If, after an upgrade, you experience problems with compiled client programs, such as Commands out of sync or unexpected core dumps, the programs were probably compiled using old header or library files. In this case, check the date of the mysql.h header file and libmysqlclient.a library used for compilation to verify that they are from the new MySQL distribution. If not, recompile the programs with the new headers and libraries. Recompilation might also be necessary for programs compiled against the shared client library if the library major version number has changed (for example, from libmysqlclient.so.17 to libmysqlclient.so.18).

The major shared client library version determines compatibility. (For example, for libmysqlclient.so.18.1.0, the major version is 18.) Libraries shipped with newer versions of MySQL are drop-in replacements for older versions that have the same major number. As long as the major library version is the same, you can upgrade the library and old applications should continue to work with it.

Undefined-reference errors might occur at runtime when you try to execute a MySQL program. If these errors specify symbols that start with mysql_ or indicate that the libmysqlclient library cannot be found, it means that your system cannot find the shared libmysqlclient.so library. The solution to this problem is to tell your system to search for shared libraries in the directory where that library is located. Use whichever of the following methods is appropriate for your system: