The header ctype.h is a part of the standard C library. It provides functions for classifying and converting characters.

All of these functions take one parameter, an int that must be either EOF or representable as an unsigned char.

The names of the classifying functions are prefixed with 'is'. Each returns an integer non-zero value (TRUE) if the character passed to it satisfies the related condition. If the condition is not satisfied then the function returns a zero value (FALSE).

These classifying functions operate as shown, assuming the default C locale:

int a;
int c = 'A';
a = isalpha(c); /* Checks if c is alphabetic (A-Z, a-z), returns non-zero here. */
a = isalnum(c); /* Checks if c  is alphanumeric (A-Z, a-z, 0-9), returns non-zero here. */
a = iscntrl(c); /* Checks is c is a control character (0x00-0x1F, 0x7F), returns zero here. */
a = isdigit(c); /* Checks if c is a digit (0-9), returns zero here. */
a = isgraph(c); /* Checks if c has a graphical representation (any printing character except space), returns non-zero here. */
a = islower(c); /* Checks if c is a lower-case letter (a-z), returns zero here. */
a = isprint(c); /* Checks if c is any printable character (including space), returns non-zero here. */
a = isupper(c); /* Checks if c is a upper-case letter (a-z), returns zero here. */
a = ispunct(c); /* Checks if c is a punctuation character, returns zero here. */
a = isspace(c); /* Checks if c is a white-space character, returns zero here. */
a = isupper(c); /* Checks if c is an upper-case letter (A-Z), returns non-zero here. */
a = isxdigit(c); /* Checks if c is a hexadecimal digit (A-F, a-f, 0-9), returns non-zero here. */

a = isblank(c); /* Checks if c is a blank character (space or tab), returns non-zero here. */

There are two conversion functions. These are named using the prefix 'to'. These functions take the same argument as those above. However the return value is not a simple zero or non-zero but the passed argument changed in some manner.

These conversion functions operate as shown, assuming the default C locale:

int a;
int c = 'A';

/* Converts c to a lower-case letter (a-z). 
 * If conversion is not possible the unchanged value is returned. 
 * Returns 'a' here. 
 */
a = tolower(c);

/* Converts c to an upper-case letter (A-Z). 
 * If conversion is not possible the unchanged value is returned. 
 * Returns 'A' here. 
 */
a = toupper(c);

The below information is quoted from cplusplus.com (opens new window) mapping how the original 127-character ASCII set is considered by each of the classifying type functions (a • indicates that the function returns non-zero for that character)

|ASCII values|characters|iscntrl|isblank|isspace|isupper|islower|isalpha|isdigit|isxdigit|isalnum|ispunct|isgraph|isprint |---|---|---|---|---|---|---|---|---|--- |0x00 .. 0x08|NUL, (other control codes)|•||||||||||| |0x09|tab ('\t')|•|•|•||||||||| |0x0A .. 0x0D|(white-space control codes: '\f','\v','\n','\r')|•||•||||||||| |0x0E .. 0x1F|(other control codes)|•||||||||||| |0x20|space (' ')||•|•|||||||||• |0x21 .. 0x2F|!"#$%&'()*+,-./||||||||||•|•|• |0x30 .. 0x39|0123456789|||||||•|•|•||•|• |0x3a .. 0x40|:;<=>?@||||||||||•|•|• |0x41 .. 0x46|ABCDEF||||•||•||•|•||•|• |0x47 .. 0x5A|GHIJKLMNOPQRSTUVWXYZ||||•||•|||•||•|• |0x5B .. 0x60|[]^_`||||||||||•|•|• |0x61 .. 0x66|abcdef|||||•|•||•|•||•|• |0x67 .. 0x7A|ghijklmnopqrstuvwxyz|||||•|•|||•||•|• |0x7B .. 0x7E|{}~bar||||||||||•|•|• |0x7F|(DEL)|•|||||||||||

#include <ctype.h>
#include <stdio.h>

typedef struct {
  size_t space;
  size_t alnum;
  size_t punct;
} chartypes;

chartypes classify(FILE *f) {
  chartypes types = { 0, 0, 0 };
  int ch;

  while ((ch = fgetc(f)) != EOF) {
    types.space += !!isspace(ch);
    types.alnum += !!isalnum(ch);
    types.punct += !!ispunct(ch);
  }

  return types;
}

The classify function reads characters from a stream and counts the number of spaces, alphanumeric and punctuation characters. It avoids several pitfalls.

• When reading a character from a stream, the result is saved as an int, since otherwise there would be an ambiguity between reading EOF (the end-of-file marker) and a character that has the same bit pattern.
• The character classification functions (e.g. isspace) expect their argument to be either representable as an unsigned char, or the value of the EOF macro. Since this is exactly what the fgetc returns, there is no need for conversion here.
• The return value of the character classification functions only distinguishes between zero (meaning false) and nonzero (meaning true). For counting the number of occurrences, this value needs to be converted to a 1 or 0, which is done by the double negation, !!.

#include <ctype.h>
#include <stddef.h>

typedef struct {
  size_t space;
  size_t alnum;
  size_t punct;
} chartypes;

chartypes classify(const char *s) {
  chartypes types = { 0, 0, 0 };
  const char *p;
  for (p= s; p != '\0'; p++) {
    types.space += !!isspace((unsigned char)*p);
    types.alnum += !!isalnum((unsigned char)*p);
    types.punct += !!ispunct((unsigned char)*p);
  }

  return types;
}

The classify function examines all characters from a string and counts the number of spaces, alphanumeric and punctuation characters. It avoids several pitfalls.

• The character classification functions (e.g. isspace) expect their argument to be either representable as an unsigned char, or the value of the EOF macro.
• The expression *p is of type char and must therefore be converted to match the above wording.
• The char type is defined to be equivalent to either signed char or unsigned char.
• When char is equivalent to unsigned char, there is no problem, since every possible value of the char type is representable as unsigned char.
• When char is equivalent to signed char, it must be converted to unsigned char before being passed to the character classification functions. And although the value of the character may change because of this conversion, this is exactly what these functions expect.
• The return value of the character classification functions only distinguishes between zero (meaning false) and nonzero (meaning true). For counting the number of occurrences, this value needs to be converted to a 1 or 0, which is done by the double negation, !!.