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fix overflow behavior of clock() function

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per Austin Group interpretation for issue #686, which cites the
requirements of ISO C, clock() cannot wrap. if the result is not
representable, it must return (clock_t)-1. in addition, the old code
was performing wrapping via signed overflow and thus invoking
undefined behavior.

since it seems impossible to accurately check for overflow with the
old times()-based fallback code, I have simply dropped the fallback
code for now, thus always returning -1 on ancient systems. if there's
a demand for making it work and somebody comes up with a way, it could
be reinstated, but the clock() function is essentially useless on
32-bit system anyway (it overflows in less than an hour).

it should be noted that I used LONG_MAX rather than ULONG_MAX, despite
32-bit archs using an unsigned type for clock_t. this discrepency with
the glibc/LSB type definitions will be fixed now; since wrapping of
clock_t is no longer supported, there's no use in it being unsigned.
This commit is contained in:
Rich Felker 2013-05-23 14:31:02 -04:00
parent 1e5eb73545
commit 05453b37fc
1 changed files with 10 additions and 7 deletions
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17
src/time/clock.c
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@ -1,15 +1,18 @@
#include <time.h>
#include <sys/times.h>
#include "syscall.h"
#include <limits.h>
int __clock_gettime(clockid_t, struct timespec *);
clock_t clock()
{
struct timespec ts;
struct tms tms;
if (!__clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts))
return ts.tv_sec*1000000 + ts.tv_nsec/1000;
__syscall(SYS_times, &tms);
return (tms.tms_utime + tms.tms_stime)*10000;
if (__clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts))
return -1;
if (ts.tv_sec > LONG_MAX/1000000
|| ts.tv_nsec/1000 > LONG_MAX-1000000*ts.tv_sec)
return -1;
return ts.tv_sec*1000000 + ts.tv_nsec/1000;
}