(mysql.info.gz) UDF return values

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 25.2.2.4 Return Values and Error Handling
 .........................................
 
 The initialization function should return `0' if no error occurred and
 `1' otherwise.  If an error occurs, `xxx_init()' should store a
 null-terminated error message in the `message' parameter.  The message
 will be returned to the client.  The message buffer is
 `MYSQL_ERRMSG_SIZE' characters long, but you should try to keep the
 message to less than 80 characters so that it fits the width of a
 standard terminal screen.
 
 The return value of the main function `xxx()' is the function value, for
 `long long' and `double' functions.  A string functions should return a
 pointer to the result and store the length of the string in the
 `length' arguments.
 
 Set these to the contents and length of the return value.  For example:
 
      memcpy(result, "result string", 13);
      *length = 13;
 
 The `result' buffer that is passed to the calc function is 255 byte
 big. If your result fits in this, you don't have to worry about memory
 allocation for results.
 
 If your string function needs to return a string longer than 255 bytes,
 you must allocate the space for it with `malloc()' in your `xxx_init()'
 function or your `xxx()' function and free it in your `xxx_deinit()'
 function.  You can store the allocated memory in the `ptr' slot in the
 `UDF_INIT' structure for reuse by future `xxx()' calls.   UDF
 calling.
 
 To indicate a return value of `NULL' in the main function, set
 `is_null' to `1':
 
      *is_null = 1;
 
 To indicate an error return in the main function, set the `error'
 parameter to `1':
 
      *error = 1;
 
 If `xxx()' sets `*error' to `1' for any row, the function value is
 `NULL' for the current row and for any subsequent rows processed by the
 statement in which `XXX()' was invoked.  (`xxx()' will not even be
 called for subsequent rows.) * In MySQL versions prior to
 3.22.10, you should set both `*error' and `*is_null':
 
      *error = 1;
      *is_null = 1;
 

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(mysql.info.gz) UDF arguments

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