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CTYPE(3)                   Library Functions Manual                   CTYPE(3)

NAME
     ctype - character classification and mapping functions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <ctype.h>

     isalpha(int c);

     isupper(int c);

     islower(int c);

     isdigit(int c);

     isxdigit(int c);

     isalnum(int c);

     isspace(int c);

     ispunct(int c);

     isprint(int c);

     isgraph(int c);

     iscntrl(int c);

     isblank(int c);

     toupper(int c);

     tolower(int c);

DESCRIPTION
     The above functions perform character tests and conversions on the
     integer c.

     See the specific manual pages for information about the test or
     conversion performed by each function.

ENVIRONMENT
     In NetBSD 11, the ctype functions will always crash with a signal on
     certain invalid inputs as a diagnostic aid for applications; see CAVEATS.
     Setting the environment variable LIBC_ALLOWCTYPEABUSE before starting a
     program will restore the old behavior of returning nonsense answers for
     these inputs, or sometimes but not always crashing, depending on factors
     such as address space layout randomization.

EXAMPLES
     To print an upper-case version of a string to stdout, the following code
     can be used:

           const char *s = "xyz";

           while (*s != '\0') {
               putchar(toupper((unsigned char)*s));
               s++;
           }

SEE ALSO
     isalnum(3), isalpha(3), isblank(3), iscntrl(3), isdigit(3), isgraph(3),
     islower(3), isprint(3), ispunct(3), isspace(3), isupper(3), isxdigit(3),
     tolower(3), toupper(3), ascii(7)

STANDARDS
     These functions, with the exception of isblank(), conform to ANSI
     X3.159-1989 ("ANSI C89").  All described functions, including isblank(),
     also conform to IEEE Std 1003.1-2001 ("POSIX.1").

CAVEATS
     The argument of these functions is of type int, but only a very
     restricted subset of values are actually valid.  The argument must either
     be the value of the macro EOF (which has a negative value), or must be a
     non-negative value within the range representable as unsigned char.
     Passing invalid values leads to undefined behavior.

     Values of type int that were returned by getc(3), fgetc(3), and similar
     functions or macros are already in the correct range, and may be safely
     passed to these ctype functions without any casts.

     Values of type char or signed char must first be cast to unsigned char,
     to ensure that the values are within the correct range.  Casting a
     negative-valued char or signed char directly to int will produce a
     negative-valued int, which will be outside the range of allowed values
     (unless it happens to be equal to EOF, but even that would not give the
     desired result).

     Because the bugs may manifest as silent misbehavior or as crashes only
     when fed input outside the US-ASCII range, the NetBSD implementation of
     the ctype functions is designed to elicit a compiler warning for code
     that passes inputs of type char in order to flag code that may pass
     negative values at runtime that would lead to undefined behavior:

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

           int
           main(int argc, char **argv)
           {

                   if (argc < 2)
                           return 1;
                   setlocale(LC_ALL, "");

                   printf("  char=%-4d isprint? %d\n",
                       (int)*argv[1],
                       isprint(*argv[1]) ? 1 : 0);
                   printf("u_char=%-4d isprint? %d\n",
                       (int)(unsigned char)*argv[1],
                       isprint((unsigned char)*argv[1]) ? 1 : 0);
                   return 0;
           }

     When compiling this program, GCC reports a warning for the line that
     passes char.  At runtime, you may get nonsense answers for some inputs
     without the cast -- if you're lucky and it doesn't crash:

     % gcc -Wall -o test test.c
     In file included from /usr/include/ctype.h:100,
                      from test.c:1:
     test.c: In function 'main':
     test.c:15:21: warning: array subscript has type 'char' [-Wchar-subscripts]
        15 |             isprint(*argv[1]) ? 1 : 0);
           |                     ^
     % LC_CTYPE=C ./test $(printf '\270')
       char=-72  isprint? 1
     u_char=184  isprint? 0
     % LC_CTYPE=C ./test $(printf '\377')
       char=-1   isprint? 0
     u_char=255  isprint? 0
     % LC_CTYPE=fr_FR.ISO8859-1 ./test $(printf '\377')
       char=-1   isprint? 0
     u_char=255  isprint? 1

     Some implementations of libc, such as glibc as of 2018, hide the
     undefined behavior by defining the functions to work for all integer
     inputs representable by either unsigned char or char, and suppress the
     warning.  However, this is not an excuse for avoiding conversion to
     unsigned char: if EOF coincides with any such value, as it does when it
     is -1 on platforms with signed char, programs that pass char will still
     necessarily confuse the classification and mapping of EOF with the
     classification and mapping of some non-EOF inputs.

NetBSD 11.99                  September 14, 2025                  NetBSD 11.99