Minimal UDF for MariaDB in C

Minimal-MariaDB-UDF-In-C.md

Assume that you've got MariaDB built and running locally. Let's say the install directory is in $MARIADB_LOCAL_INSTALL.

Enter the following into udf_minimal.c:

#include "mariadb.h"

my_bool mysum_init(UDF_INIT* initid, UDF_ARGS* args, char* message);
long long mysum(UDF_INIT* initid, UDF_ARGS* args, char* result,
              unsigned long* length, char* is_null, char* error);

my_bool mysum_init(UDF_INIT* initid, UDF_ARGS* args, char* message) {
  return 0;
}

long long mysum(UDF_INIT* initid, UDF_ARGS* args, char* result,
                unsigned long* length, char* is_null, char* error) {
  uint i = 0;
  long long sum = 0;
  for (i = 0; i < args->arg_count; i++) {
    if (args->args[i] == NULL) {
      continue;
    }

    sum += *((long long*)args->args[i]);
  }

  return sum;
}

Now build a shared library from it:

$ gcc -I$MARIADB_LOCAL_INSTALL/include -I$MARIADB_LOCAL_INSTALL/sql -I$MARIADB_LOCAL_INSTALL/build/include -shared -o udf_minimal.so udf_minimal.c
$ cp udf_minimal.so $MARIADB_LOCAL_INSTALL/dbplugins

And within the MariaDB client run:

MariaDB [(none)]> CREATE FUNCTION mysum RETURNS INTEGER SONAME 'udf_minimal.so';
Query OK, 0 rows affected (0.003 sec)

And try using the function!

MariaDB [(none)]> select mysum(20, 30);
+---------------+
| mysum(20, 30) |
+---------------+
|            50 |
+---------------+
1 row in set (0.000 sec)

Huzzah!

Sources:

• https://dev.mysql.com/doc/extending-mysql/8.0/en/adding-loadable-function.html
• https://github.com/MariaDB/server/blob/11.4/sql/udf_example.c
• https://mariadb.com/kb/en/create-function-udf/