CODEWITHSUNDEEP
c++clinuxgccx86-64
pythonic
pythonic

Reputation: 21589

undefined reference to 'some_function'?

I am trying to compile a C++ program and it gives me this error.

undefined reference to 'some_function'

Whereas I do add the file which contains some_function in the makefile. Also I include the declaration of some_function in the file where I use it. So why does the compiler still complains that it can't find it? What can be the possible reasons?

My makefile is like that

CXX = g++
CXXFILES = dlmalloc.c pthreads.cpp queue.cpp 

CXXFLAGS = -O3 -o prog -rdynamic -D_GNU_SOURCE
LIBS = -lpthread -ldl

all:
    $(CXX) $(CXXFILES) $(LIBS) $(CXXFLAGS)

clean:
    rm -f prog *.o

some_function is defined in dlmalloc.c and used inside pthreads.cpp. Does it have to do with the fact that dlmalloc.c is a C source code file and others are C++ files. Maybe I should use the extern "C" keyword with some_function here, right?

Upvotes: 1

Views: 634

Answers (1)

Mike Seymour
Mike Seymour

Reputation: 254431

Maybe I should use the extern "C" keyword here, right?

Yes. If you want to call a function compiled as C from C++ code, then you need to declare it extern "C" - but only in C++, since that's not valid syntax in C.

You can use the __cplusplus preprocessor symbol to create a header that's valid for either language:

#ifdef __cplusplus
extern "C" {
#endif
void some_function();

/* many, many more C declarations */

#ifdef __cplusplus
}
#endif

The stuff inside extern "C" { ... } is handled as C declarations. Be careful to make sure each function is always declared this way for C++, and in the odd chance that the C function is actually written in C++ (strongly discouraged!) the decaration must come before the actual definition of the function.

Upvotes: 8

Related Questions