Verbose comment

diff --git a/src/network/poll.c b/src/network/poll.c
index df92479e0334a283c6539a669372001a8780df0e..540bc1dac6dff11664c75bee5fef24ed4c44b1e4 100644 (file)
--- a/src/network/poll.c
+++ b/src/network/poll.c
@@ -38,7 +38,6 @@ int poll (struct pollfd *fds, unsigned nfds, int timeout)
     struct timeval tv = { 0, 0 };
     int val = -1;
 
-
     FD_ZERO (&rdset);
     FD_ZERO (&wrset);
     FD_ZERO (&exset);
@@ -48,8 +47,26 @@ int poll (struct pollfd *fds, unsigned nfds, int timeout)
         if (val < fd)
             val = fd;
 
-        /* I assume the OS has a solution select overflow if it does not have
-         * poll(). If it did not, we are screwed anyway. */
+        /* With POSIX, FD_SET & FD_ISSET are not defined if fd is negative or
+        * bigger or equal than FD_SETSIZE. That is one of the reasons why VLC
+        * uses poll() rather than select(). Most POSIX systems implement
+        * fd_set has a bit field with no sanity checks. This is especially bad
+        * on systems (such as BSD) that have no process open files limit by
+        * default, such that it is quite feasible to get fd >= FD_SETSIZE.
+        * The next instructions will result in a buffer overflow if run on
+        * a POSIX system, and the later FD_ISSET will do undefined memory
+        * access.
+        *
+        * With Winsock, fd_set is a table of integers. This is awfully slow.
+        * However, FD_SET and FD_ISSET silently and safely discard
+        * overflows. If it happens we will loose socket events. Note that
+        * most (if not all) Winsock SOCKET handles are actually bigger than
+        * FD_SETSIZE in terms of absolute value - they are not POSIX file
+        * descriptors. From Vista, there is a much nicer WSAPoll(), but Mingw
+        * is yet to support it.
+        *
+        * With BeOS, the situation is unknown (FIXME: document).
+        */
         if (fds[i].events & POLLIN)
             FD_SET (fd, &rdset);
         if (fds[i].events & POLLOUT)