12.2.2.1 UDF Calling Sequences for simple functions

The main function should be declared as shown here. Note that the return type and parameters differ, depending on whether you will declare the SQL function XXX() to return STRING, INTEGER, or REAL in the CREATE FUNCTION statement:

For STRING functions:

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
          char *result, unsigned long *length,
          char *is_null, char *error);

For INTEGER functions:

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions:

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

The initialisation and deinitialisation functions are declared like this:

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members are listed below. The initialisation function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.):

my_bool maybe_null

xxx_init() should set maybe_null to 1 if xxx() can return NULL. The default value is 1 if any of the arguments are declared maybe_null.

unsigned int decimals

Number of decimals. The default value is the maximum number of decimals in the arguments passed to the main function. (For example, if the function is passed 1.34, 1.345, and 1.3, the default would be 3, because 1.345 has 3 decimals.

unsigned int max_length

The maximum length of the string result. The default value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimals indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.) If you want to return a blob, you can set this to 65K or 16M; this memory is not allocated but used to decide which column type to use if there is a need to temporary store the data.

char *ptr

A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory between functions. In xxx_init(), allocate the memory and assign it to this pointer:

initid->ptr = allocated_memory;

In xxx() and xxx_deinit(), refer to initid->ptr to use or deallocate the memory.

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