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# locale.h
**`locale.h`** controls how your program formats and interprets culture-specific data: whether the decimal separator is `.` or `,`, what the currency symbol is, which characters count as letters. By default a C program runs in the `"C"` locale — ASCII, `.` decimal separator, no localisation — which is predictable but not what a German or French user expects to see.

```c
#include <locale.h>

setlocale(LC_ALL, "");              // pick up the user's environment locale
setlocale(LC_ALL, "C");             // reset to the predictable ASCII baseline
setlocale(LC_NUMERIC, "de_DE.UTF-8");  // German number format only
```

The category lets you change one aspect without touching the others:

| Category | Controls |
| `LC_ALL`      | Everything |
| `LC_NUMERIC`  | Decimal point, thousands separator |
| `LC_MONETARY` | Currency symbol and format |
| `LC_CTYPE`    | `isalpha`, `toupper` etc. |
| `LC_COLLATE`  | String comparison order |
| `LC_TIME`     | `strftime` output |

`localeconv()` returns a `struct lconv` with all the current locale's formatting strings: `decimal_point`, `thousands_sep`, `currency_symbol`, and more.

The practical rule: call `setlocale(LC_ALL, "")` at startup if you are producing output for humans. But be careful with `LC_NUMERIC` — any library that calls `setlocale` on your behalf can change the decimal separator under your feet, which will break `strtod` parsing of machine-generated data. Keep `LC_NUMERIC` as `"C"` when parsing programmatic input.

## Practice

```c
// compile: gcc -o localedemo localedemo.c
// run: ./localedemo
// description: show how locale changes decimal formatting; install de_DE if missing

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

int main(void) {
    double pi = 3.141592653589793;

    setlocale(LC_ALL, "C");
    printf("C locale:      %f\n", pi);   // 3.141593

    if (!setlocale(LC_NUMERIC, "de_DE.UTF-8") &&
        !setlocale(LC_NUMERIC, "de_DE"))
        puts("de_DE locale not available; install with: sudo locale-gen de_DE.UTF-8");
    else {
        printf("de_DE locale:  %f\n", pi);   // 3,141593 (comma!)
        printf("decimal_point = '%s'\n", localeconv()->decimal_point);
    }
    return 0;
}
```

If the German locale is installed you will see a comma instead of a dot in the second line. If not, install it with `sudo locale-gen de_DE.UTF-8` on Debian/Ubuntu. Once you have seen this in action it is hard to forget that `LC_NUMERIC` can silently change how `printf` and `strtod` work.