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Question

I SOLVED THIS ISSUE BY MYSELF

The problem was the linkage of the library. I copied the libmywrapper.so(i renamed it) file to /usr/lib and linked with -mywrapper That's it :-)

Original post:

I'm writing a wrapper library that allows to call C++ functions out of C-code. Unfortnuately it doesn't link...

wrapper.h:

#ifdef __cplusplus
extern "C"
{
#endif
    extern char* (keygen) ();
#ifdef __cplusplus
 }
#endif

wrapper.cpp:

#include "wrapper.h"
#include <someincludes>
char* keygen ()
{
    urandom u;

Makefile:

TARGET      := ./mywrapperlib.so
CXXFLAGS    := -fPIC -shared -g -Wall -std=c++0x -I../someincludes -I.
CXX         := g++
LIB         := -lsomelibs
EXT         := cpp
BUILDDIR    := build

override BUILDDIR := $(strip $(BUILDDIR))
SOURCES  := $(wildcard *.$(EXT))
OBJECTS  := $(patsubst %.$(EXT), $(BUILDDIR)/%.o, $(SOURCES))
DEPS     := $(patsubst %.$(EXT), $(BUILDDIR)/%.dep, $(SOURCES))

.PHONY: all
all: $(TARGET)

$(TARGET): $(OBJECTS) $(DEPS)
    $(CXX) $(CXXFLAGS) -o $(TARGET) $(OBJECTS) $(LIBS)

ifneq ($(MAKECMDGOALS), clean)
-include $(DEPS)
endif

$(OBJECTS): $(BUILDDIR)/%.o: %.$(EXT) $(BUILDDIR)/%.dep $(BUILDDIR)/.tag
    $(CXX) $(CXXFLAGS) -c $< -o $@

$(DEPS): $(BUILDDIR)/%.dep: %.$(EXT) $(BUILDDIR)/.tag
    mkdir -p $(dir $(@))
    $(CXX) $(CXXFLAGS) -MM $< -MT $@ -MT $(<:.$(EXT)=.o) -o $@

%.tag:
    mkdir -p $(dir $(@))
    touch $@

.PHONY: clean
clean:
    $(RM) -r $(BUILDDIR)

A test file that should use the library: test.c:

#include <wrapper.h>
int main()
{
    char* test = keygen();
}

When i try to compile it with

gcc  -o test.a -g -Iinclude -Llib/mywrapperlib.so test.c

I get

/tmp/ccB9bEot.o: In function `main':
/some/paths/test.c:7: undefined reference to `keygen'

Im very unexperienced with mixing C & C++ code and writing libraries. Now im stuck and hope that someone can help me with this problem.

EDIT:

I checked the lib with nm:

nm lib/cryptdbwrapperlib.so | grep keygen
0000000000006935 T keygen

So, i guess that the problem is the linkage...

Answer 1

It's to do with the order of your flags to gcc .

Do this:

gcc  -o test.a -g -Iinclude test.c -Llib/mywrapperlib.so
#                           ^^^^^^ ^^^^^^^^^^^^^^^^^^^^^
#                           first    second

GCC reads libraries and objects left-to-right and (basically) ignores any that aren't needed "yet". With my proposed change, test.c goes first so GCC knows that it's going to be looking for a symbol keygen ; then, when it finally sees -Llib/mywrapperlib.so it scans it for keygen , finds it, and knows that this library is required.

Answer 2

Change the function's signature in wrapper.cpp to

extern "C" char* keygen ()

Otherwise it will be compiled with a C++ style name, and hence be a different function than the one declared in the header.

Answer 3

I SOLVED THIS ISSUE BY MYSELF

The problem was the linkage of the library. I copied the libmywrapper.so(i renamed it) file to /usr/lib and linked with -mywrapper That's it :-)