Effective data transmission across different platforms depends entirely on the data type and length, which can vary due to the memory allocation methods used by each platform. Even if the same data type is used, the resulting data values may differ between platforms. Data can be made compatible across different platforms by using a converted data value from the STRING data type. Although network performance can suffer due to heavier data loads, the Tmax system provides the Field Definition Language (FDL) method in an effort to provide the ultimate solution to these data communication problems.

The Field Definition Language (FDL) is a set of C language functions for defining and manipulating storage structures called field buffers.

A field buffer contains a set of attribute-value pairs in a field: an identifier (field key) and its data value. The identifier-value pair enables parameterized interaction between FDL and data in different kinds of processes. The identifier is used and referenced when data is passed between processes. However, when data processing is required, the data value mapped to the identifier is called and used.

The following describes the major terms used in this guide:

There are many ways to represent a record. The following compares a C language structure with a field buffer.

C Language Data Structure

The following is an example of a C language data structure that contains the ID, name, address, age, and gender information of a student:

struct student {
    long stid;
    char name[10];
    char addr[40];
    short age;
    char sex;
} ;

The structure has five fields: stid, name, addr, age, and sex. Following each field is its declared data type: stid is a long type; name and addr are char types; age is a short type; and sex is a char type.

A variable named 'x' points to the student structure. 'x' allows the structure to be available for the address of a field.

x→stid, x→name, x→addr, x→age, x→sex

To pass a structure to another process, data from each field is sent even if a field is unused. A C language type structure has two disadvantages.

Field Buffer

A field buffer is a data structure that provides an alternative method for dividing records into fields. Unlike C structures, field buffers are a data structure that divides a record into fields and relates the fields. A field name is a field key that points to actual data and its type. The name can be up to 16 characters in length.

The main advantage of the field buffer is data independence. Data independence allows the field type and length to be modified without requiring recompilation of the programs that use these fields. Unlike a C language data structure, which passes all member values, the field key method is able to pass only the fields and values that are in use.

To create a field table, use the fdlc utility with a text file that contains the relevant data.

#demo.f
#name   number  type    flags   comments
INPUT   101     string  -       -
OUTPUT  102     string  -       -

$> fdlc -c -i demo.f -ji -jc webtdemo

package webtdemo;
public interface demo_fdl {
    public int INPUT = (469762149);    /* number: 101 type: string */
    public int OUTPUT = (469762150);  /* number: 102 type: string */
}

$fdlc -c -i demo.f

$fdlc -c -i demo.f -o filename.fdl

FDLFILE=/home/tmax/sample/fdl/filename.fdl