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I have a project that had c files and cpp files mixed in it. The linker throws errors at me:

undefined reference to <all my functions in the c files>

After I rename all *.c files to *.cpp the errors vanish. So can't I mix those files or is this some compiler/linker options problem?

My options:

GCC C++ Compiler:

-I/usr/include/glib-2.0 -I/usr/include/gtk-2.0 -I"/myPath" -O0 -g3 \ -Wall -c -fmessage-length=0 `pkg-config --cflags glib-2.0 gtk+-2.0

GCC C Compiler:

-I/usr/include/glib-2.0 -I/usr/include/gtk-2.0 -I"/myPath" -O0 -g3 \ -Wall -c -fmessage-length=0 -std=c99 `pkg-config --cflags glib-2.0 gtk+-2.0`

GCC C++ Linker:

-L/usr/include/glib-2.0/glib `pkg-config --libs gtk+-2.0`

Since it says C++ Linker, does that mean object files from c files cannot be linked into the project (by default)?

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2 Answers 2

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15

You need to wrap the C code like so

extern "C" { #include "sample1.h" }

A better way to do it is like in this StackOverflow question, in the C header files.

use:

#ifdef __cplusplus extern "C" { #endif

At the start of all C header files.

and use:

#ifdef __cplusplus } #endif

At the end of all C header files.

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7 Comments

Well, in most cases this will work, but it's a bit of a kludge. It's better to carefully place the extern "C"s inside the header, either creating a new header for C++ use, or detecting when you are being compiled as C++ as described in the question you linked.
@BoBTFish You and I have both linked to the same question. See the second link I posted.
I will rephrase and fix though, better make it clear.
Man, it's so hard to begin coding in C/C++ nowadays :-( Thank you, this solves a lot of problems.
@mini-me Are you implying that it was easier before?
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extern "C" is one approach. The other approach, assuming your C files are of the (almost ubiquitous) sort which are already compilable as C++, is simply to coerce the compiler into treating them as C++ (which gives them the appropriate name mangling to work with your C++ files). For GCC, this is -x c++.

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2 Comments

This looks like a better solution, doesn't it ? (since I don't have to pollute the code with wrappers), more reference
It depends. I say "almost ubiquitous" but there is the odd C library which has a variable named class... and it doesn't work for precompiled C libraries. But yeah, when it works, there's way fewer gotchas than with extern "C".

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