isdigit man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

isdigit, isdigit_l — test for a decimal digit


#include <ctype.h>

int isdigit(int c);
int isdigit_l(int c, locale_t locale);


For isdigit(): The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

The isdigit() and isdigit_l() functions shall test whether c is a character of class digit in the current locale, or in the locale represented by locale, respectively; see the Base Definitions volume of POSIX.1‐2008, Chapter 7, Locale.

The c argument is an int, the value of which the application shall ensure is a character representable as an unsigned char or equal to the value of the macro EOF. If the argument has any other value, the behavior is undefined.

The behavior is undefined if the locale argument to isdigit_l() is the special locale object LC_GLOBAL_LOCALE or is not a valid locale object handle.

Return Value

The isdigit() and isdigit_l() functions shall return non-zero if c is a decimal digit; otherwise, they shall return 0.


No errors are defined.

The following sections are informative.

Application Usage

To ensure applications portability, especially across natural languages, only these functions and the functions in the reference pages listed in the See Also section should be used for character classification.

See Also

isalnum(), isalpha(), isblank(), iscntrl(), isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(), isxdigit()

The Base Definitions volume of POSIX.1‐2008, Chapter 7, Locale, <ctype.h>, <locale.h>