isctype

isctype is a function from the C standard library interfaces that tests whether a given character belongs to a specified character class. It is commonly found on POSIX-compliant systems and in some C libraries (such as those used on Unix-like systems and some Windows environments), but it is not part of the ISO C standard. The function is useful for character classification that may depend on locale settings beyond the basic ASCII categories.

The typical declaration is int isctype(int c, int type); both parameters are integers. The first parameter represents the character to test, usually supplied as an unsigned char promoted to int, or as EOF. The second parameter is a bitmask describing the character class to test against, such as digits, letters, whitespace, punctuation, or other locale-dependent categories. The exact set and names of the bitmasks are implementation-defined, but they generally correspond to common classes like upper-case letters, lower-case letters, digits, whitespace, and punctuation. The function returns a non-zero value if the character belongs to the requested class and zero otherwise, with behavior influenced by the current locale.

Usage notes and portability considerations are important. Because isctype is not mandated by the ISO C standard, developers should verify availability in their target environments. When portability is critical, equivalent checks using standard C library functions such as isalpha, isdigit, isspace, and friends can be more reliable; are often preferred because they do not rely on non-standard variants and are locale-aware through the standard facilities. For wide characters, wide-character variants such as iswctype are available and operate with wide character types and corresponding masks.

Example (illustrative, not portable across all implementations):

#include <ctype.h>

int c = 'A';

if (isctype(c, DIGIT)) { /* handle digit */ }