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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
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:
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 |