GNU Info
(libc.info)General Time String Parsing
A More User-friendly Way to Parse Times and Dates
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The Unix standard defines another function for parsing date strings.
The interface is weird, but if the function happens to suit your
application it is just fine. It is problematic to use this function in
multi-threaded programs or libraries, since it returns a pointer to a
static variable, and uses a global variable and global state (an
environment variable).
- Variable: getdate_err
This variable of type `int' contains the error code of the last
unsuccessful call to `getdate'. Defined values are:
1
The environment variable `DATEMSK' is not defined or null.
2
The template file denoted by the `DATEMSK' environment
variable cannot be opened.
3
Information about the template file cannot retrieved.
4
The template file is not a regular file.
5
An I/O error occurred while reading the template file.
6
Not enough memory available to execute the function.
7
The template file contains no matching template.
8
The input date is invalid, but would match a template
otherwise. This includes dates like February 31st, and dates
which cannot be represented in a `time_t' variable.
- Function: struct tm * getdate (const char *STRING)
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
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.
- Function: int getdate_r (const char *STRING, struct tm *TP)
The `getdate_r' function is the reentrant counterpart of
`getdate'. It does not use the global variable `getdate_err' to
signal an error, but instead returns an error code. The same error
codes as described in the `getdate_err' documentation above are
used, with 0 meaning success.
Moreover, `getdate_r' stores the broken-down time in the variable
of type `struct tm' pointed to by the second argument, rather than
in a static variable.
This function is not defined in the Unix standard. Nevertheless
it is available on some other Unix systems as well.
The warning against using `getdate' in SUID-programs applies to
`getdate_r' as well.
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