strstr

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

int main() {
    const char *haystack = "Hello, world! This is a sample string.";
    const char *needle = "world";

    char *result = strstr(haystack, needle);

    if (result != NULL) {
        printf("'%s' found at index %ld\n", needle, result - haystack);
    } else {
        printf("'%s' not found in the string.\n", needle);
    }

    return 0;
}