Tribblix: manual page: CURLMOPT

CURLMOPT_PUSHFUNCTION(3) Introduction to Library Functions

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 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.

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 "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);
}

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).