---
name: "tolower_l"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/tolower"
---

# tolower\_l(3)

upper case to lower case letter conversion

Part of [tolower(3)](/tolower/3.md).

## NAME

**tolower**, **tolower\_l** – upper case to lower case letter conversion

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<ctype.h\>**

*int*\

**tolower**(*int c*);

*int*\

**tolower\_l**(*int c*, *locale\_t loc*);

## DESCRIPTION

The **tolower**() and **tolower\_l**() functions convert an upper-case letter to the corresponding lower-case letter. The argument must be representable as an *unsigned char* or the value of EOF.

The **tolower\_l**() function takes an explicit locale argument, whereas the **tolower**() function uses the current global or per-thread locale.

## RETURN VALUES

If the argument is an upper-case letter, the **tolower**() and **tolower\_l**() functions return the corresponding lower-case letter if there is one; otherwise, the argument is returned unchanged.

## COMPATIBILITY

The 4.4BSD extension of accepting arguments outside of the range of the *unsigned char* type in locales with large character sets is considered obsolete and may not be supported in future releases. The **towlower**() or **towlower\_l**() function should be used instead.

## SEE ALSO

[ctype(3)](/ctype/3.md), [ctype\_l(3)](/ctype_l/3.md), [islower(3)](/islower/3.md), [islower\_l(3)](/islower_l/3.md), [towlower(3)](/towlower/3.md), [towlower\_l(3)](/towlower_l/3.md), [xlocale(3)](/xlocale/3.md)

## STANDARDS

The **tolower**() function conforms to ISO/IEC 9899:1990 (“ISO C90”).