CODEWITHSUNDEEP
cchar
Amir Saniyan
Amir Saniyan

Reputation: 13779

ANSI C: Why character functions accept int argument instead of char argument?

Why character functions accept int argument instead of char argument?

<ctype.h>

int isalnum(int c); 
int isalpha(int c); 
int iscntrl(int c); 
int isdigit(int c); 
int isgraph(int c); 
int islower(int c); 
int isprint(int c); 
int ispunct(int c); 
int isspace(int c); 
int isupper(int c); 
int isxdigit(int c); 
int tolower(int c); 
int toupper(int c);

Upvotes: 12

Views: 2081

Answers (4)

Gargi Srinivas
Gargi Srinivas

Reputation: 939

Yes, it could be to accommodate EOF which is always a non-char value, though the exact value of EOF could vary with different systems but it'll never be same as any character code.

Upvotes: 0

paxdiablo
paxdiablo

Reputation: 882716

Characters and integers are rather tightly knit in C.

When you receive a character from an input stream, it must be able to represent every single character plus the end-of-file symbol.

That means a char type won't be big enough so they use a wider type.

The C99 rationale document states:

Since these functions are often used primarily as macros, their domain is restricted to the small positive integers representable in an unsigned char, plus the value of EOF. EOF is traditionally -1, but may be any negative integer, and hence distinguishable from any valid character code. These macros may thus be efficiently implemented by using the argument as an index into a small array of attributes.

The standard itself has this to say:

The header <ctype.h> declares several functions useful for classifying and mapping characters. In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

Upvotes: 13

supercat
supercat

Reputation: 81337

When C was first invented, there was no compile-time checking of function arguments. If one called foo(bar,boz), and bar and boz were of type int, the compiler would push two int values on the stack, call foo, and hope it was expecting to get two int values. Since integer types smaller than int are promoted to int when evaluating expressions, C functions which were written prior to the invention of prototypes could not pass any smaller integer type.

Upvotes: 5

Jerry Coffin
Jerry Coffin

Reputation: 490728

They have to accept EOF in addition to normal character values. They also predate the invention of function prototypes. At that time, there was no way to pass a char to a function -- it was always promoted to int first.

Upvotes: 3

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