Calling C code from C++ with using extern "C"

Question

I have 3 files with me, one c++ file, main.cpp, one c file, test.c and one header file, test.h

I wanted to try and use C code into C++ file. For the same reason, I have declared an function in test.h and defined that in test.c and using that in main.cpp

main_temp.c is just for explanation.

test.h

void test(int);

test.c

#include <stdio.h> void test(int a) { printf("%d", a);

main_temp.cpp

#include "test.h" int main() { foo(5); }

Here, I understand why this would not work. C symbol would be simple 'foo' but since C++ does more things while creating symbols, it might be 'void@test(int)' and to solve this name mangling problem, I have to treat C++ symbol as a C symbol. So, I would use extern "C" and my main.cpp becomes as like:

main.cpp

extern "C" { #include "test.h" } int main() { foo(5); }

I could not understand as to why this would not work! I get :

main.cpp:(.text+0xa): undefined reference to `test`

Can somebody share the insights?

Did you link with test.o, and how was it compiled? Show your command line steps, or your Makefile, or CMakefile, or etc. if you're using a build system. — aschepler
– aschepler, Commented Apr 14, 2020 at 20:12
So test.c isn't even a part of that program, and the definition of test really is missing. — aschepler
– aschepler, Commented Apr 14, 2020 at 20:14
@HemantBhargava You shouldn't use the g++ frontend to compile C files. You need to compile test.c with gcc and main.cpp with g++ and then link them together. — Miles Budnek
– Miles Budnek, Commented Apr 14, 2020 at 20:20

bobah · Accepted Answer · 2020-04-14 20:36:33Z

2

I trust you compile or link them together? Else that would be the cause. On gcc it would be something like:

g++ -c -o main.o main.cpp gcc -c -o test.o test.c g++ -o a.out main.o test.o

Assuming you have no bugs with compiling/linking, compile both main.cpp and test.c into object files and run nm on both. It will show what symbol main.o wants and what symbol test.o exports. It should become clear then why linker cannot do its job.

edited Apr 14, 2020 at 20:36

answered Apr 14, 2020 at 20:11

bobah

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6 Comments

aschepler Over a year ago

Lowercase -c. (And you can omit the -o x.o if you don't want a different filename or different output directory; the default is to use the same filename replacing the extension with .o)

Hemant Bhargava Over a year ago

I understand. Isn't there any way rather compile only two? Why do we need to compile separately for C and C++ files? Why can not I just do: g++ main.cpp test.c?

Miles Budnek Over a year ago

g++ main.cpp test.c will compile test.c as C++, not C.

aschepler Over a year ago

g++ is the driver for the C++ compiler, but you can tell it to switch to a different language instead: g++ main.cpp -x c test.c. The -x flag means "files later on the command line use this specific language".

Eljay Over a year ago

@MilesBudnek • For C, you should use a C compiler. For C++, you should use a C++ compiler. If you were to have FORTRAN in the mix, the for FORTRAN you should use a FORTRAN compiler.

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