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

# islower(3)

lower-case character test

## NAME

**islower** – lower-case character test

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

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

*int*\

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

## DESCRIPTION

The **islower**() function tests 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'' |

## RETURN VALUES

The **islower**() function returns zero if the character tests false and returns 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**() function should be used instead.

## SEE ALSO

[ctype(3)](/ctype/3.md), [isalnum\_l(3)](/isalnum_l/3.md), [iswlower(3)](/iswlower/3.md), [tolower(3)](/tolower/3.md), [ascii(7)](/ascii/7.md)

## STANDARDS

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