libcurl有两个接口

easy接口 同步、高效 前缀curl_easy。

multi接口 异步  前缀curl_multi。multi使用单线程

easy使用范例

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <curl/curl.h>

//回调

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

int main(void)
{
  CURL *curl_handle;
  static const char *headerfilename = "head.out";
  FILE *headerfile;
  static const char *bodyfilename = "body.out";
  FILE *bodyfile;

  curl_global_init(CURL_GLOBAL_ALL);

  /* init the curl session */
  curl_handle = curl_easy_init();

  /* set URL to get */
  curl_easy_setopt(curl_handle, CURLOPT_URL, "http://example.com");

  /* no progress meter please */
  curl_easy_setopt(curl_handle, CURLOPT_NOPROGRESS, 1L);

  /* send all data to this function  */
  curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, write_data);

  /* open the files */
  headerfile = fopen(headerfilename,"wb");
  if (headerfile == NULL) {
    curl_easy_cleanup(curl_handle);
    return -1;
  }
  bodyfile = fopen(bodyfilename,"wb");
  if (bodyfile == NULL) {
    curl_easy_cleanup(curl_handle);
    return -1;
  }

  /* we want the headers be written to this file handle */
  curl_easy_setopt(curl_handle,   CURLOPT_WRITEHEADER, headerfile);

  /* we want the body be written to this file handle instead of stdout */
  curl_easy_setopt(curl_handle,   CURLOPT_WRITEDATA, bodyfile);

  /* get it! */
  curl_easy_perform(curl_handle);

  /* close the header file */
  fclose(headerfile);

  /* close the body file */
  fclose(bodyfile);

  /* cleanup curl stuff */
  curl_easy_cleanup(curl_handle);

  return 0;
}

multi使用范例

#include <stdio.h>
#include <string.h>
#include <sys/time.h>

#include <curl/curl.h>

int main(void)
{
  CURL *curl;

  CURLM *multi_handle;
  int still_running;

  struct curl_httppost *formpost=NULL;
  struct curl_httppost *lastptr=NULL;
  struct curl_slist *headerlist=NULL;
  static const char buf[] = "Expect:";

  /* Fill in the file upload field. This makes libcurl load data from
     the given file name when curl_easy_perform() is called. */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "sendfile",
               CURLFORM_FILE, "postit2.c",
               CURLFORM_END);

  /* Fill in the filename field */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "filename",
               CURLFORM_COPYCONTENTS, "postit2.c",
               CURLFORM_END);

  /* Fill in the submit field too, even if this is rarely needed */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "submit",
               CURLFORM_COPYCONTENTS, "send",
               CURLFORM_END);

  curl = curl_easy_init();
  multi_handle = curl_multi_init();

  /* initalize custom header list (stating that Expect: 100-continue is not
     wanted */
  headerlist = curl_slist_append(headerlist, buf);
  if(curl && multi_handle) {

    /* what URL that receives this POST */
    curl_easy_setopt(curl, CURLOPT_URL, "http://www.example.com/upload.cgi");
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headerlist);
    curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);

    curl_multi_add_handle(multi_handle, curl);

    curl_multi_perform(multi_handle, &still_running);

    do {
      struct timeval timeout;
      int rc; /* select() return code */

      fd_set fdread;
      fd_set fdwrite;
      fd_set fdexcep;
      int maxfd = -1;

      long curl_timeo = -1;

      FD_ZERO(&fdread);
      FD_ZERO(&fdwrite);
      FD_ZERO(&fdexcep);

      /* set a suitable timeout to play around with */
      timeout.tv_sec = 1;
      timeout.tv_usec = 0;

      curl_multi_timeout(multi_handle, &curl_timeo);
      if(curl_timeo >= 0) {
        timeout.tv_sec = curl_timeo / 1000;
        if(timeout.tv_sec > 1)
          timeout.tv_sec = 1;
        else
          timeout.tv_usec = (curl_timeo % 1000) * 1000;
      }

      /* get file descriptors from the transfers */
      curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);

      /* In a real-world program you OF COURSE check the return code of the
         function calls.  On success, the value of maxfd is guaranteed to be
         greater or equal than -1.  We call select(maxfd + 1, ...), specially in
         case of (maxfd == -1), we call select(0, ...), which is basically equal
         to sleep. */

      rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);

      switch(rc) {
      case -1:
        /* select error */
        break;
      case 0:
      default:
        /* timeout or readable/writable sockets */
        printf("perform!\n");
        curl_multi_perform(multi_handle, &still_running);
        printf("running: %d!\n", still_running);
        break;
      }
    } while(still_running);

    curl_multi_cleanup(multi_handle);

    /* always cleanup */
    curl_easy_cleanup(curl);

    /* then cleanup the formpost chain */
    curl_formfree(formpost);

    /* free slist */
    curl_slist_free_all (headerlist);
  }
  return 0;
}