CURLMOPT_PUSHFUNCTION(3) Library Functions Manual CURLMOPT_PUSHFUNCTION(3)

CURLMOPT_PUSHFUNCTION - callback that approves or denies server pushes

#include <curl/curl.h>
int curl_push_callback(CURL *parent,
                       CURL *easy,
                       size_t num_headers,
                       struct curl_pushheaders *headers,
                       void *clientp);
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_PUSHFUNCTION,
                            curl_push_callback func);

This callback gets called when a new HTTP/2 stream is being pushed by the server (using the PUSH_PROMISE frame). If no push callback is set, all offered pushes are denied automatically.

The callback gets its arguments like this:

parent is the handle of the stream on which this push arrives. The new handle has been duplicated from the parent, meaning that it has gotten all its options inherited. It is then up to the application to alter any options if desired.

easy is a newly created handle that represents this upcoming transfer.

num_headers is the number of name+value pairs that was received and can be accessed

headers is a handle used to access push headers using the accessor functions described below. This only accesses and provides the PUSH_PROMISE headers, the normal response headers are provided in the header callback as usual.

clientp is the pointer set with CURLMOPT_PUSHDATA(3)

If the callback returns CURL_PUSH_OK, the new easy handle is added to the multi handle, the callback must not do that by itself.

The callback can access PUSH_PROMISE headers with two accessor functions. These functions can only be used from within this callback and they can only access the PUSH_PROMISE headers: curl_pushheader_byname(3) and curl_pushheader_bynum(3). The normal response headers are passed to the header callback for pushed streams just as for normal streams.

The header fields can also be accessed with curl_easy_header(3), introduced in later libcurl versions.

The application has accepted the stream and it can now start receiving data, the ownership of the CURL handle has been taken over by the application.
The callback denies the stream and no data reaches the application, the easy handle is destroyed by libcurl.
Returning this code rejects the pushed stream and returns an error back on the parent stream making it get closed with an error. (Added in 7.72.0)
*
All other return codes are reserved for future use.

NULL, no callback

HTTP

#include <string.h>
/* only allow pushes for filenames starting with "push-" */
int push_callback(CURL *parent,
                  CURL *easy,
                  size_t num_headers,
                  struct curl_pushheaders *headers,
                  void *clientp)
{
  char *headp;
  int *transfers = (int *)clientp;
  FILE *out;
  headp = curl_pushheader_byname(headers, ":path");
  if(headp && !strncmp(headp, "/push-", 6)) {
    fprintf(stderr, "The PATH is %s\n", headp);
    /* save the push here */
    out = fopen("pushed-stream", "wb");
    /* write to this file */
    curl_easy_setopt(easy, CURLOPT_WRITEDATA, out);
    (*transfers)++; /* one more */
    return CURL_PUSH_OK;
  }
  return CURL_PUSH_DENY;
}
int main(void)
{
  int counter;
  CURLM *multi = curl_multi_init();
  curl_multi_setopt(multi, CURLMOPT_PUSHFUNCTION, push_callback);
  curl_multi_setopt(multi, CURLMOPT_PUSHDATA, &counter);
}

Added in 7.44.0

Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.

CURLMOPT_PIPELINING(3), CURLMOPT_PUSHDATA(3), CURLOPT_PIPEWAIT(3), RFC7540

2024-05-22 libcurl