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We undefined them to avoid warnings about macro redefinitions. But we haven't fully followed the necessary include order, since at least 147c2482542, in 2011. Recently the combination of the include order rules not being followed and undefining _POSIX_C_SOURCE started to cause a compile failure, starting with 03023a2664f. Undefining _POSIX_C_SOURCE hides clock_gettime(), which is referenced in an inline function as of 03023a2664f, whereas it was a macro before. After seeing some evidence that undefining _POSIX_C_SOURCE et al isn't required, I tried to build postgres with plpython on most of our supported platforms (except DragonFlyBSD and Illumos, but similar systems were tested), with/without the #undefines. No compiler warning / behavioral difference. The oldest supported python version, 3.2, defines _POSIX_C_SOURCE to 200112L ad _XOPEN_SOURCE to 600, whereas newer versions of python use 200809L/700 respectively. As _POSIX_C_SOURCE/_XOPEN_SOURCE will default to the newer operating system on most platforms, it's possible that when using python 3.2 new warnings would be emitted - but that seems acceptable. It's possible that this approach won't work on some older platforms. But getting rid of most of the include-order complexity seems promising, and it's an easily revertible patch if we end up having to go another way. Discussion: https://postgr.es/m/20230124165814.2njc7gnvubn2amh6@awork3.anarazel.de
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.