Passing 4GL Global Variables to C

Overview

We’ve created a solution for accessing 4GL global variables from C code in a form of a Python script (included in Lycia Compiler).

At runtime, as soon as control has been passed to the C code, Lycia copies the values ​​of all registered global 4GL variables to the C side and vice versa. As soon as the 4GL side receives control back from the C code, the values ​​of global variables are copied to the variables on the 4GL side. Therefore, you will have two copies of certain global variables: one on the C side and one on the 4GL side. Lycia will make sure that the values ​​​​are valid at the moment control is passed to the C or 4GL side by registering all the defined 4GL global variables in C.

 

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Prerequisites

To use this solution, you will need:

• Compiler component of Lycia package installed;
• Python version 2.7 or later;
• c4gl utility from Informix for 4GL-to-C conversion.

 

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Sharing Variables

Step 1. Create a .4gl file with C function call and .c file with 4GL function call. You will find an example in the snippets below:

global_fgl.4gl

GLOBALS
    DEFINE var INT                  # Define global variable 'var'
END GLOBALS
MAIN
    LET var = 8                     # Initialize global variable 'var'
    DISPLAY "(init) var = ", var    # Display current value of global variable 'var'
    CALL add_two()                  # Calling a C function that increments the global variable 'var' by two
    DISPLAY "(main) var = ", var    # Display current value of global variable 'var'
END MAIN
FUNCTION mult_two()
    DISPLAY "(mult_two before) var = ", var    # Display current value of global variable 'var'
    LET var = var * 2
    DISPLAY "(mult_two after)  var = ", var    # Display current value of global variable 'var'
END FUNCTION
global_c.c


#include <fglapi.h>
extern int4 var;            // Define external global variable var
void add_two() {
  var = var + 2;            // Increment by two
  fgl_call(mult_two, 0);    // Calling the fgl function, which multiplies by two
  var = var + 2;            // Increment by two
}

 

global_c.c

#include <fglapi.h>
extern int4 var;            // Define external global variable var
void add_two() {
  var = var + 2;            // Increment by two
  fgl_call(mult_two, 0);    // Calling the fgl function, which multiplies by two
  var = var + 2;            // Increment by two
}

 

Step 2. For generating C globals, you need to execute the Python script globals_c.py with a 4GL file that defines globals (or a list of such files, separated with a space):

python /opt/Querix/Lycia/bin/globals_c.py global_fgl.4gl

As a result, the script will generate .c file(s) with shared global variables. The file(s) will have the same name(s) as the .4gl file(s).

Step 3. Include generated file(s) — in our example, global_fgl.c — in C static library.

 

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Need-To-Knows

• Lycia does not check the consistency of global variables between the C and 4GL sides. If they are different for any reason, it may cause the memory errors while copying variables. For existing programs, we strongly recommend to use the script to ensure that the variables are consistent. For new programs, we recommend to pass variables through the stack instead of using 4GL global variables generated from C code.
• The script doesn't support complex data types such as record within a record, ARRAY OF ARRAY, etc.

 

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