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multi: fix curl_multi_waitfds reporting of fd_count

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- Make curl_multi_waitfds consistent with the documentation.

Issue Addressed:

 - The documentation of curl_multi_waitfds indicates that users should
   be able to call curl_multi_waitfds with a NULL ufds. However, before
   this change, the function would return CURLM_BAD_FUNCTION_ARGUMENT.
 - Additionally, the documentation suggests that users can use this
   function to determine the number of file descriptors (fds) needed.
   However, the function would stop counting fds if the supplied fds
   were exhausted.

Changes Made:

 - NULL ufds Handling: curl_multi_waitfds can now accept a NULL ufds if
   size is also zero.
 - Counting File Descriptors: If curl_multi_waitfds is passed a NULL
   ufds, or the size of ufds is insufficient, the output parameter
   fd_count will return the number of fds needed. This value may be
   higher than actually needed but never lower.

Testing:

 - Test 2405 has been updated to cover the usage scenarios described
   above.

Fixes https://github.com/curl/curl/issues/15146
Closes https://github.com/curl/curl/pull/15155
This commit is contained in:
Christopher Dannemiller 2024-10-04 09:31:59 -07:00 • committed by Jay Satiro
parent 7d6edf1d8d
commit c78044c07e
7 changed files with 120 additions and 43 deletions
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7
docs/libcurl/curl_multi_waitfds.md
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@ -45,12 +45,13 @@ 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
that on return 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.
subsequent function call. In this case, *fd_count* receives a number greater
than or equal to the number of descriptors.
# %PROTOCOLS%
@ -89,7 +90,7 @@ int main(void)
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);
mc = curl_multi_waitfds(multi, ufds, fd_count, &fd_count);
if(mc != CURLM_OK) {
fprintf(stderr, "curl_multi_waitfds() failed, code %d.\n", mc);