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Add inline functions to convert between sbintime_t

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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.)
This commit is contained in:
ian 2017-07-29 17:00:23 +00:00 committed by Sebastian Huber
parent 8f4149ea93
commit 3c3c17500c
1 changed files with 52 additions and 7 deletions
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59
newlib/libc/include/sys/time.h
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@ -138,7 +138,7 @@ bintime_shift(struct bintime *_bt, int _exp)
#define SBT_1M (SBT_1S * 60) #define SBT_1M (SBT_1S * 60)
#define SBT_1MS (SBT_1S / 1000) #define SBT_1MS (SBT_1S / 1000)
#define SBT_1US (SBT_1S / 1000000) #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 #define SBT_MAX 0x7fffffffffffffffLL
static __inline int static __inline int
@ -165,6 +165,53 @@ sbttobt(sbintime_t _sbt)
return (_bt); 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: * Background information:
* *
@ -220,7 +267,7 @@ sbttots(sbintime_t _sbt)
struct timespec _ts; struct timespec _ts;
_ts.tv_sec = _sbt >> 32; _ts.tv_sec = _sbt >> 32;
_ts.tv_nsec = ((uint64_t)1000000000 * (uint32_t)_sbt) >> 32; _ts.tv_nsec = sbttons((uint32_t)_sbt);
return (_ts); return (_ts);
} }
@ -228,8 +275,7 @@ static __inline sbintime_t
tstosbt(struct timespec _ts) tstosbt(struct timespec _ts)
{ {
return (((sbintime_t)_ts.tv_sec << 32) + return (((sbintime_t)_ts.tv_sec << 32) + nstosbt(_ts.tv_nsec));
(_ts.tv_nsec * (((uint64_t)1 << 63) / 500000000) >> 32));
} }
static __inline struct timeval static __inline struct timeval
@ -238,7 +284,7 @@ sbttotv(sbintime_t _sbt)
struct timeval _tv; struct timeval _tv;
_tv.tv_sec = _sbt >> 32; _tv.tv_sec = _sbt >> 32;
_tv.tv_usec = ((uint64_t)1000000 * (uint32_t)_sbt) >> 32; _tv.tv_usec = sbttous((uint32_t)_sbt);
return (_tv); return (_tv);
} }
@ -246,8 +292,7 @@ static __inline sbintime_t
tvtosbt(struct timeval _tv) tvtosbt(struct timeval _tv)
{ {
return (((sbintime_t)_tv.tv_sec << 32) + return (((sbintime_t)_tv.tv_sec << 32) + ustosbt(_tv.tv_usec));
(_tv.tv_usec * (((uint64_t)1 << 63) / 500000) >> 32));
} }
#endif /* __BSD_VISIBLE */ #endif /* __BSD_VISIBLE */