libcurl库使用

libcurl介绍

libcurl是一个跨平台的网络协议库，支持http, https,ftp, gopher, telnet, dict, file, 和ldap 协议。libcurl同样支持HTTPS证书授权，HTTP POST,HTTP PUT, FTP 上传, HTTP基本表单上传，代理，cookies和用户认证。

基本编程框架

• 调用curl_global_init()初始化libcurl
• 调用curl_easy_init()函数获得curl的指针
• 调用curl_easy_setopt()设置传输选项
• 调用curl_easy_perform()函数完成传输任务
• 调用curl_easy_cleanup()释放内存

编程注意事项

• curl_global_init()：这个函数只能用一次。(其实在调用curl_global_cleanup函数后仍然可再用)。如果这个函数在curl_easy_init函数调用时还没调用，它讲由libcurl库自动调用，所以多线程下最好主动调用该函数以防止在线程中curl_easy_init时多次调用。虽然libcurl是线程安全的，但curl_global_init是不能保证线程安全的，所以不要在每个线程中都调用curl_global_init，应该将该函数的调用放在主线程中。
• curl_global_cleanup():在结束libcurl使用的时候，用来对curl_global_init做的工作清理。类似于close的函数。虽然libcurl是线程安全的，但curl_global_cleanup是不能保证线程安全的，所以不要在每个线程中都调用curl_global_init，应该将该函数的调用放在主线程中。
• curl_easy_init( )与curl_easy_cleanup():成对使用，用来初始化一个CURL的指针与释放CURL的指针。

常用的网站

该网站有libcurl的API的详细介绍，还有一些例子
The libcurl API

使用Postman生成代码

由于libcurl库的参数设置十分多，我们可以使Postman来生成代码。

• 配置请求的参数，点击code
  
• 左边选择语言，右边就能生成对应语言的代码。
  

例子

simple HTTP POST using the easy interface

#include 
#include 
 
int main(void)
{
  CURL *curl;
  CURLcode res;
 
  /* In windows, this will init the winsock stuff */
  curl_global_init(CURL_GLOBAL_ALL);
 
  /* get a curl handle */
  curl = curl_easy_init();
  if(curl) {
    /* First set the URL that is about to receive our POST. This URL can
       just as well be an https:// URL if that is what should receive the
       data. */
    curl_easy_setopt(curl, CURLOPT_URL, "http://postit.example.com/moo.cgi");
    /* Now specify the POST data */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "name=daniel&project=curl");
 
    /* Perform the request, res will get the return code */
    res = curl_easy_perform(curl);
    /* Check for errors */
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));
 
    /* always cleanup */
    curl_easy_cleanup(curl);
  }
  curl_global_cleanup();
  return 0;
}

HTTP PUT using CURLOPT_POSTFIELDS

#include 
#include 
#include 
#include 
 
static const char olivertwist[]=
  "Among other public buildings in a certain town, which for many reasons "
  "it will be prudent to refrain from mentioning, and to which I will assign "
  "no fictitious name, there is one anciently common to most towns, great or "
  "small: to wit, a workhouse; and in this workhouse was born; on a day and "
  "date which I need not trouble myself to repeat, inasmuch as it can be of "
  "no possible consequence to the reader, in this stage of the business at "
  "all events; the item of mortality whose name is prefixed";
 
/* ... to the head of this chapter. String cut off to stick within the C90
   509 byte limit. */
 
/*
 * This example shows an HTTP PUT operation that sends a fixed buffer with
 * CURLOPT_POSTFIELDS to the URL given as an argument.
 */
 
int main(int argc, char **argv)
{
  CURL *curl;
  CURLcode res;
  char *url;
 
  if(argc < 2)
    return 1;
 
  url = argv[1];
 
  /* In windows, this will init the winsock stuff */
  curl_global_init(CURL_GLOBAL_ALL);
 
  /* get a curl handle */
  curl = curl_easy_init();
  if(curl) {
    struct curl_slist *headers = NULL;
 
    /* default type with postfields is application/x-www-form-urlencoded,
       change it if you want */
    headers = curl_slist_append(headers, "Content-Type: literature/classic");
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
 
    /* pass on content in request body. When CURLOPT_POSTFIELDSIZE is not used,
       curl does strlen to get the size. */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, olivertwist);
 
    /* override the POST implied by CURLOPT_POSTFIELDS
     *
     * Warning: CURLOPT_CUSTOMREQUEST is problematic, especially if you want
     * to follow redirects. Be aware.
     */
    curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "PUT");
 
    /* specify target URL, and note that this URL should include a file
       name, not only a directory */
    curl_easy_setopt(curl, CURLOPT_URL, url);
 
    /* Now run off and do what you have been told! */
    res = curl_easy_perform(curl);
    /* Check for errors */
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));
 
    /* always cleanup */
    curl_easy_cleanup(curl);
 
    /* free headers */
    curl_slist_free_all(headers);
  }
 
  curl_global_cleanup();
  return 0;
}

文件上传

#include 
#include 
#include 

static size_t write_data(char *p, size_t size, size_t nmemb, void *userdata)
{
    char *buf = malloc(size * nmemb + 1);
    memcpy(buf, p, size * nmemb);
    buf[size * nmemb] = 0x00;
    log_debug(">>  %s", buf);
    free(buf);
    return size * nmemb;
}

/**
 * @brief 文件上传
 * @param path 文件路径
 * @param url url地址
 * @return 
 */
int http_file_upload(char *path, char *url)
{
    CURL *curl;
    CURLcode res;

    /* get a curl handle */
    curl = curl_easy_init();
    if (!curl) {
        return -1;
    }

    mgLog_debug("url: %s", url);
    curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);

	struct curl_slist *headers = NULL;
	headers = curl_slist_append(headers, "Content-Type: multipart/form-data");
	curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

    struct curl_httppost *post = NULL;
    struct curl_httppost *last = NULL;

    curl_formadd(&post, &last, CURLFORM_COPYNAME, "key", CURLFORM_COPYCONTENTS, "value", CURLFORM_END);
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "file", CURLFORM_FILE, path, CURLFORM_END);
    curl_easy_setopt(curl, CURLOPT_HTTPPOST, post);

    /*执行数据请求*/
    res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        log_debug("curl_easy_perform() failed: %s", curl_easy_strerror(res));
    }
    
    curl_easy_cleanup(curl);
    curl_slist_free_all(headers);
    curl_formfree(post);

    return 0;
}

void test(void)
{
	curl_global_init(CURL_GLOBAL_ALL);
	http_file_upload("/tmp/test.txt","http://test.com/upload");
	curl_global_cleanup();
}