Firebird 2.1 Language Reference Update

Notes:
External functions (UDFs)
the engine will pass a NULL argument value unchanged to the function. This leads to a NULL result,
which is correct. Without the NULL keyword (your only option in pre-2.0 versions), NULL is passed
to the function as an empty string and the result is an empty string as well.
For more information about passing NULLs to UDFs, see the note at the end of this book.
• Depending on how you declare it (see CSTRING note), this function can accept and return strings of up to
32767 characters.
• Before Firebird 2.0, the result type was CHAR(n).
• In Firebird 1.5.1 and below, the default declaration used CSTRING(80) instead of CSTRING(255).
• In InterBase, substr returned NULL if endpos lay past the end of the string.
Tip
Library: ib_udf
Added in: 1.0
Although the function arguments are slightly different, consider using the internal SQL function SUBSTRING
instead, for better compatibility and multi-byte character set support.
Changed in: 1.5.2, 2.0
Better alternative: Internal function SUBSTRING()
substrlen
Description: Returns the substring starting at startpos and having length characters (or less, if the end of
the string is reached first). Positions are 1-based. If either startpos or length is smaller than 1, an empty
string is returned. This function only works correctly with single-byte characters.
Result type: VARCHAR(n)
Syntax:
substrlen (str, startpos, length)
Declaration:
DECLARE EXTERNAL FUNCTION substrlen
CSTRING(255) NULL, SMALLINT, SMALLINT
RETURNS CSTRING(255) FREE_IT
ENTRY_POINT 'IB_UDF_substrlen' MODULE_NAME 'ib_udf'
The above declaration is from the file ib_udf2.sql. The NULL after the argument is an optional
addition that became available in Firebird 2. If the argument is declared with the NULL keyword,
the engine will pass a NULL argument value unchanged to the function. This leads to a NULL result,
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