HP SunSoft Pascal 4.0 manual Procedures and Functions as Parameters, 132

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The Pascal main program, ProcParMain.p, which calls the C procedure, proc_c, passing it the address of the Pascal procedure, proc_pas. The C procedure assigns a value to the string s, and calls the procedure whose pointer it just received. Then the Pascal procedure, proc_pas, writes a literal constant and the string it just received.

The commands to compile and execute ProcPar.c and

ProcParMain.p

program ProcParMain; type

{ Declare a procedure pointer type. }

proc_ptr = ^procedure(var s: string; i: integer);

{Declare an external C procedure which takes a procedure argument.}

procedure proc_c(p: proc_ptr); external c;

procedure proc_pas(var cstr: string; strlen: integer); var

i:integer;

begin

write('Hello from PROC_PASCAL: '); for i := 1 to strlen do

write(cstr[i])

writeln; end; { proc_pas }

begin

{Call the C routine. } proc_c(addr(proc_pas))

end. { ProcParMain }

hostname% cc -c ProcPar.c

hostname% pc ProcPar.o ProcParMain.p hostname% a.out

Hello from PROC_PASCAL: Called from C

Procedures and Functions as Parameters

It is probably clearer to pass a pointer to a procedure than to pass the procedure name itself. See “Procedure Calls: Pascal–C” on page 117.

A procedure or function passed as an argument is associated with a static link to its lexical parent’s activation record. When an outer block procedure or function is passed as an argument, Pascal passes a null pointer in the position normally occupied by the passed routine’s static link. So that procedures and

132 Pascal 4.0 User’s Guide