Header File 

ctype.h, mbstring.h 

Category 

Classification Routines 

Prototype 

int isalpha(int c); 

int __iscsymf(int c); 

int iswalpha(wint_t c); 

int _ismbcalpha(unsigned int c); 

Description 

Classifies an alphabetical character. 

isalpha 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). 

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

Return Value 

isalpha returns nonzero if c is a letter. 

__iscsymf returns true if and only if the argument c is a letter or an underscore. 

iswalpha returns nonzero if c is a wchar_t in the character set defined by the implementation. 

_ismbcalpha 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 (isalpha(c))
    printf("%c is alphabetical\n",c);
  else 
    printf("%c is not alphabetical\n",c);
  return 0;
}

Portability