C/C++ Programmer's Guide (G06.25+)

Mixed-Language Programming for TNS/R and
TNS/E Native Programs
HP C/C++ Programmer’s Guide for NonStop Systems—429301-008
8-20
Considerations When Interfacing to pTAL
Arrays of Structures
If you specify bounds when you declare a pTAL structure, you create an array of
structures. The following pTAL and C arrays of structures are equivalent. Each
declaration contains an array of ten structure occurrences:
pTAL Code C Code
STRUCT cell (
*
); struct cell
BEGIN {
INT x; short x;
STRING y; char y;
END; };
STRUCT .EXT tcell(cell)[0:9]; struct cell ccell [10];
PROC honey (c); void JOANIE
INT .EXT c (cell); (struct cell *c);
EXTERNAL;
Redefinitions and Unions
Variant records are approximated by pTAL structure redefinitions and C unions.
An pTAL redefinition declares a structure item that uses the same memory location as
an existing structure item. The existing structure item can be a simple variable, array,
substructure, or pointer that:
•
Begins on a word boundary
•
Is at the same BEGIN-END level in the structure as the redefinition
•
Is the same size or larger than the redefinition
A C union defines a set of variables that can have different data types and whose
values alternatively share the same portion of memory. The size of a union is the size
of its largest variable; the largest item need not come first.
Although a union typically begins on a word boundary, native C sometimes misaligns
numeric data that is inside a union. In a union, a numeric data item (short, long, int, or
long long) is misaligned if the data item is at an odd byte offset from the beginning of
the union, and the union itself begins at an odd byte address. If possible, you should
rearrange the data structure to ensure that unions begin on an even byte address (and
thus, NMC or CCOMP does not insert an implicit filler byte).
A union typically, but not always, begins on a word boundary. If FIELDALIGN
SHARED2 is in effect, and if the union contains only character data, a union might
begin on an odd-byte (that is, not be word-aligned).
Pointers
Pointers contain memory addresses of data. You must store an address into a pointer
before you use it. In pTAL and C pointer declarations, you specify the data type of the
data to which the pointer points. You must use pointers when sharing global variables.
You can pass pointer contents by value between pTAL and C routines.