在Ubuntu 12.04.1 LTS libssl libcrypto上的未定义引用

Question

I have got a series of errors I have added -l ssl and -l crypto . However I do not know which version library I need to install

/usr/include/boost/asio/ssl/detail/openssl_init.hpp:48: undefined reference to `SSL_library_init'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:49: undefined reference to `SSL_load_error_strings'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:50: undefined reference to `SSL_library_init'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:52: undefined reference to `CRYPTO_num_locks'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:55: undefined reference to `CRYPTO_set_locking_callback'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:56: undefined reference to `CRYPTO_set_id_callback'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:48: undefined reference to `SSL_library_init'
/usr/include/boost/asio/ssl/detail/openssl_init.hpp:49: undefined reference to `SSL_load_error_strings'

There are numerous options such as

Note, selecting 'libss7-dbg' for regex 'libssl*'
Note, selecting 'libssl0.9.8-dbg' for regex 'libssl*'
Note, selecting 'libss7-dev' for regex 'libssl*'
Note, selecting 'libssl0.9.8' for regex 'libssl*'
Note, selecting 'libss2' for regex 'libssl*'
Note, selecting 'libssm-dev' for regex 'libssl*'
Note, selecting 'libssl' for regex 'libssl*'
Note, selecting 'libssh-4' for regex 'libssl*'
Note, selecting 'libssh-2-dev' for regex 'libssl*'
Note, selecting 'libssh-2-doc' for regex 'libssl*'
Note, selecting 'libssl1.0.0' for regex 'libssl*'
Note, selecting 'libsscm3' for regex 'libssl*'
Note, selecting 'libssl-ocaml-dev' instead of 'libssl-ocaml-dev-l8h98'
Note, selecting 'libssl-ocaml' instead of 'libssl-ocaml-l8h98'
Note, selecting 'libssreflect-ocaml' instead of 'libssreflect-ocaml-kevs8'
Note, selecting 'libssreflect-ocaml-dev' instead of 'libssreflect-ocaml-dev-kevs8'
Note, selecting 'python-libssh2' instead of 'python2.7-libssh2'
libss2 is already the newest version.

and

Note, selecting 'libcryptokit-ocaml' for regex 'libcrypto*'
Note, selecting 'libcryptokit-ocaml-02n31' for regex 'libcrypto*'
Note, selecting 'libcryptgps-ocaml-dev' for regex 'libcrypto*'
Note, selecting 'libcrypt-blowfish-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-saltedhash-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-gcrypt-perl' for regex 'libcrypto*'
Note, selecting 'libcryptui' for regex 'libcrypto*'
Note, selecting 'libcrypt-generatepassword-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-dh-gmp-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-cbc-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-dsa-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-dh-perl' for regex 'libcrypto*'
Note, selecting 'libcrypt-openssl-bignum-perl' for regex 'libcrypto*'
Note, selecting 'libcryptgps-ocaml-dev' instead of 'libcryptgps-ocaml-dev-1zmb9'
Note, selecting 'libcryptokit-ocaml' instead of 'libcryptokit-ocaml-02n31'
Note, selecting 'libcryptokit-ocaml-dev' instead of 'libcryptokit-ocaml-dev-02n31'

g++ -pthread -l ssl -lcrypto IpcManagerMain.cpp -o IpcManagerMain.o  -lentity++ -lpersistence++ -lplatform++ -lreflection++ -lmonitor++ -lmeta++ -lipc++ -lbroadcast++ -lutilities++ -L/usr/lib          -lboost_serialization -lboost_thread-mt -lboost_date_time -lboost_iostreams -lboost_program_options -lboost_filesystem -lboost_system

There are more but I have shortened it. Could someone please point me in the right direction?

Answer 1

Imagine the (imaginary) linker trying to link these modules:

link moda modb modc modd

The linker doesn't (for efficiency reasons) record all the symbols it has seen in moda to try to match them with symbols in modb . For example, if moda is a huge library, then the linking process becomes very slow (and memory consuming), if all symbols of moda are remembered throughout the whole process.

As a result, the linker always remembers only the (so far) missing symbols and tries to find them in the modules that come afterwards . Note that this is only true for libraries. This is because objects of the same library/application may have circular dependency.

This means that missing symbols of modb for example, are searched for only in modc and modd and not in moda (again, if moda , modc and modd are libraries).

Therefore, if you have modx using symbols of mody , then you must mention mody after modx in the link command.

For example, if you have files main.o , funcs.o and libmatrix.a , then you need to write your link command like this:

gcc funcs.o main.o -lmatrix

otherwise, if you write gcc -lmatrix funcs.o main.o , the symbols of libmatrix.a will not be visible to funcs.o and main.o .