ISO C 23 § 7.24.2.1 and 7.24.2.2 document how rand() and srand() are
expected to behave. In particular:
  "The srand function uses the argument as a seed for a new sequence
   of pseudo-random numbers to be returned by subsequent calls to rand.
   If srand is then called with the same seed value, the sequence of
   pseudo-random numbers shall be repeated. ...
   The srand function is not required to avoid data races with other
   calls to pseudo-random sequence generation functions. ..."

The two attached programs call srand() in one thread and then rand()
in another thread. There is no data race, since the second thread
is only created after the call to srand() has returned. The behaviour
in Cygwin is that the values in the second thread ignore the srand()
call done in the first thread.

How to reproduce the bug:

$ x86_64-pc-cygwin-gcc -Wall rand-in-posix-thread.c
$ ./a

or

$ x86_64-pc-cygwin-gcc -Wall rand-in-isoc-thread.c
$ ./a

Expected output:

Value from main thread:     1583559764
Value from separate thread: 1583559764

Actual output:

Value from main thread:     1583559764
Value from separate thread: 1481765933