[redirecting to the newlib mailing list]
This is long-standing code in newlib, not actually inside Cygwin.
On Nov 10 21:19, Bruno Haible via Cygwin wrote:
> 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.
Struct _reent is a kind of per-execution unit datastructure. This
could be independent code blocks in bare-metal code, or threads in
Cygwin et al. So the _reent struct also holds the seed state of the
rand and rand48 generators for a good reason.
But that's only half the picture, because newlib actually has two ways
of storing _reent data: Either in a pre-thread struct _reent, or (if
_REENT_THREAD_LOCAL is defined) as per-member thread_local storage.
Theoretically, this could be easily fixed:
- Either we maintain a global struct _reent which could be used
from rand().
- Or, in case of _REENT_THREAD_LOCAL, by making the rand48 data globally
available as static data, rather than as thread_local data.
The rand() function would still not use locking but AFAICS that's
not actually required by POSIX or ISO C.
However, this is something I don't want to decide single-handedly,
so I'm asking how to go forward on the newlib ML.
As far as Cygwin alone is concerned, a patch like this one would suffice:
diff --git a/newlib/libc/stdlib/rand.c b/newlib/libc/stdlib/rand.c
index ba9cc80f2b21..2b48e7a725b1 100644
--- a/newlib/libc/stdlib/rand.c
+++ b/newlib/libc/stdlib/rand.c
@@ -58,6 +58,12 @@ on two different systems.
#include <stdlib.h>
#include <reent.h>
+#ifdef __CYGWIN__
+#define _RAND_REENT _GLOBAL_REENT
+#else
+#define _RAND_REENT _REENT
+#endif
+
#ifdef _REENT_THREAD_LOCAL
_Thread_local unsigned long long _tls_rand_next = 1;
#endif
@@ -65,7 +71,7 @@ _Thread_local unsigned long long _tls_rand_next = 1;
void
srand (unsigned int seed)
{
- struct _reent *reent = _REENT;
+ struct _reent *reent = _RAND_REENT;
_REENT_CHECK_RAND48(reent);
_REENT_RAND_NEXT(reent) = seed;
@@ -74,7 +80,7 @@ srand (unsigned int seed)
int
rand (void)
{
- struct _reent *reent = _REENT;
+ struct _reent *reent = _RAND_REENT;
/* This multiplier was obtained from Knuth, D.E., "The Art of
Computer Programming," Vol 2, Seminumerical Algorithms, Third
>
> 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
>
> #include <assert.h>
> #include <pthread.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> static void *
> rand_invocator_thread (void *arg)
> {
> printf ("Value from separate thread: %d\n", rand ());
> return NULL;
> }
>
> int
> main ()
> {
> unsigned int seed = 19891109;
>
> srand (seed);
> printf ("Value from main thread: %d\n", rand ());
> srand (seed);
> pthread_t t;
> assert (pthread_create (&t, NULL, rand_invocator_thread, NULL) == 0);
> assert (pthread_join (t, NULL) == 0);
>
> return 0;
> }
> #include <assert.h>
> #include <threads.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> static int
> rand_invocator_thread (void *arg)
> {
> printf ("Value from separate thread: %d\n", rand ());
> return 0;
> }
>
> int
> main ()
> {
> unsigned int seed = 19891109;
>
> srand (seed);
> printf ("Value from main thread: %d\n", rand ());
> srand (seed);
> thrd_t t;
> assert (thrd_create (&t, rand_invocator_thread, NULL) == thrd_success);
> assert (thrd_join (t, NULL) == thrd_success);
>
> return 0;
> }
>
> --
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