unsigned int arg_count

The number of arguments. Check this value in the initialization function if you require your function to be called with a particular number of arguments. For example:

if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}

For other UDF_ARGS member values that are arrays, array references are zero-based. That is, refer to array members using index values from 0 to args->arg_count – 1.

enum Item_result *arg_type

A pointer to an array containing the types for each argument. The possible type values are STRING_RESULT, INT_RESULT, and REAL_RESULT.

To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example:

if (args->arg_type[0] != STRING_RESULT ||
    args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}

As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify that the first two arguments should be coerced to string and integer, respectively, do this in xxx_init():

args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;

char **args

args->args communicates information to the initialization function about the general nature of the arguments passed to your function. For a constant argument i, args->args[i] points to the argument value. (See below for instructions on how to access the value properly.) For a nonconstant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A nonconstant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with nonconstant arguments.

For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed.

If argument i represents NULL, args->args[i] is a null pointer (0). If the argument is not NULL, functions can refer to it as follows:

unsigned long *lengths

For the initialization function, the lengths array indicates the maximum string length for each argument. You should not change these. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

char *maybe_null

For the initialization function, the maybe_null array indicates for each argument whether the argument value might be null (0 if no, 1 if yes).