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132 Pascal 4.0 User’s Guide

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

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.

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 }

The commands to compile and

execute ProcPar.c and

ProcParMain.p

hostname% cc -c ProcPar.c

hostname% pc ProcPar.o ProcParMain.p

hostname% a.out

Hello from PROC_PASCAL: Called from C

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