使用libcurl为什么无法获取到数据？

Question

我的代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>
size_t memory_callback(void *data,size_t size,size_t nmemb,void *content){
    size_t realsize=size*nmemb;
    //char *content=(char *)user;
    content=realloc(content,realsize+1);
    if(content==NULL){
        fprintf(stderr,"not enouth memory");
        return -1;
    }
    memcpy(content,data,realsize);
    //printf("%s",content);//这里可以得到数据
    return  realsize;
}

size_t get_content(char *url,char *content){
    CURL *curl_handle;
    CURLcode res;
    curl_global_init(CURL_GLOBAL_ALL);
    curl_handle = curl_easy_init();
    curl_easy_setopt(curl_handle, CURLOPT_URL, url);
    curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2)");
    curl_easy_setopt(curl_handle, CURLOPT_VERBOSE, 0L);
    curl_easy_setopt(curl_handle, CURLOPT_NOPROGRESS, 1L);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, memory_callback);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)content);
    res = curl_easy_perform(curl_handle);
    curl_easy_cleanup(curl_handle);
    curl_global_cleanup();
    return res;
}


int main(int argc,char *argv[]){
    char *content=NULL;
    get_content("http://www.baidu.com",content);
    free(content);
    printf("%s",content);//这里的content难道不是memcpy之后的吗？  
    return 0;
}

在main中无法输出数据？这是什么原因 不是传递的是指针吗？ 当在函数中更改了指针 会反映到实际的变量中。

Answer 1

是指针的问题, 如果你想在函数中改变指针的值需要在传入指向指针的指针.

帮你改了下. 有点强迫症, 顺便改了改了你的code style.

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

size_t memory_callback(void *data, size_t size, size_t nmemb, void *content)
{
    size_t realsize = size * nmemb;
    char *p = *(char **)content;
    size_t len = p ? strlen(p) : 0;
    *(char **)content = realloc(p, len + realsize + 1);
    p = *(char **)content;

    if (NULL == p)
    {
        fprintf(stderr,"not enouth memory");
        return -1;
    }

    memcpy(p + len, data, realsize);
    p[len + realsize] = '\0';

    return  realsize;
}

size_t get_content(char *url, char **content)
{
    CURL *curl_handle;
    CURLcode res;
    curl_global_init(CURL_GLOBAL_ALL);
    curl_handle = curl_easy_init();
    curl_easy_setopt(curl_handle, CURLOPT_URL, url);
    curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2)");
    curl_easy_setopt(curl_handle, CURLOPT_VERBOSE, 0L);
    curl_easy_setopt(curl_handle, CURLOPT_NOPROGRESS, 1L);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, memory_callback);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)content);
    res = curl_easy_perform(curl_handle);
    curl_easy_cleanup(curl_handle);
    curl_global_cleanup();

    return res;
}

int main(int argc, char *argv[])
{
    char *content = NULL;

    get_content("http://www.baidu.com", &content);
    printf("%s\n", content);
    if (content) free(content);


    return 0;
}