C: Differences between strchr() and index()

Question

I am doing something in C which requires use of the strings (as most programs do).

Looking in the manpages, I found, at string(3):

SYNOPSIS

#include <strings.h>

char * index(const char *s, int c)

(...)

#include <string.h>

char * strchr(const char *s, int c)

So I curiously looked at both strchr(3) and index(3)...

And I found that both do the following:

The strchr()/index() function locates the first occurrence of c in the string pointed to by s. The terminating null character is considered to be part of the string; therefore if c is '\0', the functions locate the terminating '\0'.

So, the manpage is basically a copy & paste.

Besides, I suppose that, because of some obfuscated necessity, the second parameter has type int, but is, in fact, a char. I think I am not wrong, but can anyone explain to me why is it an int, not a char?

If they are both the same, which one is more compatible across versions, and if not, which's the difference?

Answer 1

strchr() is part of the C standard library. index() is a now deprecated POSIX function. The POSIX specification recommends implementing index() as a macro that expands to a call to strchr().

Since index() is deprecated in POSIX and not part of the C standard library, you should use strchr().

The second parameter is of type int because these functions predate function prototypes. See also https://stackoverflow.com/a/5919802/ for more information on this.