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## SQLite C Interface

## Create Or Redefine SQL Functions

>     int sqlite3_create_function(
>       sqlite3 *db,
>       const char *zFunctionName,
>       int nArg,
>       int eTextRep,
>       void *pApp,
>       void (*xFunc)(sqlite3_context*,int,sqlite3_value**),
>       void (*xStep)(sqlite3_context*,int,sqlite3_value**),
>       void (*xFinal)(sqlite3_context*)
>     );
>     int sqlite3_create_function16(
>       sqlite3 *db,
>       const void *zFunctionName,
>       int nArg,
>       int eTextRep,
>       void *pApp,
>       void (*xFunc)(sqlite3_context*,int,sqlite3_value**),
>       void (*xStep)(sqlite3_context*,int,sqlite3_value**),
>       void (*xFinal)(sqlite3_context*)
>     );

These two functions (collectively known as "function creation routines") are used to add SQL functions or aggregates or to redefine the behavior of existing SQL functions or aggregates. The only difference between the two is that the second parameter, the name of the (scalar) function or aggregate, is encoded in UTF-8 for sqlite3_create_function() and UTF-16 for sqlite3_create_function16().

The first parameter is the database connection to which the SQL function is to be added. If a single program uses more than one database connection internally, then SQL functions must be added individually to each database connection.

The second parameter is the name of the SQL function to be created or redefined. The length of the name is limited to 255 bytes, exclusive of the zero-terminator. Note that the name length limit is in bytes, not characters. Any attempt to create a function with a longer name will result in SQLITE_ERROR being returned.

The third parameter (nArg) is the number of arguments that the SQL function or aggregate takes. If this parameter is -1, then the SQL function or aggregate may take any number of arguments between 0 and the limit set by sqlite3_limit(SQLITE_LIMIT_FUNCTION_ARG). If the third parameter is less than -1 or greater than 127 then the behavior is undefined.

The fourth parameter, eTextRep, specifies what text encoding this SQL function prefers for its parameters. Any SQL function implementation should be able to work work with UTF-8, UTF-16le, or UTF-16be. But some implementations may be more efficient with one encoding than another. An application may invoke sqlite3_create_function() or sqlite3_create_function16() multiple times with the same function but with different values of eTextRep. When multiple implementations of the same function are available, SQLite will pick the one that involves the least amount of data conversion. If there is only a single implementation which does not care what text encoding is used, then the fourth argument should be SQLITE_ANY.

The fifth parameter is an arbitrary pointer. The implementation of the function can gain access to this pointer using sqlite3_user_data().

The seventh, eighth and ninth parameters, xFunc, xStep and xFinal, are pointers to C-language functions that implement the SQL function or aggregate. A scalar SQL function requires an implementation of the xFunc callback only, NULL pointers should be passed as the xStep and xFinal parameters. An aggregate SQL function requires an implementation of xStep and xFinal and NULL should be passed for xFunc. To delete an existing SQL function or aggregate, pass NULL for all three function callbacks.

It is permitted to register multiple implementations of the same functions with the same name but with either differing numbers of arguments or differing preferred text encodings. SQLite will use the implementation that most closely matches the way in which the SQL function is used. A function implementation with a non-negative nArg parameter is a better match than a function implementation with a negative nArg. A function where the preferred text encoding matches the database encoding is a better match than a function where the encoding is different. A function where the encoding difference is between UTF16le and UTF16be is a closer match than a function where the encoding difference is between UTF8 and UTF16.

Built-in functions may be overloaded by new application-defined functions. The first application-defined function with a given name overrides all built-in functions in the same database connection with the same name. Subsequent application-defined functions of the same name only override prior application-defined functions that are an exact match for the number of parameters and preferred encoding.

An application-defined function is permitted to call other SQLite interfaces. However, such calls must not close the database connection nor finalize or reset the prepared statement in which the function is running.

Requirements: H16103 H16106 H16109 H16112 H16118 H16121 H16127 H16130 H16133 H16136 H16139 H16142

See also lists of Objects, Constants, and Functions.

This page last modified 2009/08/19 14:11:24 UTC