---
name: "isalnum"
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/isalnum"
---

# isalnum(3)

alphanumeric character test

## NAME

**isalnum**, **isalnum\_l** – alphanumeric character test

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

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

*int*\

**isalnum**(*int c*);

*int*\

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

## DESCRIPTION

The **isalnum**() and **isalnum\_l**() functions test for any character for which [isalpha(3)](/isalpha/3.md), [isalpha\_l(3)](/isalpha_l/3.md) or [isdigit(3)](/isdigit/3.md), [isdigit\_l(3)](/isdigit_l/3.md) is true. 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):

| | | | | |
| --- | --- | --- | --- | --- |
| 060 \`\`0'' | 061 \`\`1'' | 062 \`\`2'' | 063 \`\`3'' | 064 \`\`4'' |
| 065 \`\`5'' | 066 \`\`6'' | 067 \`\`7'' | 070 \`\`8'' | 071 \`\`9'' |
| 101 \`\`A'' | 102 \`\`B'' | 103 \`\`C'' | 104 \`\`D'' | 105 \`\`E'' |
| 106 \`\`F'' | 107 \`\`G'' | 110 \`\`H'' | 111 \`\`I'' | 112 \`\`J'' |
| 113 \`\`K'' | 114 \`\`L'' | 115 \`\`M'' | 116 \`\`N'' | 117 \`\`O'' |
| 120 \`\`P'' | 121 \`\`Q'' | 122 \`\`R'' | 123 \`\`S'' | 124 \`\`T'' |
| 125 \`\`U'' | 126 \`\`V'' | 127 \`\`W'' | 130 \`\`X'' | 131 \`\`Y'' |
| 132 \`\`Z'' | 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 **isalnum\_l**() function takes an explicit locale argument, whereas the **isalnum**() function uses the current global or per-thread locale.

## RETURN VALUES

The **isalnum**() and **isalnum\_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 **iswalnum**() or **iswalnum\_l**() function should be used instead.

## SEE ALSO

[ctype(3)](/ctype/3.md), [ctype\_l(3)](/ctype_l/3.md), [isalpha(3)](/isalpha/3.md), [isalpha\_l(3)](/isalpha_l/3.md), [isdigit(3)](/isdigit/3.md), [isdigit\_l(3)](/isdigit_l/3.md), [iswalnum(3)](/iswalnum/3.md), [iswalnum\_l(3)](/iswalnum_l/3.md), [xlocale(3)](/xlocale/3.md), [ascii(7)](/ascii/7.md)

## STANDARDS

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

## HISTORY

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