#include <string.h>

void * __attribute__((const)) memchr(const void * mem, int c, size_t num){
	const char * p = mem;
	while(num--){
		if(* p == (char) c) return (void *) p;
		p++;
	}
	return NULL;
}

WARNING :
string/memchr.c: In function 'memchr':
string/memchr.c:18:30: warning: cast discards 'const' qualifier from pointer target type [-Wcast-qual]
   if(* p == (char) c) return (void *) p;