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CURLMOPT_PUSHFUNCTION(3)            libcurl           CURLMOPT_PUSHFUNCTION(3)

NAME
       CURLMOPT_PUSHFUNCTION - callback that approves or denies server pushes

SYNOPSIS
       #include <curl/curl.h>

       char *curl_pushheader_bynum(struct curl_pushheaders *h, size_t num);
       char *curl_pushheader_byname(struct curl_pushheaders *h, const char *name);

       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 will be 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 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 will be provided in
       the header callback as usual.

       clientp is the pointer set with CURLMOPT_PUSHDATA(3)

       If the callback returns CURL_PUSH_OK, the 'easy' handle will be 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. The normal response
       headers will be passed to the header callback for pushed streams just
       as for normal streams.

       curl_pushheader_bynum
              Returns the header at index num (or NULL). The returned pointer
              points to a "name:value" string that will be freed when this
              callback returns.

       curl_pushheader_byname
              Returns the value for the given header name (or NULL). This is a
              shortcut so that the application does not have to loop through
              all headers to find the one it is interested in. The data
              pointed will be freed when this callback returns. If more than
              one header field use the same name, this returns only the first
              one.

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 for this will reach
              the application, the easy handle will be destroyed by libcurl.

       CURL_PUSH_ERROROUT (2)
              Returning this will reject the pushed stream and return 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
       HTTP(S) (HTTP/2 only)

EXAMPLE
       /* only allow pushes for file names 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;
       }

       curl_multi_setopt(multi, CURLMOPT_PUSHFUNCTION, push_callback);
       curl_multi_setopt(multi, CURLMOPT_PUSHDATA, &counter);

AVAILABILITY
       Added in 7.44.0

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

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

ibcurl 8.1.2                    April 26, 2023        CURLMOPT_PUSHFUNCTION(3)