libc/gen/ctype.3

.\"	$NetBSD: ctype.3,v 1.31 2019/01/15 07:01:01 wiz Exp $
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.\"     @(#)ctype.3	6.5 (Berkeley) 4/19/91
.\"
.Dd January 15, 2019
.Dt CTYPE 3
.Os
.Sh NAME
.Nm ctype
.Nd character classification and mapping functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In ctype.h
.Fn isalpha "int c"
.Fn isupper "int c"
.Fn islower "int c"
.Fn isdigit "int c"
.Fn isxdigit "int c"
.Fn isalnum "int c"
.Fn isspace "int c"
.Fn ispunct "int c"
.Fn isprint "int c"
.Fn isgraph "int c"
.Fn iscntrl "int c"
.Fn isblank "int c"
.Fn toupper "int c"
.Fn tolower "int c"
.Sh DESCRIPTION
The above functions perform character tests and conversions on the integer
.Ar c .
.Pp
See the specific manual pages for information about the
test or conversion performed by each function.
.Sh EXAMPLES
To print an upper-case version of a string to stdout,
the following code can be used:
.Bd -literal -offset indent
const char *s = "xyz";

while (*s != '\e0') {
    putchar(toupper((unsigned char)*s));
    s++;
}
.Ed
.Sh SEE ALSO
.Xr isalnum 3 ,
.Xr isalpha 3 ,
.Xr isblank 3 ,
.Xr iscntrl 3 ,
.Xr isdigit 3 ,
.Xr isgraph 3 ,
.Xr islower 3 ,
.Xr isprint 3 ,
.Xr ispunct 3 ,
.Xr isspace 3 ,
.Xr isupper 3 ,
.Xr isxdigit 3 ,
.Xr tolower 3 ,
.Xr toupper 3 ,
.Xr ascii 7
.Sh STANDARDS
These functions, with the exception of
.Fn isblank ,
conform to
.St -ansiC .
All described functions, including
.Fn isblank ,
also conform to
.St -p1003.1-2001 .
.Sh CAVEATS
The argument of these functions is of type
.Vt int ,
but only a very restricted subset of values are actually valid.
The argument must either be the value of the macro
.Dv EOF
(which has a negative value),
or must be a non-negative value within the range representable as
.Vt unsigned char .
Passing invalid values leads to undefined behavior.
.Pp
Values of type
.Vt int
that were returned by
.Xr getc 3 ,
.Xr fgetc 3 ,
and similar functions or macros
are already in the correct range, and may be safely passed to these
.Nm ctype
functions without any casts.
.Pp
Values of type
.Vt char
or
.Vt signed char
must first be cast to
.Vt unsigned char ,
to ensure that the values are within the correct range.
Casting a negative-valued
.Vt char
or
.Vt signed char
directly to
.Vt int
will produce a negative-valued
.Vt int ,
which will be outside the range of allowed values
(unless it happens to be equal to
.Dv EOF ,
but even that would not give the desired result).
.Pp
Because the bugs may manifest as silent misbehavior or as crashes only
when fed input outside the US-ASCII range, the
.Nx
implementation of the
.Nm
functions is designed to elicit a compiler warning for code that passes
inputs of type
.Vt char
in order to flag code that may pass negative values at runtime that
would lead to undefined behavior:
.Bd -literal -offset indent
#include <ctype.h>
#include <locale.h>
#include <stdio.h>

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

	if (argc < 2)
		return 1;
	setlocale(LC_ALL, "");
	printf("%d %d\en", *argv[1], isprint(*argv[1]));
	printf("%d %d\en", (int)(unsigned char)*argv[1],
	    isprint((unsigned char)*argv[1]));
	return 0;
}
.Ed
.Pp
When compiling this program, GCC reports a warning for the line that
passes
.Vt char .
At runtime, you may get nonsense answers for some inputs without the
cast \(em if you're lucky and it doesn't crash:
.Bd -literal -offset indent
% gcc -Wall -o test test.c
test.c: In function 'main':
test.c:12:2: warning: array subscript has type 'char'
% LC_CTYPE=C ./test $(printf '\e270')
-72 5
184 0
% LC_CTYPE=C ./test $(printf '\e377')
-1 0
255 0
% LC_CTYPE=fr_FR.ISO8859-1 ./test $(printf '\e377')
-1 0
255 2
.Ed
.Pp
Some implementations of libc, such as glibc as of 2018, attempt to
avoid the worst of the undefined behavior by defining the functions to
work for all integer inputs representable by either
.Vt unsigned char
or
.Vt char ,
and suppress the warning.
However, this is not an excuse for avoiding conversion to
.Vt unsigned char :
if
.Dv EOF
coincides with any such value, as it does when it is \-1 on platforms
with signed
.Vt char ,
programs that pass
.Vt char
will still necessarily confuse the classification and mapping of
.Dv EOF
with the classification and mapping of some non-EOF inputs.