Ubuntu Manpage: CURLMOPT_PUSHFUNCTION - callback that approves or denies server pushes

Provided by: libcurl4-doc_7.81.0-1ubuntu1.19_all

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 *userp);

       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
duphandle()d 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.

userp 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 *userp)
       {
         char *headp;
         int *transfers = (int *)userp;
         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.