This is a simple reproducer to show different setuptools behavior when doing a single threaded build versus when doing a parallel build.

Non-parallel build:

1. ./setup.py build -j1

2. objdump -d build/lib*/mod1*.so | grep -A5 myfunction 0000000000001135 : 1135: 55 push %rbp 1136: 48 89 e5 mov %rsp,%rbp 1139: b8 0c 00 00 00 mov $0xc,%eax 113e: 5d pop %rbp 113f: c3 ret

3. objdump -d build/lib*/mod2*.so | grep -A5 myfunction 0000000000001135 : 1135: 55 push %rbp 1136: 48 89 e5 mov %rsp,%rbp 1139: b8 2a 00 00 00 mov $0x2a,%eax 113e: 5d pop %rbp 113f: c3 ret

As expected, myfunction() returns value 12 for mod1 and value 42 for mod2.

Now, do a parallel build and check again.

1. ./setup.py build -j2

2. objdump -d build/lib*/mod1*.so | grep -A5 myfunction 0000000000001135 : 1135: 55 push %rbp 1136: 48 89 e5 mov %rsp,%rbp 1139: b8 2a 00 00 00 mov $0x2a,%eax 113e: 5d pop %rbp 113f: c3 ret

3. objdump -d build/lib*/mod2*.so | grep -A5 myfunction 0000000000001135 : 1135: 55 push %rbp 1136: 48 89 e5 mov %rsp,%rbp 1139: b8 2a 00 00 00 mov $0x2a,%eax 113e: 5d pop %rbp 113f: c3 ret

In both cases value 42 (0x2a) is returned.