[vlc-commits] [Git][videolan/vlc][master] compat: fix jrand48()

[Steve Lhomme (@robUx4)]

Steve Lhomme pushed to branch master at VideoLAN / VLC


Commits:
a632b0df by Romain Vimont at 2023-01-23T15:01:48+00:00
compat: fix jrand48()

According to the manpage, jrand48() must return a signed long integer
uniformly distributed over the interval [-2^31, 2^31).

The old implementation first computed iterate48(), which returns a
positive 48-bit integer (as 'uint64_t', then cast to 'int64_t'). Once
right-shifted by 16 bits, the result is guaranteed to be a 32-bit
positive integer as 'int64_t'.

It is then (implicitly) cast to 'long' to match the return type.

When the 32th bit is 0 (i.e. the value fits in 31-bit), then everything
is ok. However, when the 32nd bit is 1, then there are two cases (which
are both incorrect):
 - if the 'long' type is 32-bit on the platform, then conversion from
   'int64_t' to 'long' is undefined;
 - if the 'long' type is more than 32-bit, then the resulting value will
   be a positive integer in the interval [0, 2^32), whereas jrand48()
   must return a value in the interval [-2^31, 2^31).

- - - - -


1 changed file:

- compat/nrand48.c


Changes:

=====================================
compat/nrand48.c
=====================================
@@ -53,7 +53,12 @@ double erand48 (unsigned short subi[3])
 
 long jrand48 (unsigned short subi[3])
 {
-    return ((int64_t)iterate48 (subi)) >> 16;
+    union {
+        uint32_t u;
+        int32_t i;
+    } v;
+    v.u = iterate48(subi) >> 16;
+    return v.i;
 }
 
 long nrand48 (unsigned short subi[3])



View it on GitLab: https://code.videolan.org/videolan/vlc/-/commit/a632b0dfe2a99a46055caf6cc945f6dd366433f4

-- 
View it on GitLab: https://code.videolan.org/videolan/vlc/-/commit/a632b0dfe2a99a46055caf6cc945f6dd366433f4
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