lib: add curl_multi_waitfds

New function call, similar to curl_multi_fdset()

Closes #13135

docs/libcurl/Makefile.inc

@@ -87,6 +87,7 @@ man_MANS = \
  curl_multi_timeout.3 \
  curl_multi_wakeup.3 \
  curl_multi_wait.3 \
+ curl_multi_waitfds.3 \
  curl_pushheader_bynum.3 \
  curl_pushheader_byname.3 \
  curl_share_cleanup.3 \

docs/libcurl/curl_multi_fdset.md

@@ -10,6 +10,7 @@ See-also:
   - curl_multi_perform (3)
   - curl_multi_timeout (3)
   - curl_multi_wait (3)
+  - curl_multi_waitfds (3)
   - select (2)
 Protocol:
   - All

docs/libcurl/curl_multi_waitfds.md

@@ -0,0 +1,107 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: curl_multi_waitfds
+Section: 3
+Source: libcurl
+See-also:
+  - curl_multi_perform (3)
+  - curl_multi_poll (3)
+  - curl_multi_wait (3)
+  - curl_multi_fdset (3)
+---
+
+# NAME
+
+curl_multi_waitfds - extracts file descriptor information from a multi handle
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+#include <stdlib.h>
+
+CURLMcode curl_multi_waitfds(CURLM *multi,
+                             struct curl_waitfd *ufds,
+                             unsigned int size,
+                             unsigned int *fd_count);
+~~~
+
+# DESCRIPTION
+
+This function extracts *curl_waitfd* structures which are similar to
+*poll(2)*'s *pollfd* structure from a given multi_handle.
+
+These structures can be used for polling on multi_handle file descriptors in a
+fashion similar to curl_multi_poll(3). The curl_multi_perform(3)
+function should be called as soon as one of them is ready to be read from or
+written to.
+
+libcurl fills provided *ufds* array up to the *size*.
+If a number of descriptors used by the multi_handle is greater than the
+*size* parameter then libcurl returns CURLM_OUT_OF_MEMORY error.
+
+If the *fd_count* argument is not a null pointer, it points to a variable
+that on returns specifies the number of descriptors used by the multi_handle to
+be checked for being ready to read or write.
+
+The client code can pass *size* equal to zero just to get the number of the
+descriptors and allocate appropriate storage for them to be used in a
+subsequent function call.
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURLMcode mc;
+  struct curl_waitfd *ufds;
+
+  CURLM *multi = curl_multi_init();
+
+  do {
+    /* call curl_multi_perform() */
+
+    /* get the count of file descriptors from the transfers */
+    unsigned int fd_count = 0;
+
+    mc = curl_multi_waitfds(multi, NULL, 0, &fd_count);
+
+    if(mc != CURLM_OK) {
+      fprintf(stderr, "curl_multi_waitfds() failed, code %d.\n", mc);
+      break;
+    }
+
+    if(!fd_count)
+      continue; /* no descriptors yet */
+
+    /* Allocate storage for our descriptors.
+    * Note that a better approach can be used to minimize allocations and
+    * deallocations, if needed, like pre-allocated or grow-only array.
+    */
+    ufds = (struct curl_waitfd*)malloc(fd_count * sizeof(struct curl_waitfd));
+
+    /* get wait descriptors from the transfers and put them into array. */
+    mc = curl_multi_waitfds(multi, ufds, fd_count, NULL);
+
+    if(mc != CURLM_OK) {
+      fprintf(stderr, "curl_multi_waitfds() failed, code %d.\n", mc);
+      free(ufds);
+      break;
+    }
+
+    /* Do polling on descriptors in ufds */
+
+    free(ufds);
+  } while (!mc);
+}
+~~~
+
+# AVAILABILITY
+
+Added in 8.8.0
+
+# RETURN VALUE
+
+**CURLMcode** type, general libcurl multi interface error code. See
+libcurl-errors(3)