setlocale(3)

💖 setlocale(3) — Library Functions Manual 💖

okay bestie, so we're diving into setlocale(3) — yes the (3) means it lives in section 3 of the manual, the Library Functions Manual, aka where the C library girlies hang out ✨

🌸 NAME 🌸

setlocale — set the current locale 💅

📚 LIBRARY 📚

Standard C library (libc, -lc) 🫶

✨ SYNOPSIS ✨

#include <locale.h>

char *setlocale(int category, const char *_Nullable locale);

🦋 DESCRIPTION 🦋

so setlocale() is the function you call to set or query your program's current locale. that's it, that's the whole job 🎀

If locale is not NULL, your program's current locale gets modified according to the args. The category argument is the one deciding which parts of the program's current locale should be modified. Here's the whole roster, no skips:

• LC_ALL — All of the locale 👑
• LC_ADDRESS — Formatting of addresses and geography-related items (*)
• LC_COLLATE — String collation
• LC_CTYPE — Character classification
• LC_IDENTIFICATION — Metadata describing the locale (*)
• LC_MEASUREMENT — Settings related to measurements (metric versus US customary) (*)
• LC_MESSAGES — Localizable natural-language messages
• LC_MONETARY — Formatting of monetary values
• LC_NAME — Formatting of salutations for persons (*)
• LC_NUMERIC — Formatting of nonmonetary numeric values
• LC_PAPER — Settings related to the standard paper size (*)
• LC_TELEPHONE — Formats to be used with telephone services (*)
• LC_TIME — Formatting of date and time values

The categories marked with an asterisk in the above table are GNU extensions, okay? not universal, not portable, they're the bonus DLC. For further information on these locale categories, see locale(7) 🧚

Now the locale argument — it's a pointer to a character string holding the required setting of category. That string is either a well-known constant like "C" or "da_DK" (see below bestie, we get there), or an opaque string that was returned by another call of setlocale() 💫

If locale is an empty string, "", then every part of the locale that should be modified gets set according to the environment variables. The details are implementation-dependent — that's the vibe. For glibc, first (regardless of category), the environment variable LC_ALL is inspected, next the environment variable with the same name as the category (see the table above), and finally the environment variable LANG. The first existing environment variable is used. If its value is not a valid locale specification, the locale is unchanged, and setlocale() returns NULL 😭

The locale "C" or "POSIX" is a portable locale; it exists on all conforming systems. universal icon behavior fr fr 🌈

A locale name is typically of the form language[_territory][.codeset][@modifier], where language is an ISO 639 language code, territory is an ISO 3166 country code, and codeset is a character set or encoding identifier like ISO-8859-1 or UTF-8. For a list of all supported locales, try "locale -a" (see locale(1)) 🔥

If locale is NULL, the current locale is only queried, not modified. it's just a lil vibe check, no changes bestie 🙈

On startup of the main program, the portable "C" locale is selected as default. A program may be made portable to all locales by calling:

setlocale(LC_ALL, "");

after program initialization, and then:

• using the values returned from a localeconv(3) call for locale-dependent information;
• using the multibyte and wide character functions for text processing if MB_CUR_MAX > 1;
• using strcoll(3) and strxfrm(3) to compare strings; and
• using wcscoll(3) and wcsxfrm(3) to compare wide-character strings.

that's the portability starter pack, no cap 💅

🫶 RETURN VALUE 🫶

A successful call to setlocale() returns an opaque string that corresponds to the locale set. This string may be allocated in static storage — noted. The string returned is such that a subsequent call with that string and its associated category will restore that part of the process's locale. The return value is NULL if the request cannot be honored 😔

⭐ ATTRIBUTES ⭐

For an explanation of the terms used in this section, see attributes(7).

• Interface: setlocale() — Attribute: Thread safety — Value: MT-Unsafe const:locale env

ok so the thread-safety tea is: MT-Unsafe const:locale env. write that down 📝

👑 STANDARDS 👑

C11, POSIX.1-2008.

🎀 Categories 🎀

• LC_ALL
• LC_COLLATE
• LC_CTYPE
• LC_MONETARY
• LC_NUMERIC
• LC_TIME

→ C11, POSIX.1-2008.

• LC_MESSAGES

→ POSIX.1-2008.

• Others: → GNU.

💜 HISTORY 💜

POSIX.1-2001, C89.

🎀 Categories 🎀

• LC_ALL
• LC_COLLATE
• LC_CTYPE
• LC_MONETARY
• LC_NUMERIC
• LC_TIME

→ C89, POSIX.1-2001.

• LC_MESSAGES

→ POSIX.1-2001.

• Others: → GNU.

🌈 SEE ALSO 🌈

locale(1), localedef(1), isalpha(3), localeconv(3), nl_langinfo(3), rpmatch(3), strcoll(3), strftime(3), charsets(7), locale(7)

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and that's the whole page bestie, you ate that 💅✨ this was setlocale(3) from Linux man-pages 6.20, dated 2026-02-08 — go forth and localize, you gorgeous main character 🦋🎀💖