bpo-11410: Standardize and use symbol visibility attributes across POSIX and Windows. by vsajip · Pull Request #16347 · python/cpython

vsajip

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Because exporting symbols is orthogonal to other things. Consider graminit.c - it's a very low-level module which needs to export a symbol, but doesn't really use any other Python-specific or system-specific stuff. If you replace "exports.h" with "pyport.h" in graminit.c, you get an error when compiling:

gcc -pthread -c -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE -o Python/graminit.o Python/graminit.c
In file included from Python/graminit.c:3:0:
./Include/pyport.h:105:9: error: unknown type name ‘ssize_t’
 typedef ssize_t         Py_ssize_t;
         ^
./Include/pyport.h:117:9: error: unknown type name ‘size_t’
 typedef size_t Py_uhash_t;
         ^
Makefile:1711: recipe for target 'Python/graminit.o' failed

Including <stddef.h> before "pyport.h" doesn't fix things:

gcc -pthread -c -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include    -DPy_BUILD_CORE -o Python/graminit.o Python/graminit.c
In file included from Python/graminit.c:3:0:
./Include/pyport.h:105:9: error: unknown type name ‘ssize_t’
 typedef ssize_t         Py_ssize_t;
         ^
Makefile:1711: recipe for target 'Python/graminit.o' failed

So you end up having to include "Python.h" instead, which appears to be overkill as it exposes a lot of things to code in graminit.c which aren't needed by that code.