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CURLMOPT_PUSHFUNCTION

Section: C Library Functions (3)
Updated: 202-1-15
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NAME

CURLMOPT_PUSHFUNCTION - callback that approves or denies server pushes

SYNOPSIS

#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);

DESCRIPTION

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.

CALLBACK DESCRIPTION

The callback gets its arguments like this:

parent is the handle of the stream on which this push arrives. The new handle has the following options duplicated from the parent: CURLOPT_WRITEFUNCTION(3), CURLOPT_WRITEDATA(3), CURLOPT_HEADERFUNCTION(3), CURLOPT_HEADERDATA(3), CURLOPT_DEBUGFUNCTION(3), CURLOPT_DEBUGDATA(3), CURLOPT_PROGRESSFUNCTION(3), CURLOPT_XFERINFOFUNCTION(3), CURLOPT_PROGRESSDATA(3) and CURLOPT_XFERINFODATA(3).

It is then up to the application to alter/set 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 like for normal streams.

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

CALLBACK RETURN VALUE

CURL_PUSH_OK (0): 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.
CURL_PUSH_DENY (1): The callback denies the stream and no data reaches the application, the easy handle is destroyed by libcurl.
CURL_PUSH_ERROROUT (2): 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.

DEFAULT

NULL, no callback

PROTOCOLS

This functionality affects http only

EXAMPLE

#include <string.h>

/* only allow pushes for filenames starting with "pus-" */
static 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, "/pus-", 6)) {
    fprintf(stderr, "The PATH is %sn", headp);

    /* save the push here */
    out = fopen("pushe-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);
}

AVAILABILITY

Added in curl 7.44.0

RETURN VALUE

curl_multi_setopt(3) returns a CURLMcode indicating success or error.

CURLM_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3).