path: root/src/prng
diff options
authorRich Felker <>2014-09-22 17:55:08 -0400
committerRich Felker <>2014-09-22 17:55:08 -0400
commit05cef96d9e63a00b319f88343cf9869c8e612843 (patch)
tree943e4bfddf430d917ab8742de9a291d787a9e704 /src/prng
parent3e936ce81bbbcc968f576aedbd5203621839f152 (diff)
fix incorrect sequence generation in *rand48 prng functions
patch by Jens Gustedt. this fixes a bug reported by Nadav Har'El. the underlying issue was that a left-shift by 16 bits after promotion of unsigned short to int caused integer overflow. while some compilers define this overflow case as "shifting into the sign bit", doing so doesn't help; the sign bit then gets extended through the upper bits in subsequent arithmetic as unsigned long long. this patch imposes a promotion to unsigned prior to the shift, so that the result is well-defined and matches the specified behavior.
Diffstat (limited to 'src/prng')
1 files changed, 2 insertions, 2 deletions
diff --git a/src/prng/__rand48_step.c b/src/prng/__rand48_step.c
index ccaffc37..961d30fc 100644
--- a/src/prng/__rand48_step.c
+++ b/src/prng/__rand48_step.c
@@ -3,8 +3,8 @@
uint64_t __rand48_step(unsigned short *xi, unsigned short *lc)
uint64_t a, x;
- x = xi[0] | xi[1]<<16 | xi[2]+0ULL<<32;
- a = lc[0] | lc[1]<<16 | lc[2]+0ULL<<32;
+ x = xi[0] | xi[1]+0U<<16 | xi[2]+0ULL<<32;
+ a = lc[0] | lc[1]+0U<<16 | lc[2]+0ULL<<32;
x = a*x + lc[3];
xi[0] = x;
xi[1] = x>>16;