Why do I get "undefined reference" errors when linking against OpenSSL?

Question

My compilation process throws around errors like

..undefined reference to `BN_cmp'

although I include <openssl/bn.h>

and run gcc -lssl -lcrypto test.c -o test

can someone help? (openssl libssl1.0.0, libssl-dev are installed)

Solution

The solution is as simple as adding the -l flags at the end:

gcc test.c -o test -lssl -lcrypto

The order matters because ld since Ubuntu 11.04 is invoked with the -as-needed switch by default, so that files/libraries which depend on other libraries must come before these other libraries, i.e. test.c needs libcrypto, so it must come before -lcrypto.

For more information, see Toolchain Transition in Natty Narwhal.