multi: make curl_multi_wait avoid malloc in the typical case

bagder · bagder · commit 5f1163517e15 · 2017-04-03T23:20:02.000+02:00
When only a few additional file descriptors are used, avoid the malloc. Closes #1377
diff --git a/lib/multi.c b/lib/multi.c
@@ -974,6 +974,8 @@ CURLMcode curl_multi_fdset(struct Curl_multi *multi,
   return CURLM_OK;
 }
 
+#define NUM_POLLS_ON_STACK 10
+
 CURLMcode curl_multi_wait(struct Curl_multi *multi,
                           struct curl_waitfd extra_fds[],
                           unsigned int extra_nfds,
@@ -987,8 +989,10 @@ CURLMcode curl_multi_wait(struct Curl_multi *multi,
   unsigned int nfds = 0;
   unsigned int curlfds;
   struct pollfd *ufds = NULL;
+  bool ufds_malloc = FALSE;
   long timeout_internal;
   int retcode = 0;
+  struct pollfd a_few_on_stack[NUM_POLLS_ON_STACK];
 
   if(!GOOD_MULTI_HANDLE(multi))
     return CURLM_BAD_HANDLE;
@@ -1028,9 +1032,14 @@ CURLMcode curl_multi_wait(struct Curl_multi *multi,
   nfds += extra_nfds; /* add the externally provided ones */
 
   if(nfds || extra_nfds) {
-    ufds = malloc(nfds * sizeof(struct pollfd));
-    if(!ufds)
-      return CURLM_OUT_OF_MEMORY;
+    if(nfds > NUM_POLLS_ON_STACK) {
+      ufds = malloc(nfds * sizeof(struct pollfd));
+      if(!ufds)
+        return CURLM_OUT_OF_MEMORY;
+      ufds_malloc = TRUE;
+    }
+    else
+      ufds = &a_few_on_stack[0];
   }
   nfds = 0;
 
@@ -1108,7 +1117,8 @@ CURLMcode curl_multi_wait(struct Curl_multi *multi,
     }
   }
 
-  free(ufds);
+  if(ufds_malloc)
+    free(ufds);
   if(ret)
     *ret = retcode;
   return CURLM_OK;

-Original file line number
+Diff line change
   return CURLM_OK;
+}
 +#define NUM_POLLS_ON_STACK 10
++
 CURLMcode curl_multi_wait(struct Curl_multi *multi,
                           struct curl_waitfd extra_fds[],
                           unsigned int extra_nfds,
   unsigned int nfds = 0;
   unsigned int curlfds;
   struct pollfd *ufds = NULL;
 +  bool ufds_malloc = FALSE;
   long timeout_internal;
   int retcode = 0;
 +  struct pollfd a_few_on_stack[NUM_POLLS_ON_STACK];Collapse commentComment on line R995
tbodt commented on Apr 23, 2017 
tbodt
on Apr 23, 2017
Contributor
alloca is also an option...
rumpelsepp replied on Apr 23, 2017 
rumpelsepp
on Apr 23, 2017
See BUGS section: http://man7.org/linux/man-pages/man3/alloca.3.html
zeusk replied on Apr 23, 2017 
zeusk
on Apr 23, 2017
but wouldn't this approach also be subject to the same limitation?
bagder replied on Apr 23, 2017 
bagder
on Apr 23, 2017
MemberAuthor
alloca is not portable enough so it would require a set of extra (and ugly) #ifdefs
tbodt replied on Apr 24, 2017 
tbodt
on Apr 24, 2017
Contributor
yeah, I was going to suggest VLAs but realized they aren't portable enough either
Code has comments. Press enter to view.
   if(!GOOD_MULTI_HANDLE(multi))
     return CURLM_BAD_HANDLE;
   nfds += extra_nfds; /* add the externally provided ones */
   if(nfds || extra_nfds) {
 -    ufds = malloc(nfds * sizeof(struct pollfd));
 -    if(!ufds)
 -      return CURLM_OUT_OF_MEMORY;
 +    if(nfds > NUM_POLLS_ON_STACK) {
 +      ufds = malloc(nfds * sizeof(struct pollfd));
 +      if(!ufds)
 +        return CURLM_OUT_OF_MEMORY;
 +      ufds_malloc = TRUE;
 +    }
 +    else
 +      ufds = &a_few_on_stack[0];
+  }
   nfds = 0;
+    }
+  }
 -  free(ufds);
 +  if(ufds_malloc)
 +    free(ufds);
   if(ret)
     *ret = retcode;
   return CURLM_OK;