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strptime's %z specifier parses a string consisting of a sign ('+'
or '-'), two hours digits, and optionally two minutes digits, into a
tm.tm_gmtoff field containing the signed number of seconds the time
zone is offset from UTC time.
The time/tst-strptime2.c program passes a short list of strings through
strptime, validating that either the gmtoff value returned matches an
expected value, or that strptime returns an expected NULL for invalid
strings (for example, when the minutes portion of the string is outside
of the range 00 to 59, or the sign is missing before the hours digits).
In review of strptime fixes, Carlos O'Donell expressed a wish that
the test function iterate through the entire range of all possible
numeric strings (-9999 to +9999) which could be passed to strptime %z,
and validate the correct response.
Specifically, the test will look for a NULL response from strptime
when:
* sign ('+' or '-') is not present before the first digit (invalid
format).
* A sign and no digits are found (invalid format).
* A sign and one digit are found (invalid format).
* A sign and three digits are found (invalid format).
* A sign and four digits (-9999 to +9999) are found but the last
two digits (minutes) are in the range 60 to 99.
The test will look for a success response from strptime with
tm.tm_gmtoff matching the calculated tm_gmtoff value when:
* A sign and four digits are found (-9999 to +9999), and the last
two digits (minutes) are in the range 00 to 59.
* A sign and two digit strings are found (-99 to +99).
The test's iteration over the possible digit values results in 22223
test strings prepared, tested, and passed by strptime.
The test supports a --verbose command line option which will show
the test results of every test input, and a final summary of all
tests. Here is some sample output:
PASS: input "1113472456 1030", expected: invalid, return value NULL
PASS: input "1113472456 +", expected: invalid, return value NULL
PASS: input "1113472456 -", expected: invalid, return value NULL
PASS: input "1113472456 +0", expected: invalid, return value NULL
PASS: input "1113472456 -0", expected: invalid, return value NULL
PASS: input "1113472456 +1", expected: invalid, return value NULL
...
PASS: input "1113472456 +9", expected: invalid, return value NULL
PASS: input "1113472456 -9", expected: invalid, return value NULL
PASS: input "1113472456 +00", expected: valid, tm.tm_gmtoff 0
PASS: input "1113472456 -00", expected: valid, tm.tm_gmtoff 0
PASS: input "1113472456 +01", expected: valid, tm.tm_gmtoff 3600
PASS: input "1113472456 -01", expected: valid, tm.tm_gmtoff -3600
PASS: input "1113472456 +02", expected: valid, tm.tm_gmtoff 7200
...
PASS: input "1113472456 +99", expected: valid, tm.tm_gmtoff 356400
PASS: input "1113472456 -99", expected: valid, tm.tm_gmtoff -356400
PASS: input "1113472456 +000", expected: invalid, return value NULL
PASS: input "1113472456 -000", expected: invalid, return value NULL
PASS: input "1113472456 +001", expected: invalid, return value NULL
...
PASS: input "1113472456 +999", expected: invalid, return value NULL
PASS: input "1113472456 -999", expected: invalid, return value NULL
PASS: input "1113472456 +0000", expected: valid, tm.tm_gmtoff 0
PASS: input "1113472456 -0000", expected: valid, tm.tm_gmtoff 0
PASS: input "1113472456 +0001", expected: valid, tm.tm_gmtoff 60
PASS: input "1113472456 -0001", expected: valid, tm.tm_gmtoff -60
...
PASS: input "1113472456 +0059", expected: valid, tm.tm_gmtoff 3540
PASS: input "1113472456 -0059", expected: valid, tm.tm_gmtoff -3540
PASS: input "1113472456 +0060", expected: invalid, return value NULL
PASS: input "1113472456 -0060", expected: invalid, return value NULL
...
PASS: input "1113472456 +0099", expected: invalid, return value NULL
PASS: input "1113472456 -0099", expected: invalid, return value NULL
PASS: input "1113472456 +0100", expected: valid, tm.tm_gmtoff 3600
PASS: input "1113472456 -0100", expected: valid, tm.tm_gmtoff -3600
PASS: input "1113472456 +0101", expected: valid, tm.tm_gmtoff 3660
...
PASS: input "1113472456 +9999", expected: invalid, return value NULL
PASS: input "1113472456 -9999", expected: invalid, return value NULL
PASS: 22223 input strings: 0 fail, 22223 pass
Any failing test will result in printing the failed line to stdout, and
will trigger the printing of the summary line at the of all tests. For
example:
FAIL: input "1113472456 1030", expected: invalid, return value NULL,
got: valid, tm.tm_gmtoff 37800
FAIL: 22223 input strings: 1 fail, 22222 pass
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Topic: strptime supports a %z input field descriptor, which parses a
time zone offset from UTC time into the broken-out time field tm_gmtoff.
Problems:
1) In the current implementation, the minutes portion calculation is
correct only for minutes evenly divisible by 3. This is because the
minutes value is converted to decimal time, but inadequate precision
leads to rounding which calculates results that are too low for
some values.
For example, due to rounding, a +1159 offset string results in an
incorrect tm_gmtoff of 43128 (== 11 * 3600 + 58.8 * 60) seconds,
instead of 43140 (== 11 * 3600 + 59 * 60) seconds. In contrast,
a +1157 offset (minutes divisible by 3) does not cause the bug,
and results in a correct tm_gmtoff of 43020.
2) strptime's %z specifier will not parse time offsets less than
-1200 or greater than +1200, or if only hour digits are present, less
than -12 or greater than +12. It will return NULL for offsets outside
that range. These limits do not meet historical and modern use cases:
* Present day exceeds the +1200 limit:
- Pacific/Auckland (New Zealand) summer time is +1300.
- Pacific/Kiritimati (Christmas Island) is +1400.
- Pacific/Apia (Samoa) summer time is +1400.
* Historical offsets exceeded +1500/-1500.
* POSIX supports -2459 to +2559.
* Offsets up to +/-9959 may occasionally be useful.
* Paul Eggert's notes provide additional detail:
- https://sourceware.org/ml/libc-alpha/2014-12/msg00068.html
- https://sourceware.org/ml/libc-alpha/2014-12/msg00072.html
3) tst-strptime2, part of the 'make check' test suite, does not test
for the above problems.
Corrective actions:
1) In time/strptime_l.c, calculate the offset from the hour and
minute portions directly, without the rounding errors introduced by
decimal time.
2) Remove the +/-1200 range limit, permitting strptime to parse offsets
from -9959 through +9959.
3) Add zone offset values to time/tst-strptime2.c.
* Test minutes evenly divisible by three (+1157) and not evenly
divisible by three (+1158 and +1159).
* Test offsets near the old and new range limits (-1201, -1330, -2459,
-2500, -99, -9959, +1201, +1330, +1400, +1401, +2559, +2600, +99,
and +9959)
The revised strptime passes all old and new tst-strptime2 tests.
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