The "locale.h" header file in C programming provides facilities for handling locale-specific information, such as character sets, date and time formats, and currency symbols.

Here are some of the commonly used functions defined in the "locale.h" header file:

1. "setlocale": Sets the current locale for the program, either by specifying a locale name or by using the system's default locale.

2. "localeconv": Returns a pointer to a structure containing information about the formatting conventions for numbers and currency symbols in the current locale.

3. "nl_langinfo": Returns a pointer to a string containing locale-specific information for a specific category, such as the date and time format or the decimal point character.

4. "isprint_l", "toupper_l", "tolower_l", and other similar functions: These are locale-dependent versions of the standard C library functions, which take an additional locale parameter that specifies the locale to use for the operation.

Here is an example of using the "locale.h" header file in a C program:

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

int main() {
    setlocale(LC_ALL, "");

    struct lconv *lc = localeconv();
    printf("Currency symbol: %s\n", lc->currency_symbol);
    printf("Thousands separator: %c\n", lc->thousands_sep);
    printf("Decimal point: %c\n", lc->decimal_point);

    printf("Date format: %s\n", nl_langinfo(D_FMT));
    printf("Time format: %s\n", nl_langinfo(T_FMT));

    char ch = 'a';
    printf("Uppercase of %c is %c\n", ch, toupper_l(ch, NULL));
    printf("Lowercase of %c is %c\n", ch, tolower_l(ch, NULL));

    return 0;
}

In this example, the "locale.h" header file functions are used to print information about the formatting conventions for numbers, currency symbols, dates, and times in the current locale, as well as demonstrating the use of the locale-dependent versions of the standard C library functions. The output of the program will vary depending on the current locale setting.