Description of problem:

Fedora's libdate-tz.so library is built with USE_OS_TZDB=1 defined, so that must also be defined when users include it. That isn't obvious to users.

Version-Release number of selected component (if applicable):

date-devel-3.0.1^20210518git052eeba-3.fc36.x86_64
date-devel-3.0.1^20210518git052eeba-4.fc37.x86_64

How reproducible:

Always.

Steps to Reproduce:

Compile this as 'g++ -g leap.cc -ldate-tz' and run it:

#include <date/date.h>
#include <date/tz.h>
#include <iostream>
#include <chrono>

using namespace std::literals::chrono_literals;

int main()
{
    auto t = date::sys_days{date::July/1/2015} - 500ms;
    std::cout << t.time_since_epoch().count() << '\n';
    auto u = date::clock_cast<date::utc_clock>(t);
    std::cout << u.time_since_epoch().count() << '\n';
    auto [is_leap, elapsed] = date::is_leap_second(u);
    std::cout << is_leap << ' ' << elapsed.count() << '\n';
}




Actual results:

$ g++ -g leap.cc -ldate-tz 
$ ./a.out
1435708799500
Segmentation fault (core dumped)



Expected results:

With the macro defined the program runs correctly:

$ g++ -g leap.cc -ldate-tz -DUSE_OS_TZDB=1
$ ./a.out
1435708799500
1435708824500
0 25


Additional info:

The date.pc pkg-config file has the correct macro definitions:

Cflags: -I${includedir} -DUSE_OS_TZDB=1 -DONLY_C_LOCALE=0

And it looks like the cmake files do too.

But users are not required to use pkg-config or cmake, so the simplest way to use this library fails at runtime with a mysterious crash.

Could the tz.h file be patched during the RPM build to set it correctly:

#ifndef USE_OS_TZDB
# define USE_OS_TZDB 1
#endif

so that the header automatically matches how the fedora package was built, and can be used out of the box?

Another option would be to just use #warning to tell the user they should define USE_OS_TZDB appropriately, but I think it should Just Work automatically.

If that's not acceptable, then there should be clear documentation in an easy to find location saying that USE_OS_TZDB=1 must be defined before including date/tz.h

Maybe it could be added to the %description of the libdate-tz sub-package:

--- a/date.spec
+++ b/date.spec
@@ -58,6 +58,8 @@ an easy way to access all of the data in this database, using the types
 from "date.h" and <chrono>. The IANA database also includes data on leap
 seconds, and this library provides utilities to compute with that
 information as well.
+This library is compiled with -DUSE_OS_TZDB=1 so you must use the same
+macro definition before including date/tz.h in your own code.
 
 
 %package        devel