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29.4.2.3 UDF Argument Processing
The args
parameter points to a
UDF_ARGS
structure that has the members
listed here:
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 toargs->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
,REAL_RESULT
, andDECIMAL_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; }
Arguments of type
DECIMAL_RESULT
are passed as strings, so you should handle them the same way asSTRING_RESULT
values.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 toxxx()
. For example, to specify that the first two arguments should be coerced to string and integer, respectively, do this inxxx_init()
:args->arg_type[0] = STRING_RESULT; args->arg_type[1] = INT_RESULT;
Exact-value decimal arguments such as
1.3
orDECIMAL
column values are passed with a type ofDECIMAL_RESULT
. However, the values are passed as strings. If you want to receive a number, use the initialization function to specify that the argument should be coerced to aREAL_RESULT
value:args->arg_type[2] = REAL_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 argumenti
,args->args[i]
points to the argument value. (See later for instructions on how to access the value properly.) For a nonconstant argument,args->args[i]
is0
. A constant argument is an expression that uses only constants, such as3
or4*7-2
orSIN(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
representsNULL
,args->args[i]
is a null pointer (0). If the argument is notNULL
, functions can refer to it as follows:An argument of type
STRING_RESULT
is given as a string pointer plus a length, to enable handling of binary data or data of arbitrary length. The string contents are available asargs->args[i]
and the string length isargs->lengths[i]
. Do not assume that the string is null-terminated.For an argument of type
INT_RESULT
, you must castargs->args[i]
to along long
value:long long int_val; int_val = *((long long*) args->args[i]);
For an argument of type
REAL_RESULT
, you must castargs->args[i]
to adouble
value:double real_val; real_val = *((double*) args->args[i]);
For an argument of type
DECIMAL_RESULT
, the value is passed as a string and should be handled like aSTRING_RESULT
value.ROW_RESULT
arguments are not implemented.
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 typesINT_RESULT
orREAL_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).char **attributes
args->attributes
communicates information about the names of the UDF arguments. For argumenti
, the attribute name is available as a string inargs->attributes[i]
and the attribute length isargs->attribute_lengths[i]
. Do not assume that the string is null-terminated.By default, the name of a UDF argument is the text of the expression used to specify the argument. For UDFs, an argument may also have an optional
[AS]
clause, in which case the argument name isalias_name
alias_name
. Theattributes
value for each argument thus depends on whether an alias was given.Suppose that a UDF
my_udf()
is invoked as follows:In this case, the
attributes
andattribute_lengths
arrays will have these values:args->attributes[0] = "expr1" args->attribute_lengths[0] = 5 args->attributes[1] = "alias1" args->attribute_lengths[1] = 6 args->attributes[2] = "alias2" args->attribute_lengths[2] = 6
unsigned long *attribute_lengths
The
attribute_lengths
array indicates the length of each argument name.
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