docs/libcurl/opts/CURLMOPT_PUSHFUNCTION.3

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.TH CURLMOPT_PUSHFUNCTION 3 "November 04, 2020" "libcurl 7.78.0" "curl_multi_setopt options"

.SH NAME
CURLMOPT_PUSHFUNCTION \- callback that approves or denies server pushes
.SH SYNOPSIS
.nf
#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);
.fi
.SH 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.
.SH CALLBACK DESCRIPTION
The callback gets its arguments like this:

\fIparent\fP 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.

\fIeasy\fP is a newly created handle that represents this upcoming transfer.

\fInum_headers\fP is the number of name+value pairs that was received and can
be accessed

\fIheaders\fP 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.

\fIuserp\fP is the pointer set with \fICURLMOPT_PUSHDATA(3)\fP

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.
.IP 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.
.IP curl_pushheader_byname
Returns the value for the given header name (or NULL). This is a shortcut so
that the application doesn't 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.
.SH CALLBACK RETURN VALUE
.IP "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.
.IP "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.
.IP "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 curl 7.72.0)
.IP *
All other return codes are reserved for future use.
.SH DEFAULT
NULL, no callback
.SH PROTOCOLS
HTTP(S) (HTTP/2 only)
.SH EXAMPLE
.nf
/* 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);
.fi
.SH AVAILABILITY
Added in 7.44.0
.SH RETURN VALUE
Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.
.SH "SEE ALSO"
.BR CURLMOPT_PUSHDATA "(3), " CURLMOPT_PIPELINING "(3), " CURLOPT_PIPEWAIT "(3), "
.BR RFC 7540