate the system localization files for it.