c - setlocale() not working as expect

Question

The following program use setlocale() to take locale settings from environment variable, and print time.

locale_test.c:

// locale test

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

// locale test
void locale_test() {
    // use environment variable to get locale setting,
    setlocale(LC_ALL, "");

    char buf[26];
    time_t now = time(NULL);
    ctime_r(&now, buf);
    printf("time: %s", buf);

    char *time_locale = setlocale(LC_TIME, NULL);
    printf("current LC_TIME locale: %s\n", time_locale);
}

int main(int argc, char *argv[]) {
    locale_test();
    return 0;
}

Execute:

• LC_ALL=fr_FR LC_TIME=fr_FR ./a.out, output is:

  time: Wed Oct 14 13:16:35 2015
  current LC_TIME locale: C

• LC_ALL=en_US LC_TIME=en_US ./a.out, output is:

  time: Wed Oct 14 13:17:12 2015
  current LC_TIME locale: C

The question is:

• The result of 2 execution are expected to be different format, due to different locale, but they are the same, why?
• When try to get current LC_TIME, it just print C, but not the value specified from environment variable.

---

Update - Solution:

According to comment & answer, following changes are made:

• Use correct locale name from result of locale -a
• Use strftime() to generate the time string instead of ctime()

The new program:

// locale test

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

// locale test
void locale_test() {
    // use environment variable to get locale setting,
    if(setlocale(LC_ALL, "") == NULL) {
        printf("error while setlocale()\n");
    }

    // get current LC_TIME
    char *time_locale = setlocale(LC_TIME, NULL);
    if(time_locale == NULL) {
        printf("error while setlocale()\n");
    } else {
        printf("LC_TIME: %s\n", time_locale);
    }

    // print time in locale,
    size_t buf_size = 50;
    char buf[buf_size];
    // char *format = "%F %T %z";
    char *format = "%A, %d %B %Y, %H:%M:%S %Z";

    time_t now = time(NULL);
    struct tm tm_now;
    localtime_r(&now, &tm_now);

    strftime(buf, buf_size, format, &tm_now);
    printf("time: %s\n", buf);
}

int main(int argc, char *argv[]) {
    locale_test();
    return 0;
}

Then execute:

• LC_ALL=en_US.utf8 LC_TIME=en_US.utf8 ./a.out, output is:

  LC_TIME: en_US.utf8
  time: Wednesday, 14 October 2015, 16:36:36 CST

• LC_ALL=zh_CN.utf8 LC_TIME=zh_CN.utf8 ./a.out, output is:

  LC_TIME: zh_CN.utf8
  time: 星期三, 14 十月 2015, 16:38:10 CST

• LC_ALL=en_US.utf8 LC_TIME=en_US.utf8 ./a.out, output is:

  LC_TIME: ja_JP.utf8
  time: 水曜日, 14 10月 2015, 16:40:05 CST

That's work as expect.

Answer 1

You have to check the return value of setlocale, because it will fail if the locale string is invalid, the locale you want is not installed, or some other reason.

if (setlocale(LC_ALL, "") == NULL) {
    puts("Unable to set locale");
}

You can check what locales are available with locale -a. Maybe you do have a French locale installed, but you have to set it with fr_FR.utf8.

Then again, ctime output does not depend on the locale; the standard specifies its exact format, and it will always be in English. For localized output use strftime instead.