23.3.2.5. Compiling and Installing User-Defined Functions

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file sql/udf_example.c that is included in MySQL source distributions.

If a UDF will be referred to in statements that will be replicated to slave servers, you must ensure that every slave also has the function available. Otherwise, replication will fail on the slaves when they attempt to invoke the function.

The immediately following instructions are for Unix. Instructions for Windows are given later in this section.

The udf_example.c file contains the following functions:

metaphon() returns a metaphon string of the string argument. This is something like a soundex string, but it is more tuned for English.
myfunc_double() returns the sum of the ASCII values of the characters in its arguments, divided by the sum of the length of its arguments.
myfunc_int() returns the sum of the length of its arguments.
sequence([const int]) returns a sequence starting from the given number or 1 if no number has been given.
lookup() returns the IP address for a host name.
reverse_lookup() returns the host name for an IP address. The function may be called either with a single string argument of the form 'xxx.xxx.xxx.xxx' or with four numbers.
avgcost() returns an average cost. This is an aggregate function.

A dynamically loadable file should be compiled as a sharable object file, using a command something like this:

shell> gcc -shared -o udf_example.so
        udf_example.c

If you are using gcc with CMake (which is how MySQL is configured), you should be able to create udf_example.so with a simpler command:

shell> make udf_example

After you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from udf_example.c using gcc directly produces a file named udf_example.so. Copy the shared object to the server's plugin directory and name it udf_example.so. This directory is given by the value of the plugin_dir system variable.

On some systems, the ldconfig program that configures the dynamic linker does not recognize a shared object unless its name begins with lib. In this case you should rename a file such as udf_example.so to libudf_example.so.

On Windows, you can compile user-defined functions by using the following procedure:

Obtain a MySQL source distribution. See Section 2.1.3, "How to Get MySQL".
Obtain the CMake build utility, if necessary, from http://www.cmake.org. (Version 2.6 or later is required).
In the source tree, look in the sql directory. There are files named udf_example.def udf_example.c there. Copy both files from this directory to your working directory.

Create a CMake makefile (CMakeLists.txt) with these contents:

PROJECT(udf_example)# Path for MySQL include directoryINCLUDE_DIRECTORIES("c:/mysql/include")ADD_DEFINITIONS("-DHAVE_DLOPEN")ADD_LIBRARY(udf_example MODULE udf_example.c udf_example.def)TARGET_LINK_LIBRARIES(udf_example wsock32)

Create the VC project and solution files:
```
cmake -G "<Generator>"
```
Invoking cmake --help shows you a list of valid Generators.
Create udf_example.dll:
```
devenv udf_example.sln /build Release
```

After the shared object file has been installed, notify mysqld about the new functions with the following statements. If object files have a suffix different from .so on your system, substitute the correct suffix throughout (for example, .dll on Windows).

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME
        'udf_example.so';mysql> CREATE FUNCTION myfunc_double RETURNS
        REAL SONAME 'udf_example.so';mysql> CREATE FUNCTION
        myfunc_int RETURNS INTEGER SONAME 'udf_example.so';mysql> CREATE
        FUNCTION sequence RETURNS INTEGER SONAME 'udf_example.so';mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';mysql> CREATE FUNCTION reverse_lookup    ->        RETURNS STRING SONAME 'udf_example.so';mysql> CREATE AGGREGATE FUNCTION avgcost    ->        RETURNS REAL SONAME 'udf_example.so';

To delete functions, use DROP FUNCTION:

mysql> DROP FUNCTION metaphon;mysql> DROP FUNCTION myfunc_double;mysql> DROP FUNCTION myfunc_int;mysql> DROP
        FUNCTION sequence;mysql> DROP FUNCTION
        lookup;mysql> DROP FUNCTION reverse_lookup;mysql> DROP FUNCTION avgcost;

The CREATE FUNCTION and DROP FUNCTION statements update the func system table in the mysql database. The function's name, type and shared library name are saved in the table. You must have the INSERT or DELETE privilege for the mysql database to create or drop functions, respectively.

You should not use CREATE FUNCTION to add a function that has previously been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise, the server continues to use the old version.

An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION. All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable.