How does strchr implementation work

I tried to write my own implementation of the strchr() method.

It now looks like this:

char *mystrchr(const char *s, int c) {
    while (*s != (char) c) {
        if (!*s++) {
            return NULL;
        }
    }
    return (char *)s;
}

The last line originally was

return s;

But this didn't work because s is const. I found out that there needs to be this cast (char *), but I honestly don't know what I am doing there :( Can someone explain?

I believe this is actually a flaw in the C Standard's definition of the strchr() function. (I'll be happy to be proven wrong.) (Replying to the comments, it's arguable whether it's really a flaw; IMHO it's still poor design. It can be used safely, but it's too easy to use it unsafely.)

Here's what the C standard says:

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

The strchr function locates the first occurrence of c (converted to a char) in the string pointed to by s. The terminating null character is considered to be part of the string.

Which means that this program:

#include <stdio.h>
#include <string.h>

int main(void) {
    const char *s = "hello";
    char *p = strchr(s, 'l');
    *p = 'L';
    return 0;
}

even though it carefully defines the pointer to the string literal as a pointer to const char, has undefined behavior, since it modifies the string literal. gcc, at least, doesn't warn about this, and the program dies with a segmentation fault.

The problem is that strchr() takes a const char* argument, which means it promises not to modify the data that s points to -- but it returns a plain char*, which permits the caller to modify the same data.

Here's another example; it doesn't have undefined behavior, but it quietly modifies a const qualified object without any casts (which, on further thought, I believe has undefined behavior):

#include <stdio.h>
#include <string.h>

int main(void) {
    const char s[] = "hello";
    char *p = strchr(s, 'l');
    *p = 'L';
    printf("s = \"%s\"\n", s);
    return 0;
}

Which means, I think, (to answer your question) that a C implementation of strchr() has to cast its result to convert it from const char* to char*, or do something equivalent.

This is why C++, in one of the few changes it makes to the C standard library, replaces strchr() with two overloaded functions of the same name:

const char * strchr ( const char * str, int character );
      char * strchr (       char * str, int character );

Of course C can't do this.

An alternative would have been to replace strchr by two functions, one taking a const char* and returning a const char*, and another taking a char* and returning a char*. Unlike in C++, the two functions would have to have different names, perhaps strchr and strcchr.

(Historically, const was added to C after strchr() had already been defined. This was probably the only way to keep strchr() without breaking existing code.)

strchr() is not the only C standard library function that has this problem. The list of affected function (I think this list is complete but I don't guarantee it) is:

void *memchr(const void *s, int c, size_t n);
char *strchr(const char *s, int c);
char *strpbrk(const char *s1, const char *s2);
char *strrchr(const char *s, int c);
char *strstr(const char *s1, const char *s2);

(all declared in <string.h>) and:

void *bsearch(const void *key, const void *base,
    size_t nmemb, size_t size,
    int (*compar)(const void *, const void *));

(declared in <stdlib.h>). All these functions take a pointer to const data that points to the initial element of an array, and return a non-const pointer to an element of that array.