Header File 

ctype.h, mbstring.h 

Category 

Classification Routines 

Prototype 

int isalnum(int c); 

int __iscsym(int c); 

int iswalnum(wint_t c); 

int _ismbcalnum(unsigned int c); 

Description 

Tests for an alphanumeric character. 

isalnum is a macro that classifies ASCII-coded integer values by table lookup. The macro is affected by the current locale’s LC_CTYPE category. For the default C locale, c is a letter (A to Z or a to z) or a digit (0 to 9). 

You can make this macro available as a function by undefining (#undef) it. 

Return Value 

It is a predicate returning nonzero for true and 0 for false. 

isalnum returns nonzero if c is a letter or a digit. 

__iscsym returns nonzero if c is a letter, underscore, or digit. 

iswalnum returns nonzero if iswalpha or iswdigit return true for c. 

_ismbcalnum returns true if and only if the argument c is a single-byte ASCII English letter. 

Example  

#include <stdio.h>
#include <ctype.h>
int main(void)
{
  char c = 'C';
  
  if (isalnum(c))
    printf("%c is alphanumeric\n",c);
  else 
    printf("%c is not alphanumeric\n",c);
  return 0;
}

Portability