---
name: "tolower"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/tolower"
---

# tolower(3)

upper case to lower case letter conversion

## 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*);

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

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

*int*\

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

## DESCRIPTION

The **tolower**() function converts an upper-case letter to the corresponding lower-case letter. The argument must be representable as an *unsigned char* or the value of EOF.

Although the **tolower**() function uses the current locale, the **tolower\_l**() function may be passed a locale directly. See [xlocale(3)](/xlocale/3.md) for more information.

## RETURN VALUES

If the argument is an upper-case letter, the **tolower**() function returns 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**() function should be used instead.

## SEE ALSO

[ctype(3)](/ctype/3.md), [islower(3)](/islower/3.md), [towlower(3)](/towlower/3.md), [xlocale(3)](/xlocale/3.md)

## STANDARDS

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