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deps/curl/docs/libcurl/opts/CURLMOPT_PUSHFUNCTION.md

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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLMOPT_PUSHFUNCTION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLMOPT_PIPELINING (3)
+  - CURLMOPT_PUSHDATA (3)
+  - CURLOPT_PIPEWAIT (3)
+  - RFC 7540
+---
+
+# NAME
+
+CURLMOPT_PUSHFUNCTION - callback that approves or denies server pushes
+
+# SYNOPSIS
+
+~~~c
+#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
+
+HTTP(S) (HTTP/2 only)
+
+# EXAMPLE
+
+~~~c
+#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 7.44.0
+
+# RETURN VALUE
+
+Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.