curl_easy_recv(3) Library Functions Manual curl_easy_recv(3)

curl_easy_recv - receives raw data on an "easy" connection

#include <curl/curl.h>
CURLcode curl_easy_recv(CURL *curl, void *buffer, size_t buflen, size_t *n);

This function receives raw data from the established connection. You may use it together with curl_easy_send(3) to implement custom protocols using libcurl. This functionality can be particularly useful if you use proxies and/or SSL encryption: libcurl takes care of proxy negotiation and connection setup.

buffer is a pointer to your buffer memory that gets populated by the received data. buflen is the maximum amount of data you can get in that buffer. The variable n points to receives the number of received bytes.

To establish the connection, set CURLOPT_CONNECT_ONLY(3) option before calling curl_easy_perform(3) or curl_multi_perform(3). Note that curl_easy_recv(3) does not work on connections that were created without this option.

The call returns CURLE_AGAIN if there is no data to read - the socket is used in non-blocking mode internally. When CURLE_AGAIN is returned, use your operating system facilities like select(2) to wait for data. The socket may be obtained using curl_easy_getinfo(3) with CURLINFO_ACTIVESOCKET(3).

Wait on the socket only if curl_easy_recv(3) returns CURLE_AGAIN. The reason for this is libcurl or the SSL library may internally cache some data, therefore you should call curl_easy_recv(3) until all data is read which would include any cached data.

Furthermore if you wait on the socket and it tells you there is data to read, curl_easy_recv(3) may return CURLE_AGAIN if the only data that was read was for internal SSL processing, and no other data is available.


int main(void)
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    curl_easy_setopt(curl, CURLOPT_URL, "");
    /* Do not do the transfer - only connect to host */
    curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 1L);
    res = curl_easy_perform(curl);
    if(res == CURLE_OK) {
      char buf[256];
      size_t nread;
      long sockfd;
      /* Extract the socket from the curl handle - we need it for waiting. */
      res = curl_easy_getinfo(curl, CURLINFO_ACTIVESOCKET, &sockfd);
      /* read data */
      res = curl_easy_recv(curl, buf, sizeof(buf), &nread);

Added in 7.18.2.

On success, returns CURLE_OK, stores the received data into buffer, and the number of bytes it actually read into *n.

On failure, returns the appropriate error code.

The function may return CURLE_AGAIN. In this case, use your operating system facilities to wait until data can be read, and retry.

Reading exactly 0 bytes indicates a closed connection.

If there is no socket available to use from the previous transfer, this function returns CURLE_UNSUPPORTED_PROTOCOL.

curl_easy_getinfo(3), curl_easy_perform(3), curl_easy_send(3), curl_easy_setopt(3)

2024-05-22 libcurl