c - Which components use the locale variables?

Question

I have read that every process has a set of locale variables associated with it. For example, these are the locale variables associated with the bash process on my system:

$ locale
LANG="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_CTYPE="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_ALL=

I want to know who actually uses these locale variables.

Do the C standard functions (for example: fwrite()) and the Linux system calls use them? Does the behavior of some C standard functions or some Linux system call differ depending on the value of some locale variable?

Or is it only certain programs that can use these locale variables? For example, I can write a program that will display messages to the user in a different language depending on the value of the LANG locale variable.

Answer 1

By default, C's standard library functions use the "C" locale. You can switch it to the user locale to enable locale-specific:

• Character handling
• Collating
• Date/time formatting
• Numeric editing
• Monetary formatting
• Messaging

POSIX setlocale documentation contains an incomplete list of locale-dependent functions affected by it:

catopen, exec, fprintf, fscanf, isalnum, isalpha, isblank, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, iswalnum, iswalpha, iswblank, iswcntrl, iswctype, iswdigit, iswgraph, iswlower, iswprint, iswpunct, iswspace, iswupper, iswxdigit, isxdigit, localeconv, mblen, mbstowcs, mbtowc, newlocale, nl_langinfo, perror, psiginfo, setlocale, strcoll, strerror, strfmon, strftime, strsignal, strtod, strxfrm, tolower, toupper, towlower, towupper, uselocale, wcscoll, wcstod, wcstombs, wcsxfrm, wctomb

E.g.:

printf("%'d
", 1000000000);
printf("Setting LC_ALL to %s
", getenv("LANG"));
setlocale(LC_ALL, ""); // Set user-preferred locale.
printf("%'d
", 1000000000);

Outputs:

1000000000
Setting LC_ALL to en_US.UTF-8
1,000,000,000