C中的String.indexOf函数

char *strstr(const char *haystack, const char *needle);

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

int main() {
    char haystack[] = "This is a sample string.";
    char needle[] = "sample";
    char *result;

    result = strstr(haystack, needle);
    if (result != NULL) {
        printf("Found '%s' in '%s'\n", needle, haystack);
        printf("'%s' starts at position %ld\n", needle, (unsigned long)(result - haystack));
    } else {
        printf("'%s' not found in '%s'\n", needle, haystack);
    }

    return 0;
}

Found 'sample' in 'This is a sample string.'
'sample' starts at position 10