C语言strstr函数源代码

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Here's the source code for the C language strstr() function:

char *strstr(const char *haystack, const char *needle) {
    char *p = (char *) haystack;
    char *q = (char *) needle;

    while (*p) {
        if (*p == *q) {
            // found the first character of the needle
            size_t len = strlen(q);
            if (strncmp(p, q, len) == 0) {
                // found the complete needle in haystack
                return p;
            }
        }

        // didn't find the complete needle yet, keep looking
        p++;
    }

    // couldn't find the needle in haystack
    return NULL;
}

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