"undefined reference to" errors when linking static C library with C++ code

Question

I have a test file (just for the link test) where I overload the new/delete operators with my own malloc/free library called libxmalloc.a. But I keep getting "undefined reference to" error as following when linking the static library, even I change the order of test.o and -lxmalloc. But everything works well with other C programs linking this library. I'm so confused with this issue and appreciate any clue.

Error Msg:

g++ -m64 -O3 -I/usr/include/ethos -I/usr/include/nacl/x86_64 -c -o test.o test.cpp g++ -m64 -O3 -L. -o demo test.o -lxmalloc test.o: In function `operator new(unsigned long)': test.cpp:(.text+0x1): undefined reference to `malloc(unsigned long)' test.o: In function `operator delete(void*)': test.cpp:(.text+0x11): undefined reference to `free(void*)' test.o: In function `operator new[](unsigned long)': test.cpp:(.text+0x21): undefined reference to `malloc(unsigned long)' test.o: In function `operator delete[](void*)': test.cpp:(.text+0x31): undefined reference to `free(void*)' test.o: In function `main': test.cpp:(.text.startup+0xc): undefined reference to `malloc(unsigned long)' test.cpp:(.text.startup+0x19): undefined reference to `malloc(unsigned long)' test.cpp:(.text.startup+0x24): undefined reference to `free(void*)' test.cpp:(.text.startup+0x31): undefined reference to `free(void*)' collect2: ld returned 1 exit status make: *** [demo] Error 1 

My test.cpp file:

#include <dual/xalloc.h> #include <dual/xmalloc.h> void* operator new (size_t sz) {     return malloc(sz); } void operator delete (void *ptr) {     free(ptr); } void* operator new[] (size_t sz) {     return malloc(sz); } void operator delete[] (void *ptr) {     free(ptr); } int main(void) {     int *iP = new int;     int *aP = new int[3];     delete iP;     delete[] aP;     return 0; } 

My Makefile:

CFLAGS += -m64 -O3 -I/usr/include/ethos -I/usr/include/nacl/x86_64 CXXFLAGS += -m64 -O3 LIBDIR += -L. LIBS += -lxmalloc all: demo demo: test.o     $(CXX) $(CXXFLAGS) $(LIBDIR) -o demo test.o $(LIBS) test.o: test.cpp $(CXX) $(CFLAGS) -c -o $@ $< clean: - rm -f *.o demo 

Answer 1

But everything works well with other C programs linking this library.

Did you notice that C and C++ compilation create different symbol names on object file level? It's called 'name mangling'.
The (C++) linker would show undefined references as demangled symbols in the error message, which might confuse you. If you inspect your test.o file with nm -u you'll see that the referenced symbol names don't match with those provided in your library.

If you want to use functions linked in as externals that were compiled using the plain C compiler, you'll need their function declarations enclosed in an extern "C" {} block which suppresses C++ name mangling for everything declared or defined inside, e.g.:

extern "C"  {     #include <dual/xalloc.h>     #include <dual/xmalloc.h> } 

Even better, you might wrap your function declarations in your header files like this:

#if defined (__cplusplus) extern "C" { #endif  /*  * Put plain C function declarations here ...  */   #if defined (__cplusplus) } #endif