./configure --with-pydebug && make -j

-> % ./python -m unittest -v test.test_ctypes.test_libc.LibTest.test_csqrt 
test_csqrt (test.test_ctypes.test_libc.LibTest.test_csqrt) ... FAIL

======================================================================
FAIL: test_csqrt (test.test_ctypes.test_libc.LibTest.test_csqrt)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/mikhail.efimov/projects/cpython/Lib/test/test_ctypes/test_libc.py", line 30, in test_csqrt
    self.assertEqual(lib.my_csqrt(4), 2+0j)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: (5e-324+6.95324111713477e-310j) != (2+0j)

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

-> % cat /etc/issue
Debian GNU/Linux 10 \n \l

-> % gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 8.3.0-6' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-8 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 8.3.0 (Debian 8.3.0-6) 

-> % cat test_csqrt.c
#include <complex.h>
#include <stdio.h>

int complex my_csqrt(double complex a)
{
    double complex z1 = a;
    double complex z2 = csqrt(a);
    printf("my_csqrt (%.10f%+.10fi) = %.10f%+.10fi\n",
        creal(z1), cimag(z1), creal(z2), cimag(z2));
    return 0;
}

int main() {
    my_csqrt(4.0);
    my_csqrt(4.0+4.0j);
    my_csqrt(-1+0.01j);
    my_csqrt(-1-0.01j);
    return 0;
}

-> % gcc -lm test_csqrt.c -o test_csqrt && ./test_csqrt
my_csqrt (4.0000000000+0.0000000000i) = 2.0000000000+0.0000000000i
my_csqrt (4.0000000000+4.0000000000i) = 2.1973682269+0.9101797211i
my_csqrt (-1.0000000000+0.0100000000i) = 0.0049999375+1.0000124996i
my_csqrt (-1.0000000000-0.0100000000i) = 0.0049999375-1.0000124996i

-> % cat ctypes_fix/test.py      
import ctypes
libm = ctypes.CDLL('libm.so.6')
libm.clog.restype = ctypes.c_double_complex
libm.clog.argtypes = ctypes.c_double_complex,
clog_5 = libm.clog(5.0)
clog_1000_2j = libm.clog(1000.0+2j)
print(f"{clog_5=}")
print(f"{clog_1000_2j=}")

-> % ./python ctypes_fix/test.py
clog_5=(5e-324+6.9529453382261e-310j)
clog_1000_2j=(5e-324+6.9529453382261e-310j)

-> % cat ctypes_fix/test2.py   
import ctypes
libm = ctypes.CDLL('libm.so.6')
libm.clog.restype = ctypes.c_double
libm.clog.argtypes = ctypes.c_double,
clog_5 = libm.clog(5.0)
clog_1000 = libm.clog(1000.0)
print(f"{clog_5=}")
print(f"{clog_1000=}")

-> % ./python ctypes_fix/test2.py
clog_5=1.6094379124341003
clog_1000=6.907755278982137