g++ with python.h, how to compile

Question

I compile one test code with g++ without any issue.

#include "Python.h"  

int main(int argc, char** argv)  
{  
    Py_Initialize();  
    PyRun_SimpleString("import pylab");  
    PyRun_SimpleString("pylab.plot(range(5))");  
    PyRun_SimpleString("pylab.show()");  
    Py_Exit(0);  
} 

g++ -o test test.cpp -I/usr/include/python2.7/ -lpython2.7 works fine and runs.

But when I try to embed this code into another project, it fails. It really confuses me.

Makefile is like the following.

CXX=g++  
CXXFLAGS=-DIB_USE_STD_STRING -Wall -Wno-switch -g  
ROOT_DIR=..  
BASE_SRC_DIR=${ROOT_DIR}/PosixSocketClient  
INCLUDES=-I${ROOT_DIR}/Shared/ -I${BASE_SRC_DIR} -I/usr/include/python2.7  
LIBRARY=-L/usr/lib/python2.7/config  
TARGET=eu  

$(TARGET):  
    $(CXX) $(CXXFLAGS) $(INCLUDES) -o EClientSocketBase.o -c   $(BASE_SRC_DIR)/EClientSocketBase.cpp  
    $(CXX) $(CXXFLAGS) $(INCLUDES) -o EPosixClientSocket.o -c   $(BASE_SRC_DIR)/EPosixClientSocket.cpp  
    $(CXX) $(CXXFLAGS) $(INCLUDES) -o PosixTestClient.o -c PosixTestClient.cpp  
    $(CXX) $(CXXFLAGS) $(INCLUDES) -o Main.o -c Main.cpp
    $(CXX) $(LIBRARY) -lpython2.7 -o $@ EClientSocketBase.o EPosixClientSocket.o PosixTestClient.o Main.o 

clean:  
    rm -f $(TARGET) *.o  

This project compiles fine and runs, the only change I made was adding the test code in the Main.cpp file. warning/error message shows:

In file included from /usr/include/python2.7/Python.h:8:0,
from Main.cpp:15:
/usr/include/python2.7/pyconfig.h:1158:0: warning: "_POSIX_C_SOURCE" redefined [enabled by default]
/usr/include/features.h:163:0: note: this is the location of the previous definition
/usr/include/python2.7/pyconfig.h:1180:0: warning: "_XOPEN_SOURCE" redefined [enabled by default]
/usr/include/features.h:165:0: note: this is the location of the previous definition
g++ -L/usr/lib/ -lpython2.7 -ldl -lutil -o eu EClientSocketBase.o EPosixClientSocket.o PosixTestClient.o Main.o
Main.o: In function main':
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:81: undefined reference to main':
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:81: undefined reference to main':
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:81: undefined reference to Py_Initialize'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:82: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:83: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:83: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:83: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:84: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:85: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:85: undefined reference to PyRun_SimpleStringFlags'
/home/bbc/TWS/IBJts/cpp/eu-ats/Main.cpp:85: undefined reference to Py_Exit'
collect2: ld returned 1 exit status
make: * [eu] Error 1

any help? thank you!

Answer 1

Take a look at Lucas's comment for the answer:

"To get rid of the _POSIX_C_SOURCE warning, make sure to include Python.h before all other header files."

I had the same problem. I use Boost Python, so for me I moved the include of boost/python.hpp to the first line in my.cpp file.

(Lukas, post your comment as an answer so the person who asked can mark it as the right answer, and the question won't remain 'unanswered' in StackOverflow.)

Answer 2

This is a bug in Python: https://bugs.python.org/issue1045893

If you include first Python.h, the compiler won't complain but GNU libc's /usr/include/features.h will override it anyway when _GNU_SOURCE is defined:

# undef _POSIX_C_SOURCE # define _POSIX_C_SOURCE 200809L # undef _XOPEN_SOURCE # define _XOPEN_SOURCE 700