GLOBALS variables usage

This topic describes differences between I4GL and FGL regading global variables definitions and usage.

Avoid GLOBALS

Important:

Using GLOBALS is strongly discouraged. This feature is supported for backward compatibility, to compile legacy source code. Instead of GLOBALS, use PUBLIC symbols in modules to be imported with IMPORT FGL.

Why not globals? Global variables visible and modifiable across the entire program code is an old concept that reduces modularity and flexibility. GLOBALS implements specific semantics that restrain source code organization; for example, it is allowed to specify a file path such as GLOBALS "../../common/myglobals.4gl". The globals file can define functions after the GLOBALS block that are ignored when including such file.

Conversely, the IMPORT FGL instruction brings more flexibility and allows a better organization of your source code, by grouping public variables in modules where other symbols (functions, types, contants) belong to the same domain.

Mismatching global variable definitions

The c4gl C-code compiler of IBM® Informix® 4GL has a weakness that allows global variable declarations of the same variable with different data types. Each different declaration found by the c4gl compiler defines a distinct global variable, which can be used separately. This can actually be very confusing (the same global variable name can, for example, reference a DATE value in module A and an INTEGER value in module B).

IBM Informix 4GL RDS (fglpc / fglgo) does not allow multiple global variable declaration with different types. The fglgo runner raises error -1337 if this happens.

The next code example shows two .4gl modules defining the same global variable with different data types:

Main.4gl:

GLOBALS
  DEFINE v INTEGER
END GLOBALS
...
MAIN
  ...
  LET v = 123
  ...
END MAIN

Module.4gl:

GLOBALS
  DEFINE v DATE
END GLOBALS
...
FUNCTION test()
  ...
  LET v = TODAY
  ...
END FUNCTION

The fglcomp tool compiles both modules separately without problem, but when linking with fgllink or fglrun -l, the linker raises error -1337.

You must review your code and use the same data type for all global variables having the same name.

Duplicated global variable definition

IBM Informix 4GL produces a compilation error when same variable and type is defined in a GLOBALS / END GLOBALS section in the current module, and the globals file included with a GLOBALS "filename" instruction that defines the same variable.

Genero BDL accepts the duplicated global variable definition, as long as the types match. This is supported for backward compatiblity with older Genero and I4GL versions.

GLOBALS
    DEFINE var1 INTEGER
END GLOBALS

GLOBALS "myglobals.4gl"
GLOBALS
    DEFINE var1 INTEGER
END GLOBALS
MAIN
    DISPLAY "var1 = ", var1
END MAIN

As a general advice, avoid the use of global variables and use modules variables with PUBLIC scope.