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Use gmtime_r() and localtime_r() instead of gmtime() and localtime(), for thread-safety. There are a few affected calls in libpq and ecpg's libpgtypes, which are probably effectively bugs, because those libraries already claim to be thread-safe. There is one affected call in the backend. Most of the backend otherwise uses the custom functions pg_gmtime() and pg_localtime(), which are implemented differently. While we're here, change the call in the backend to gmtime*() instead of localtime*(), since for that use time zone behavior is irrelevant, and this side-steps any questions about when time zones are initialized by localtime_r() vs localtime(). Portability: gmtime_r() and localtime_r() are in POSIX but are not available on Windows. Windows has functions gmtime_s() and localtime_s() that can fulfill the same purpose, so we add some small wrappers around them. (Note that these *_s() functions are also different from the *_s() functions in the bounds-checking extension of C11. We are not using those here.) On MinGW, you can get the POSIX-style *_r() functions by defining _POSIX_C_SOURCE appropriately before including <time.h>. This leads to a conflict at least in plpython because apparently _POSIX_C_SOURCE gets defined in some header there, and then our replacement definitions conflict with the system definitions. To avoid that sort of thing, we now always define _POSIX_C_SOURCE on MinGW and use the POSIX-style functions here. Reviewed-by: Stepan Neretin <sncfmgg@gmail.com> Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi> Reviewed-by: Thomas Munro <thomas.munro@gmail.com> Discussion: https://www.postgresql.org/message-id/flat/eba1dc75-298e-4c46-8869-48ba8aad7d70@eisentraut.org