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

# islower(3)

lower-case character test

## NAME

**islower**, **islower\_l** – lower-case character test

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

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

*int*\

**islower**(*int c*);

*int*\

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

## DESCRIPTION

The **islower**() and **islower\_l**() functions test for any lower-case letters. The value of the argument must be representable as an *unsigned char* or the value of EOF.

In the ASCII character set, this includes the following characters (with their numeric values shown in octal):

| | | | | |
| --- | --- | --- | --- | --- |
| 141 \`\`a'' | 142 \`\`b'' | 143 \`\`c'' | 144 \`\`d'' | 145 \`\`e'' |
| 146 \`\`f'' | 147 \`\`g'' | 150 \`\`h'' | 151 \`\`i'' | 152 \`\`j'' |
| 153 \`\`k'' | 154 \`\`l'' | 155 \`\`m'' | 156 \`\`n'' | 157 \`\`o'' |
| 160 \`\`p'' | 161 \`\`q'' | 162 \`\`r'' | 163 \`\`s'' | 164 \`\`t'' |
| 165 \`\`u'' | 166 \`\`v'' | 167 \`\`w'' | 170 \`\`x'' | 171 \`\`y'' |
| 172 \`\`z'' |  |  |  |  |

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

## RETURN VALUES

The **islower**() and **islower\_l**() functions return zero if the character tests false and return non-zero if the character tests true.

## 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 **iswlower**() or **iswlower\_l**() function should be used instead.

## SEE ALSO

[ctype(3)](/ctype/3.md), [ctype\_l(3)](/ctype_l/3.md), [iswlower(3)](/iswlower/3.md), [iswlower\_l(3)](/iswlower_l/3.md), [tolower(3)](/tolower/3.md), [tolower\_l(3)](/tolower_l/3.md), [xlocale(3)](/xlocale/3.md), [ascii(7)](/ascii/7.md)

## STANDARDS

The **islower**() function conforms to ISO/IEC 9899:1990 (“ISO C90”). The **islower\_l**() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

## HISTORY

The **islower**() function first appeared in Version 7 AT&T UNIX.