The interface to getdate is the simplest possible for a function
to parse a string and return the value. string is the input
string and the result is returned in a statically-allocated variable.
The details about how the string is processed are hidden from the user.
In fact, they can be outside the control of the program. Which formats
are recognized is controlled by the file named by the environment
variable DATEMSK . This file should contain
lines of valid format strings which could be passed to strptime .
The getdate function reads these format strings one after the
other and tries to match the input string. The first line which
completely matches the input string is used.
Elements not initialized through the format string retain the values
present at the time of the getdate function call.
The formats recognized by getdate are the same as for
strptime . See above for an explanation. There are only a few
extensions to the strptime behavior:
- If the
%Z format is given the broken-down time is based on the
current time of the timezone matched, not of the current timezone of the
runtime environment.
Note: This is not implemented (currently). The problem is that
timezone names are not unique. If a fixed timezone is assumed for a
given string (say EST meaning US East Coast time), then uses for
countries other than the USA will fail. So far we have found no good
solution to this.
- If only the weekday is specified the selected day depends on the current
date. If the current weekday is greater or equal to the
tm_wday
value the current week's day is chosen, otherwise the day next week is chosen.
- A similar heuristic is used when only the month is given and not the
year. If the month is greater than or equal to the current month, then
the current year is used. Otherwise it wraps to next year. The first
day of the month is assumed if one is not explicitly specified.
- The current hour, minute, and second are used if the appropriate value is
not set through the format.
- If no date is given tomorrow's date is used if the time is
smaller than the current time. Otherwise today's date is taken.
It should be noted that the format in the template file need not only
contain format elements. The following is a list of possible format
strings (taken from the Unix standard):
%m
%A %B %d, %Y %H:%M:%S
%A
%B
%m/%d/%y %I %p
%d,%m,%Y %H:%M
at %A the %dst of %B in %Y
run job at %I %p,%B %dnd
%A den %d. %B %Y %H.%M Uhr
As you can see, the template list can contain very specific strings like
run job at %I %p,%B %dnd . Using the above list of templates and
assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the
following results for the given input.
Input | Match | Result
| Mon | %a | Mon Sep 22 12:19:47 EDT 1986
| Sun | %a | Sun Sep 28 12:19:47 EDT 1986
| Fri | %a | Fri Sep 26 12:19:47 EDT 1986
| September | %B | Mon Sep 1 12:19:47 EDT 1986
| January | %B | Thu Jan 1 12:19:47 EST 1987
| December | %B | Mon Dec 1 12:19:47 EST 1986
| Sep Mon | %b %a | Mon Sep 1 12:19:47 EDT 1986
| Jan Fri | %b %a | Fri Jan 2 12:19:47 EST 1987
| Dec Mon | %b %a | Mon Dec 1 12:19:47 EST 1986
| Jan Wed 1989 | %b %a %Y | Wed Jan 4 12:19:47 EST 1989
| Fri 9 | %a %H | Fri Sep 26 09:00:00 EDT 1986
| Feb 10:30 | %b %H:%S | Sun Feb 1 10:00:30 EST 1987
| 10:30 | %H:%M | Tue Sep 23 10:30:00 EDT 1986
| 13:30 | %H:%M | Mon Sep 22 13:30:00 EDT 1986
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The return value of the function is a pointer to a static variable of
type struct tm , or a null pointer if an error occurred. The
result is only valid until the next getdate call, making this
function unusable in multi-threaded applications.
The errno variable is not changed. Error conditions are
stored in the global variable getdate_err . See the
description above for a list of the possible error values.
Warning: The getdate function should never be
used in SUID-programs. The reason is obvious: using the
DATEMSK environment variable you can get the function to open
any arbitrary file and chances are high that with some bogus input
(such as a binary file) the program will crash.
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