Communication between two programs using file - file-io

Question

I want two separate programs written in Pascal communicate between themselves using additional text file. It works fine for the first 2-3 messages but then it gives run-time error in either sender.pas or receiver.pas Do you know where my mistake is or do you have any suggestion?
First program receiver.pas
var
f : text;
s : string;
begin
{I-}
Assign(f,'main.in');
while true do
begin
reset(f);
while IOResult<>0 do //
begin // Wait until the file
close(f); // is closed by sender
reset(f); //
end; //
if eof(f) then
close(f)
else
begin
readln(f,s);
close(f);
rewrite(f);
close(f);
writeln(s);
end;
end;
{I+}
end.
second program sender.pas
var
f : text;
s : string;
begin
{I-}
Assign(f,'main.in');
while true do
begin
readln(s);
rewrite(f); //
while IOResult<>0 do //
begin // Wait until the file
close(f); // is closed by receiver
rewrite(f); //
end; //
writeln(f,s);
close(f);
end;
{I+}
end.

Answer 1

Some things I noticed:
If rewrite fails, afaik the file was not opened and you don't have to close it? Closing an unopened file might cause runtime errors (though I assume assign will init it safely)
depending on how you use these, there might be in the logic. Namely that after closing a file is directly ready for opening by other apps. In general closed filehandles linger several 100ms till several seconds (depending on filesystem busines). This can cause starvation problems in such schemes. (while it would work in plain dos, which didn't linger, at least not that much)
The reader will crash if the file doesn't exist.
I assume that the {I-}/{I+} is a typo and that your sourcecode really reads {$I-} and {$I+} (note the dollar?)
In Windows pascal versions, read only access is not always locking. Put filemode:=2 as first line in everything.
What compiler is this? Delphi, Free Pascal? Which version?
My new (Free Pascal) receiver code becomes:
uses sysutils;
var
f : text;
s : string;
begin
filemode := 2; // read-only
{$I-}
Assign(f,'main.in');
while true do
begin
reset(f);
while IOResult<>0 do //
begin // Wait until the file
close(f); // is closed by sender
sleep(1000);
reset(f);
end;
if eof(f) then
begin
close(f);
sleep(1000);
end
else
begin
readln(f,s);
close(f);
rewrite(f);
close(f);
writeln(s);
end;
end;
{$I+}
end.
The new sender code is:
uses sysutils;
var
f : text;
s : string;
begin
filemode := 2; // read-only
{$I-}
Assign(f,'main.in');
while true do
begin
reset(f);
while IOResult<>0 do //
begin // Wait until the file
close(f); // is closed by sender
sleep(1000);
reset(f);
end;
if eof(f) then
begin
close(f);
sleep(1000);
end
else
begin
readln(f,s);
close(f);
rewrite(f);
close(f);
writeln(s);
end;
end;
{$I+}
end.

Answer 2

A couple of things: Make sure you check IOResult after every file operation, not just rewrite/reset - don't call close if your reset/rewrite failed - and you probably want something like a Sleep(250) in those retry loops.