usleep: Document Cygwin bug

[Bruno Haible]

In a CI run of libunistring (on GitHub), I see a test failure:

  ../../tests/test-usleep.c:35: assertion 'start < time (NULL)' failed
  FAIL test-usleep.exe (exit status: 134)

This is the failing code:

  time_t start = time (NULL);
  ASSERT (usleep (1000000) == 0);
  ASSERT (start < time (NULL));

It means that either usleep() has slept for less than 1 second, or that
time (NULL) returned the same value after 1 second or more had elapsed.

Since we have no better way to measure time than time() and gettimeofday(),
I'll blame usleep.


2024-06-07  Bruno Haible  <bruno@clisp.org>

        usleep: Document Cygwin bug.
        * doc/pastposix-functions/usleep.texi: Mention Cygwin 3.5.3 bug.

diff --git a/doc/pastposix-functions/usleep.texi 
b/doc/pastposix-functions/usleep.texi
index 5d26adbc0b..3de259c4cc 100644
--- a/doc/pastposix-functions/usleep.texi
+++ b/doc/pastposix-functions/usleep.texi
@@ -20,4 +20,7 @@
 According to POSIX, the @code{usleep} function may interfere with the program's
 use of the @code{SIGALRM} signal.  On Linux, it doesn't; on other platforms,
 it may.
+@item
+On some systems, @code{usleep} may sleep for less than the indicated duration:
+Cygwin 3.5.3.
 @end itemize