12/2010			User Defined Functions


UDFs are programs that can be called from wgrib2.  Data to the UDF
will be transfered by a file and the data from the UDF will be transfered
by another file.

Some possible UDF could be

	program to calculate the depth of the 20C isotherm
	program to calculate the specific humidity from the RH, temp, pressure
	program to calculate the wind chill



UDF will be implemented in 3 phases.



				Phase 1: Implemented

Calls to external programs by the -sys option.  

				Phase 2: Implemented

Create calling argument file with -udf_arg
Call udf with -udf
Return parameter overwrite the "data".


				Phase 3: to do

Modify RPN.c so the register 0..9 are replaced by register with names.


				Phase 4: to do

Change call to udf so it can call with named parameters.
Change call to udf so it can return named parameters.
The -udf syntax will be a extended, not replaced


Pass input/output by files.

File protocol.
			Binary
			Either header or no-header format

			Each element has a 3 element structure

			{
				int desc[3];		// 4 byte integer
							// desc[0] = strlen(name) + 1
							// desc[1] = number of elements
							// desc[2] = type  1=char, 128+4=4 byte int  256+4=4 byte float
							// size of data element is desc[2] % 8
				char name[desc[0]]	// 0 terminated character string, 

				char data[N]		// N = desc[1] * (desc[2] % 8)

			}