libcurl库的使用(通过libcurl库下载url图像)

1. 从http://curl.haxx.se/download.html下载libcurl源码，解压缩；
2. 通过CMake(cmake-gui)生成vs2013 x64位 CURL.sln；
3. 打开CURL.sln，编译会生成libcurl.dll动态库；
4. 在CURL.sln基础上，添加一个testlibcurl控制台工程；
5. testlibcurl.cpp：

#include "stdafx.h"
#include <iostream>
#include <curl/curl.h>

size_t callbackfunction(void *ptr, size_t size, size_t nmemb, void* userdata)
{
    FILE* stream = (FILE*)userdata;
    if (!stream) {
        printf("!!! No stream\n");
        return 0;
    }

    size_t written = fwrite((FILE*)ptr, size, nmemb, stream);
    return written;
}

bool download_jpeg(char* url)
{
    FILE* fp = fopen("out.jpg", "wb");
    if (!fp) {
        printf("!!! Failed to create file on the disk\n");
        return false;
    }

    CURL* curlCtx = curl_easy_init();
    curl_easy_setopt(curlCtx, CURLOPT_URL, url);
    curl_easy_setopt(curlCtx, CURLOPT_WRITEDATA, fp);
    curl_easy_setopt(curlCtx, CURLOPT_WRITEFUNCTION, callbackfunction);
    curl_easy_setopt(curlCtx, CURLOPT_FOLLOWLOCATION, 1);

    CURLcode rc = curl_easy_perform(curlCtx);
    if (rc) {
        printf("!!! Failed to download: %s\n", url);
        return false;
    }

    long res_code = 0;
    curl_easy_getinfo(curlCtx, CURLINFO_RESPONSE_CODE, &res_code);
    if (!((res_code == 200 || res_code == 201) && rc != CURLE_ABORTED_BY_CALLBACK)) {
        printf("!!! Response code: %d\n", res_code);
        return false;
    }

    curl_easy_cleanup(curlCtx);
    fclose(fp);

    return true;
}

size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream)
{
    size_t written = fwrite(ptr, size, nmemb, stream);
    return written;
}

bool download_jpeg2(char* url)
{
    CURL *curl;
    FILE *fp;
    CURLcode res;
    char* outfilename = "out2.jpg";

    curl = curl_easy_init();
    if (curl) {
        fp = fopen(outfilename, "wb");
        curl_easy_setopt(curl, CURLOPT_URL, url);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
        res = curl_easy_perform(curl);
        /* always cleanup */
        curl_easy_cleanup(curl);
        fclose(fp);
    }
    else {
        printf("!!!curl init failed\n");
        return false;
    }

    return true;
}

int main(int argc, char* argv[])
{
    char* url = "http://f.hiphotos.baidu.com/image/pic/item/d043ad4bd11373f0671f5d95a60f4bfbfbed0493.jpg";

#if 1
    if (!download_jpeg(url)) {
        printf("!! Failed to download file: %s\n", url);
        return -1;
    }
#else
    if (!download_jpeg2(url)) {
        printf("!! Failed to download file: %s\n", url);
        return -1;
    }
#endif

    std::cout << "ok!" << std::endl;
	return 0;
}

参考文献：

1. http://stackoverflow.com/questions/10112959/download-an-image-from-server-curl-however-taking-suggestions-c
2. http://stackoverflow.com/questions/1636333/download-file-using-libcurl-in-c-c
3. https://www.hackthissite.org/articles/read/1078
4. http://curl.askapache.com/c/example.html