include/ruby/internal/intern/select.h: add doxygen

Must not be a bad idea to improve documents. [ci skip]

include/ruby/internal/intern/select.h

@@ -35,9 +35,11 @@
 # include "ruby/internal/intern/select/largesize.h"
 #elif defined(_WIN32)
 # include "ruby/internal/intern/select/win32.h"
+# /** Does nothing (defined for compatibility). */
 # define rb_fd_resize(n, f) ((void)(f))
 #else
 # include "ruby/internal/intern/select/posix.h"
+# /** Does nothing (defined for compatibility). */
 # define rb_fd_resize(n, f) ((void)(f))
 #endif
 
@@ -45,7 +47,39 @@ RBIMPL_SYMBOL_EXPORT_BEGIN()
 
 struct timeval;
 
-int rb_thread_fd_select(int, rb_fdset_t *, rb_fdset_t *, rb_fdset_t *, struct timeval *);
+/**
+ * Waits for multiple file descriptors at once.  This is basically a wrapper of
+ * system-provided select() with releasing GVL, to allow other Ruby threads run
+ * in parallel.
+ *
+ * @param[in]      nfds       Max FD in everything passed, plus one.
+ * @param[in,out]  rfds       Set of FDs to wait for reads.
+ * @param[in,out]  wfds       Set of FDs to wait for writes.
+ * @param[in,out]  efds       Set of FDs to wait for OOBs.
+ * @param[in,out]  timeout    Max blocking duration.
+ * @retval         -1         Failed, errno set.
+ * @retval          0         Timeout exceeded.
+ * @retval         otherwise  Total number of file descriptors returned.
+ * @post           `rfds` contains readable FDs.
+ * @post           `wfds` contains writable FDs.
+ * @post           `efds` contains exceptional FDs.
+ * @post           `timeout` is the time left.
+ * @note           All pointers are allowed to be null pointers.
+ *
+ * Although backend  threads can run in  parallel of this function,  touching a
+ * file descriptor  from multiple threads  could be problematic.   For instance
+ * what happens  when a  thread closes  a file descriptor  that is  selected by
+ * someone else, vastly varies among operating systems.  You would better avoid
+ * touching an fd from more than one threads.
+ *
+ * @internal
+ *
+ * Although  any file  descriptors are  possible here,  it makes  completely no
+ * sense to pass  a descriptor that is  not `O_NONBLOCK`.  If you  want to know
+ * the reason for  this limitatuon in detail, you might  find this thread super
+ * interesting: https://lkml.org/lkml/2004/10/6/117
+ */
+int rb_thread_fd_select(int nfds, rb_fdset_t *rfds, rb_fdset_t *wfds, rb_fdset_t *efds, struct timeval *timeout);
 
 RBIMPL_SYMBOL_EXPORT_END()