cmake compiling but not linking a new source file in a library (libonion)

Question

I am a cmake newbie (on Debian/Sid/Linux/x86-64)

I forked libonion on https://github.com/bstarynk/onion to enable customization of malloc with Boehm's garbage collector; see this mail thread.

I added two files there onion/src/low_util.c and onion_src/low_util.h (which is #include-d successfully in several other patched files.

It is compiled but not linked.

 set(SOURCES onion.c codecs.c dict.c low_util.c request.c response.c handler.c 
                    log.c sessions.c sessions_mem.c shortcuts.c block.c mime.c url.c ${POLLER_C}
        listen_point.c request_parser.c http.c ${HTTPS_C} websocket.c ${RANDOM_C} ${SQLITE3_C})

later:

 SET(INCLUDES_ONION block.h codecs.h dict.h handler.h http.h https.h listen_point.h low_util.h log.h mime.h onion.h poller.h request.h response.h server.h sessions.h shortcuts.h types.h types_internal.h url.h websocket.h ${SQLITE3_H})
 MESSAGE(STATUS "Found include files ${INCLUDES_ONION}")

but when I build, my file low_util.c got compiled but not linked.

Linking C executable otemplate
CMakeFiles/opack.dir/__/__/src/onion/dict.c.o: In function `onion_dict_new':
dict.c:(.text+0x1bc): undefined reference to `onionlow_calloc'
CMakeFiles/opack.dir/__/__/src/onion/dict.c.o: In function `onion_dict_node_data_free':
dict.c:(.text+0x2ec): undefined reference to `onionlow_free'
CMakeFiles/opack.dir/__/__/src/onion/dict.c.o: In function `onion_dict_node_add':

Notice that libonion is a library (in C, providing HTTP service) and that I just want to add a low_util.c file (wrapping malloc & pthread_create etc... to make Boehm's GC happy: it is calling GC_malloc and GC_pthread_create ....) with its low_util.h header. Surprisingly, they get compiled, but do not seems to be linked. And I am not familiar with cmake and I am not familiar with how D.Moreno (the main author of libonion) has organized his cmake files.

Any clues?

Answer 1

Apply the following patch to make it link. The two executables which are being linked with the symbols generated from the .c file you added are missing and are added in the patch.

http://pastebin.com/mDMRiUQu

Answer 2

Based on what you posted, its hard to tell what could be wrong. The cake source code above says that a variable ${SOURCES} is equivalent to onion.c codecs.c dict.c low_util.c ... ${SQLITE3_C}, and a variable ${INCLUDE_ONION} is equivalent to block.h codecs.h dict.h ... ${SQLITE3_H}. You did not provide any targets or the files included in those targets.

A brief list of things that may help:

• where do you define the top level library or executable? If your making a library, you will need the command add_library(). If you are making an executable, you will need the add_executable() command.
• Use the command target_link_libraries() to resolve dependencies. Rather than placing all of the source files in a single library, group similar together in a single target (a target is defined by the add_* commands), and use this command to link the targets after compilation.
• Use the find_package() to get any libraries which are defined on your system but not in you project. Then, link to that library using the target_link_libraries() command.

In this case, if the onion_dict_* functions are defined within the same library, your not including those files in library. When you use add_library or add_executable, ensure you add those files to the list. If the functions are within your project but not in the same library, use the target_link_libraries() command to link to the library which contains the correct files. If those commands are defined in an external library, then first find the library using find_package(), and then link to the library using target_link_libraries().