c++curl详解

一、c curl ca

在使用 c++curl 前，需要安装c curl的CA认证。CA认证可以增加通信的安全性，同时也是一种标准的质量评估，例如当前web server中使用https协议,而客户端需要提供对应的CA证书才能访问。

CURL *curl;
CURLcode res;
curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
  curl_easy_setopt(curl, CURLOPT_CAINFO, "./cacert.pem");
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  curl_easy_cleanup(curl);
}

二、c curl创建session

c++curl提供了一种直接创建curl session对象的方法。

#include 
int main(void)
{
  CURL *curl;
  CURLcode res;
  curl_global_init(CURL_GLOBAL_DEFAULT);
  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
    res = curl_easy_perform(curl);
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
    curl_easy_cleanup(curl);
  }
  curl_global_cleanup();
  return 0;
}

三、c curl 提速优化

使用c++curl进行访问的时候，可能会出现访问速度变慢的情况。在这种情况下，可以尝试使用提速优化的方法，例如使用DNS缓存。

CURL *curl;
CURLcode res;
struct curl_slist *host = NULL;
curl = curl_easy_init();
if(curl) {
  host = curl_slist_append(host, "Host: example.com");
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, host);
  curl_easy_setopt(curl, CURLOPT_RESOLVE, "example.com:80:127.0.0.1");
  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  curl_easy_cleanup(curl);
  curl_slist_free_all(host);
}

四、c curl下载附件

c++curl可以用来下载附件。下载时需要注意一些常见的问题，例如重定向、错误处理和断点续传。

CURL *curl;
CURLcode res;
curl_off_t offset = 0;
curl = curl_easy_init();
if(curl) {
  const char *url = "http://example.com/archive.zip";
  FILE *fp = fopen("all.zip", "wb");
  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fwrite);
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
  curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
  curl_easy_setopt(curl, CURLOPT_RESUME_FROM_LARGE, offset);
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  curl_easy_cleanup(curl);
  fclose(fp);
}

五、c curl代码案例

c++curl实现了大部分cURL命令行工具中的功能，因此可以轻松地使用c++curl访问外部资源。下面是一个最简单的cURL 使用 c++curl 访问 example.com的代码案例。

#include 
#include 
int main(void)
{
  CURL *curl;
  CURLcode res;
  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
    res = curl_easy_perform(curl);
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
    curl_easy_cleanup(curl);
  }
  return 0;
}

六、c curl发送json

c++curl可以用来访问RESTful api服务器，可以使用json来进行数据交换。在发送json数据时，需要使用POST请求，并将json数据放到POST的body中。

CURL *curl;
CURLcode res;
struct curl_slist *headers = NULL;
const char *json = "{\"key\":\"value\"}";
headers = curl_slist_append(headers, "Accept: application/json");
headers = curl_slist_append(headers, "Content-Type: application/json");
curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/api");
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, json);
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  curl_easy_cleanup(curl);
  curl_slist_free_all(headers);
}

七、c curl库

c++curl提供了多个库，可以用来进行对不同协议的支持，例如FTP、SMTP和POP3。在使用这些库时，需要相应地对curl_session进行设置。

#include 
#include 
int main(void)
{
  CURL *curl;
  CURLcode res;
  struct curl_easy *easy_handle;
  struct curl_slist *headers = NULL;
  const char data[] = "data to be sent";
  curl = curl_easy_init();
  if(curl) {
    headers = curl_slist_append(headers, "Host: example.com");
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
    curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
    res = curl_easy_perform(curl);
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
    curl_easy_cleanup(easy_handle);
  }
  return 0;
}

八、c curl 数据不全

在使用c++curl访问外部资源时，可能会遇到数据不完整的情况。这可能是因为网络传输速度过慢，服务器过忙，还可能是因为使用的协议版本不支持等原因。为了解决这个问题，可以尝试使用TIMEOUT设置读取超时。

CURL *curl;
CURLcode res;
curl_socket_t sockfd;
struct timeval timeout;
timeout.tv_sec = 5;
timeout.tv_usec = 0;
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_TIMEOUT_MS, 2000);
  res = curl_easy_perform(curl);
  if(res != CURLE_OK)
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  curl_easy_getinfo(curl, CURLINFO_ACTIVESOCKET, &sockfd);
  setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
  curl_easy_cleanup(curl);
}
curl_global_cleanup();

九、c curl上传图片

c++curl可以用来进行文件上传操作，支持多种文件格式，例如图片、视频和zip。在将文件上传到web服务器时，需要使用POST请求方式，并将上传文件放到POST的body中。

CURL *curl;
CURLcode res;
curl_mime *form = NULL;
curl_mimepart *field = NULL;
struct curl_slist *headers = NULL;
curl = curl_easy_init();
if(curl) {
  headers = curl_slist_append(headers, "Accept: application/json");
  headers = curl_slist_append(headers, "Content-Type: multipart/form-data");
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
  curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/upload");
  form = curl_mime_init(curl);
  field = curl_mime_addpart(form);
  curl_mime_name(field, "image");
  curl_mime_filedata(field, "/path/to/image.png");
  curl_mime_type(field, "image/png");
  curl_easy_setopt(curl, CURLOPT_MIMEPOST, form);
  res = curl_easy_perform(curl);
  if(res != CURLE_OK) {
    fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
  }
  curl_easy_cleanup(curl);
  curl_mime_free(form);
  curl_slist_free_all(headers);
}