app failing to link to mysql libraries (CentOS)

问题:

The link step gets undefined references for all my mysql calls:

~/private/WDI/git$ make
c++ -L/usr/lib/x86_64-linux-gnu -lmysqlclient -lpthread -lz -lm -lrt -ldl -o tom tom.o Block.o IPC.o ConnectMxctl.o CI_Metadata.o Log.o
tom.o: In function `connect_to_table_managers()':
/home/chap/private/WDI/git/tom.cpp:128: undefined reference to `mysql_num_rows'
/home/chap/private/WDI/git/tom.cpp:132: undefined reference to `mysql_num_fields'
/home/chap/private/WDI/git/tom.cpp:133: undefined reference to `mysql_fetch_row'
/home/chap/private/WDI/git/tom.cpp:153: undefined reference to `mysql_num_rows'
/home/chap/private/WDI/git/tom.cpp:157: undefined reference to `mysql_fetch_row'
/home/chap/private/WDI/git/tom.cpp:167: undefined reference to `mysql_free_result'

The link libraries in the c++ command were generated by mysql_config --libs. Here's a partial listing of the directory:

/usr/lib/x86_64-linux-gnu$ ll libmysql*
-rw-r--r-- 1 root root 4838468 Jul 23 23:28 libmysqlclient.a
lrwxrwxrwx 1 root root      16 Jul 23 23:28 libmysqlclient_r.a -> libmysqlclient.a
lrwxrwxrwx 1 root root      17 Jul 23 23:28 libmysqlclient_r.so -> libmysqlclient.so
lrwxrwxrwx 1 root root      20 Jul 23 23:28 libmysqlclient_r.so.18 -> libmysqlclient.so.18
lrwxrwxrwx 1 root root      24 Jul 23 23:28 libmysqlclient_r.so.18.0.0 -> libmysqlclient.so.18.0.0
lrwxrwxrwx 1 root root      20 Jul 23 23:28 libmysqlclient.so -> libmysqlclient.so.18
lrwxrwxrwx 1 root root      24 Jul 23 23:28 libmysqlclient.so.18 -> libmysqlclient.so.18.0.0
-rw-r--r-- 1 root root 3408144 Jul 23 23:28 libmysqlclient.so.18.0.0

So things appear to be in their proper places. Why would these symbols be unresolvable?

回答1:

You should put most independent library in the end of the command, so compiler can find symbol and definitions properly:

c++ -L/usr/lib/x86_64-linux-gnu -o tom tom.o Block.o IPC.o \
  ConnectMxctl.o CI_Metadata.o Log.o \
  -lmysqlclient -lpthread -lz -lm -lrt -ldl 

By doing it this way, compiler will continue searching undefined symbols from next linking files. For exampile if IPO.o has uses symbol(type/class/struct/function etc) defined in CI_Metadata.o, compiler can find it because you put CI_Metadata.o after IPO.o. Most libraries are independent, that's why they are in the end of compile/link command.