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[PATCH] timespec: use cast and not conditional, as truncation isn't poss
[PATCH] timespec: use cast and not conditional, as truncation isn't possible
Mon, 26 Jul 2010 10:59:33 -0700
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* lib/timespec.h (timespec_cmp): Use cast to pacify gcc -Wconversion
instead of a conditional. Comment about the situation in more detail.
This undoes most of the 2009-10-29 patch.
ChangeLog | 7 +++++++
lib/timespec.h | 32 ++++++++++++++++++++++++++++----
2 files changed, 35 insertions(+), 4 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 8fb3e77..897387c 100644
@@ -1,3 +1,10 @@
+2010-07-26 Paul R. Eggert <address@hidden>
+ timespec: use cast and not conditional, as truncation isn't possible
+ * lib/timespec.h (timespec_cmp): Use cast to pacify gcc -Wconversion
+ instead of a conditional. Comment about the situation in more detail.
+ This undoes most of the 2009-10-29 patch.
2010-07-18 Bruno Haible <address@hidden>
unistr/u8-strchr: Optimize non-ASCII argument case.
diff --git a/lib/timespec.h b/lib/timespec.h
index 81b3423..175de7f 100644
@@ -22,15 +22,39 @@
# include <time.h>
/* Return negative, zero, positive if A < B, A == B, A > B, respectively.
- Assume the nanosecond components are in range, or close to it. */
+ For each time stamp T, this code assumes that either:
+ * T.tv_nsec is in the range 0..999999999; or
+ * T.tv_sec corresponds to a valid leap second on a host that supports
+ leap seconds, and T.tv_nsec is in the range 1000000000..1999999999; or
+ * T.tv_sec is the minimum time_t value and T.tv_nsec is -1; or
+ T.tv_sec is the maximum time_t value and T.tv_nsec is 2000000000.
+ This allows for special struct timespec values that are less or
+ greater than all possible valid time stamps.
+ In all these cases, it is safe to subtract two tv_nsec values and
+ convert the result to integer without worrying about overflow on
+ any platform of interest to the GNU project, since all such
+ platforms have 32-bit int or wider.
+ Replacing "(int) (a.tv_nsec - b.tv_nsec)" with something like
+ "a.tv_nsec < b.tv_nsec ? -1 : a.tv_nsec > b.tv_nsec" would cause
+ this function to work in some cases where the above assumption is
+ violated, but not in all cases (e.g., a.tv_sec==1, a.tv_nsec==-2,
+ b.tv_sec==0, b.tv_nsec==999999999) and is arguably not worth the
+ extra instructions. Using a subtraction has the advantage of
+ detecting some invalid cases on platforms that detect integer
+ The (int) cast avoids a gcc -Wconversion warning. */
static inline int
timespec_cmp (struct timespec a, struct timespec b)
return (a.tv_sec < b.tv_sec ? -1
: a.tv_sec > b.tv_sec ? 1
- : a.tv_nsec < b.tv_nsec ? -1
- : a.tv_nsec > b.tv_nsec ? 1
- : 0);
+ : (int) (a.tv_nsec - b.tv_nsec));
void gettime (struct timespec *);
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