Embedding Python in C, linking fails with undefined reference to `Py_Initialize'

Question

I am trying to compile the example from the docs https://docs.python.org/2.7/extending/embedding.html and my code looks exactly like the one under 5.1:

#include <Python.h>

int
main(int argc, char *argv[])
{
  Py_SetProgramName(argv[0]);
  Py_Initialize();
  PyRun_SimpleString("from time import time, ctime\n"
                     "print 'Today is', ctime(time())\n");

  Py_Finalize();
  return 0;
}

I use the following command to compile it which works fine for me and gives me the desired object file:

gcc -c $(python2.7-config --cflags) embedpy.c

To link it I use the following command which ends up in the following error:

gcc $(/usr/bin/python2.7-config --ldflags) embedpy.o
embedpy.o: In function `main':
/home/miguellissimo/embedpy.c:6: undefined reference to `Py_SetProgramName'
/home/miguellissimo/embedpy.c:7: undefined reference to `Py_Initialize'
/home/miguellissimo/embedpy.c:8: undefined reference to `PyRun_SimpleStringFlags'
/home/miguellissimo/embedpy.c:11: undefined reference to `Py_Finalize'
collect2: error: ld returned 1 exit status

I can't find out what I am doing wrong or what I forget to get the example working.

PS: The python2.7-config command gives the following output on my Xubuntu machine:

>>> python2.7-config --cflags 
-I/usr/include/python2.7 -I/usr/include/x86_64-linux-gnu/python2.7  -fno-stri
ct-aliasing -D_FORTIFY_SOURCE=2 -g -fstack-protector --param=ssp-buffer-size=
4 -Wformat -Werror=format-security  -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-pr
ototypes

>>> python2.7-config --ldflags
-L/usr/lib/python2.7/config-x86_64-linux-gnu -L/usr/lib -lpthread -ldl  -luti
l -lm  -lpython2.7 -Xlinker -export-dynamic -Wl,-O1 -Wl,-Bsymbolic-functions 

Answer 1

Libraries have to come after the object files when you are linking, so do:

gcc  embedpy.o $(/usr/bin/python2.7-config --ldflags)

Answer 2

Also add --embed to python3-config

On Ubuntu 20.04, Python 3.8, I also needed to pass --embed to python3-config as in:

gcc -std=c99 -ggdb3 -O0 -pedantic-errors -Wall -Wextra \
  -fpie $(python3-config --cflags --embed) -o 'eval.out' \
  'eval.c' $(python3-config --embed --ldflags)

otherwise -lpython3.8 is not added which leads to missing definitions.

This is my test program:

eval.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

int main(int argc, char *argv[]) {
    (void)argc;
    wchar_t *program = Py_DecodeLocale(argv[0], NULL);
    if (program == NULL) {
        fprintf(stderr, "Fatal error: cannot decode argv[0]\n");
        exit(1);
    }
    Py_SetProgramName(program);
    Py_Initialize();
    PyRun_SimpleString(argv[1]);
    if (Py_FinalizeEx() < 0) {
        exit(120);
    }
    PyMem_RawFree(program);
    return 0;
}

test run:

./eval.out 'print(2 ** 3)'

Answer 3

Got the same error on WSL, Ubuntu 18.04, Python3.8, g++ 7.5. Thanks to Ciro Santilli comment, adding --embed option to python3.8-config solved unresolved symbols problem, but after that I got the following error:

g++ `python3.8-config --cflags --embed` -o cpython.out cpython.cpp `python3.8-config --ldflags --embed`
lto1: fatal error: bytecode stream in file ‘/home/rpovelik/installed/miniconda3/envs/cython/lib/python3.8/config-3.8-x86_64-linux-gnu/libpython3.8.a’ generated with LTO version 6.0 instead of the expected 6.2
compilation terminated.
lto-wrapper: fatal error: g++ returned 1 exit status
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status

I suppose some people are going to face the same issue so I inspected the similar symptoms here where conda people said something like "don't use default compilers - g++ - because it could cause compatibility issues - use conda specific compilers".

By the way, adding -fno-lto solved my problem with system-wide g++ 7.5. Possibly you can try to change version of the compiler.