curl_multi_fdset does not accept NULL sets

[sfan5]

I did this

	int max_fd;
	mres = curl_multi_fdset(m_multi, nullptr, nullptr, nullptr, &max_fd);

The context here is that I am attempting to emulate curl_multi_poll on libcurl older than 7.66.0.
The idea I had is:

• check if libcurl has any fds it waits for
• if yes, just call curl_multi_wait
• if no, sleep for the wanted timeout

This actually means reporting this bug is pointless since the fallback code I am writing can never make use of a fix. Maybe you still want to update the docs 🤷

I expected the following

No crash.

The documentation for curl_multi_fdset says (emphasis mine):

If the read_fd_set argument is not a null pointer, it points to an object of type fd_set that on returns specifies the file descriptors to be checked for being ready to read.
If the write_fd_set argument is not a null pointer, it points to an object of type fd_set that on return specifies the file descriptors to be checked for being ready to write.
If the exc_fd_set argument is not a null pointer, it points to an object of type fd_set that on return specifies the file descriptors to be checked for error conditions pending.

Checking the code reveals no attempt is made to check the sets against null:

curl/lib/multi.c

Lines 1178 to 1189 in d7b6ce6

	for(i = 0; i < ps.num; i++) {
	if(!FDSET_SOCK(ps.sockets[i]))
	/* pretend it doesn't exist */
	continue;
	if(ps.actions[i] & CURL_POLL_IN)
	FD_SET(ps.sockets[i], read_fd_set);
	if(ps.actions[i] & CURL_POLL_OUT)
	FD_SET(ps.sockets[i], write_fd_set);
	if((int)ps.sockets[i] > this_max_fd)
	this_max_fd = (int)ps.sockets[i];
	}
	}

As far as I know FD_SET(..., NULL) is not specified to be safe either.

curl/libcurl version

curl 8.5.0 (x86_64-pc-linux-gnu) libcurl/8.5.0 OpenSSL/3.2.0 zlib/1.3 brotli/1.1.0 zstd/1.5.5 libidn2/2.3.4 libpsl/0.21.2 (+libidn2/2.3.4) libssh2/1.11.0 nghttp2/1.58.0

operating system

Arch Linux x86_64

[bagder]

It seems it has not supported that since at least 2006 so, yes, let's fix the docs.