c - what does "representable" mean in C11? -

according c11 wg14 draft version n1570:

the header <ctype.h> declares several functions useful classifying , mapping characters. in cases argument int, value of shall representable unsigned char or shall equal value of macro eof. if argument has other value, behavior undefined.

is undefined behaviour?:

#include <ctype.h> #include <limits.h> #include <stdlib.h>  int main(void) {   char c = char_min; /* let assume char signed , char_min < 0 */   return isspace(c) ? exit_failure : exit_success; } 

does standard allow pass char isspace() (char int)? in other words, char after conversion int representable unsigned char?

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here's how wiktionary defines "representable":

capable of being represented.

is char capable of being represented unsigned char? yes. §6.2.6.1/4:

values stored in non-bit-field objects of other object type consist of n × char_bit bits, n size of object of type, in bytes. value may copied object of type unsigned char [n] (e.g., memcpy); resulting set of bytes called object representation of value.

sizeof(char) == 1 therefore object representation unsigned char[1] i.e., char capable of being represented unsigned char. wrong?

concrete example, can represent [-2, -1, 0, 1] [0, 1, 2, 3]. if can't why?

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related: according §6.3.1.3 isspace((unsigned char)c) portable if int_max >= uchar_max otherwise implementation-defined.

under assumption char signed undefined behavior, otherwise defined since char_min have value 0. easier see intention , meaning of:

the value of shall representable unsigned char or shall equal value of macro eof

if read section 7.4 character handling <ctype.h> rationale international standard—programming languages—c says (emphasis mine going forward):

since these functions used macros, their domain restricted small positive integers representable in unsigned char, plus value of eof. eof traditionally -1, may negative integer, , hence distinguishable valid character code. these macros may efficiently implemented using argument index small array of attributes.

so valid values are:

1. positive integers can fit unsigned char
2. eof implementation defined negative number

even though c99 rationale since particular wording referring not change c99 c11 , rationale still fits.

we can find why interface uses int argument opposed char, section 7.1.4 use of library functions, says:

all library prototypes specified in terms of “widened” types argument formerly declared char written int. ensures library functions can called or without prototype in scope, maintaining backwards compatibility pre-c89 code. note, however, since functions printf , scanf use variable-length argument lists, must called in scope of prototype.