libcurl curl_easy_perform返回OK，但未调用响应回调函数

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>

size_t write_callback(void *ptr, size_t size, size_t nmemb, void *userdata) {
    size_t realsize = size * nmemb;
    char **response = (char **)userdata;
    *response = realloc(*response, realsize + 1);
    if (*response == NULL) {
        fprintf(stderr, "Failed to allocate memory\n");
        return 0;
    }
    memcpy(*response, ptr, realsize);
    (*response)[realsize] = 0;
    return realsize;
}

int main(void) {
    CURL *curl;
    CURLcode res;
    char *response = NULL;

    curl_global_init(CURL_GLOBAL_DEFAULT);
    curl = curl_easy_init();
    if(curl) {
        curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response);

        res = curl_easy_perform(curl);
        if(res != CURLE_OK) {
            fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
        } else {
            printf("Response: %s\n", response);
        }

        curl_easy_cleanup(curl);
    }

    if (response) {
        free(response);
    }
    curl_global_cleanup();
    return 0;
}