Linker error on Linux: “undefined reference to”

问题:

I am able to make a shared library without problems. I create libcbitcoin.so (with no errors) and attempt to link against it with an executable as well as OpenSSL libraries. I use this command:

gcc -L/media/sf_BitEagle_Projects/cbitcoin/build/bin -lcbitcoin \
-Wl-rpath,/media/sf_BitEagle_Projects/cbitcoin/build/bin -lssl -lcrypto \
-L/usr/local/ssl/lib/ -o /media/sf_BitEagle_Projects/cbitcoin/build/bin/testCBAddress \
/media/sf_BitEagle_Projects/cbitcoin/build/obj/testCBAddress.o \
/media/sf_BitEagle_Projects/cbitcoin/build/obj/CBOpenSSLCrypto.o

The bin directory is the location of the library. The obj directory has the object files I wish to link into an executable. In the command I use the -L, -l and -rpath options which I thought was all that is needed for linking in linux. It seems I am wrong since I get errors like:

/media/sf_BitEagle_Projects/cbitcoin/test/testCBAddress.c:40:
undefined reference to `CBNewByteArrayFromString'

CBNewByteArrayFromString is found in the library. For some reason it is not being linked. OpenSSL too:

/media/sf_BitEagle_Projects/cbitcoin/dependencies/crypto/CBOpenSSLCrypto.c:37:
undefined reference to `SHA1'

How do I get the linking to work?

GCC version: gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

On Linux Mint 13

Thank you.

回答1:

Put the libraries after the object files on the link command line:

gcc /media/sf_BitEagle_Projects/cbitcoin/build/obj/testCBAddress.o \
    /media/sf_BitEagle_Projects/cbitcoin/build/obj/CBOpenSSLCrypto.o \
    -L/media/sf_BitEagle_Projects/cbitcoin/build/bin \
    -lcbitcoin -Wl-rpath,/media/sf_BitEagle_Projects/cbitcoin/build/bin \
    -L/usr/local/ssl/lib/ -lssl -lcrypto \
    -o /media/sf_BitEagle_Projects/cbitcoin/build/bin/testCBAddress

If you don't do that, the linker may decide that it needs nothing from a particular library at the stage of the link where it scans the library, and then it won't rescan the library later after it finds some undefined symbols in the object files. If you put the object files first, you don't run into this problem.

回答2:

I think it caused by can NOT find symbol, gcc will first go through from left, try to put the lib file at the end