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* [newlib-cygwin] Add inline functions to convert between sbintime_t
@ 2018-12-04 9:03 Sebastian Huber
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From: Sebastian Huber @ 2018-12-04 9:03 UTC (permalink / raw)
To: newlib-cvs
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=3c3c17500c568785bc19beeae137456f501228f6
commit 3c3c17500c568785bc19beeae137456f501228f6
Author: ian <ian@FreeBSD.org>
Date: Sat Jul 29 17:00:23 2017 +0000
Add inline functions to convert between sbintime_t
and decimal time units. Use them in some existing code that is
vulnerable to roundoff errors.
The existing constant SBT_1NS is a honeypot, luring unsuspecting folks into
writing code such as long_timeout_ns*SBT_1NS to generate the argument for a
sleep call. The actual value of 1ns in sbt units is ~4.3, leading to a
large roundoff error giving a shorter sleep than expected when multiplying
by the trucated value of 4 in SBT_1NS. (The evil honeypot aspect becomes
clear after you waste a whole day figuring out why your sleeps return early.)
Diff:
---
newlib/libc/include/sys/time.h | 59 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 52 insertions(+), 7 deletions(-)
diff --git a/newlib/libc/include/sys/time.h b/newlib/libc/include/sys/time.h
index 2cf543d..f263a53 100644
--- a/newlib/libc/include/sys/time.h
+++ b/newlib/libc/include/sys/time.h
@@ -138,7 +138,7 @@ bintime_shift(struct bintime *_bt, int _exp)
#define SBT_1M (SBT_1S * 60)
#define SBT_1MS (SBT_1S / 1000)
#define SBT_1US (SBT_1S / 1000000)
-#define SBT_1NS (SBT_1S / 1000000000)
+#define SBT_1NS (SBT_1S / 1000000000) /* beware rounding, see nstosbt() */
#define SBT_MAX 0x7fffffffffffffffLL
static __inline int
@@ -165,6 +165,53 @@ sbttobt(sbintime_t _sbt)
return (_bt);
}
+/*
+ * Decimal<->sbt conversions. Multiplying or dividing by SBT_1NS results in
+ * large roundoff errors which sbttons() and nstosbt() avoid. Millisecond and
+ * microsecond functions are also provided for completeness.
+ */
+static __inline int64_t
+sbttons(sbintime_t _sbt)
+{
+
+ return ((1000000000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+nstosbt(int64_t _ns)
+{
+
+ return ((_ns * (((uint64_t)1 << 63) / 500000000) >> 32));
+}
+
+static __inline int64_t
+sbttous(sbintime_t _sbt)
+{
+
+ return ((1000000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+ustosbt(int64_t _us)
+{
+
+ return ((_us * (((uint64_t)1 << 63) / 500000) >> 32));
+}
+
+static __inline int64_t
+sbttoms(sbintime_t _sbt)
+{
+
+ return ((1000 * _sbt) >> 32);
+}
+
+static __inline sbintime_t
+mstosbt(int64_t _ms)
+{
+
+ return ((_ms * (((uint64_t)1 << 63) / 500) >> 32));
+}
+
/*-
* Background information:
*
@@ -220,7 +267,7 @@ sbttots(sbintime_t _sbt)
struct timespec _ts;
_ts.tv_sec = _sbt >> 32;
- _ts.tv_nsec = ((uint64_t)1000000000 * (uint32_t)_sbt) >> 32;
+ _ts.tv_nsec = sbttons((uint32_t)_sbt);
return (_ts);
}
@@ -228,8 +275,7 @@ static __inline sbintime_t
tstosbt(struct timespec _ts)
{
- return (((sbintime_t)_ts.tv_sec << 32) +
- (_ts.tv_nsec * (((uint64_t)1 << 63) / 500000000) >> 32));
+ return (((sbintime_t)_ts.tv_sec << 32) + nstosbt(_ts.tv_nsec));
}
static __inline struct timeval
@@ -238,7 +284,7 @@ sbttotv(sbintime_t _sbt)
struct timeval _tv;
_tv.tv_sec = _sbt >> 32;
- _tv.tv_usec = ((uint64_t)1000000 * (uint32_t)_sbt) >> 32;
+ _tv.tv_usec = sbttous((uint32_t)_sbt);
return (_tv);
}
@@ -246,8 +292,7 @@ static __inline sbintime_t
tvtosbt(struct timeval _tv)
{
- return (((sbintime_t)_tv.tv_sec << 32) +
- (_tv.tv_usec * (((uint64_t)1 << 63) / 500000) >> 32));
+ return (((sbintime_t)_tv.tv_sec << 32) + ustosbt(_tv.tv_usec));
}
#endif /* __BSD_VISIBLE */
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