redirecting problems

scatterbrain · 07-26-2004, 12:17 PM

Hi:

My program reads in from stdin, example:

./myprogram < data

When I try to redirect the output to another file it works fine, example:

./myprogram < data > new.data

But when I try to redirect the output back into the same input file I get an error from my program for some reason, example:

./myprogram < data > data

What I don't get is why my program would care what file I'm redirecting the output to? Because it doesnt seem to like receiving input and directing output to the same file.

Any ideas? Thanks..

aluser · 07-26-2004, 12:27 PM

You can't get there from here. The problem is that when the shell sees "> foo", it truncates foo immediately, before running the command line. Your program will see an empty file, and I'm guessing that the error comes from that. Posting what the error is would help though.

example:

Code:

13:24 aluser@alf:/tmp$ echo blah > foo
13:24 aluser@alf:/tmp$ cat < foo > foo
13:24 aluser@alf:/tmp$ cat foo
13:24 aluser@alf:/tmp$

The contents of foo disappear with the cat < foo > foo line.

scatterbrain · 07-26-2004, 12:30 PM

I see, so is there any way around that besides creating a temp file then renaming it?

aluser · 07-26-2004, 12:42 PM

either that or take a filename as an argument, read the whole thing into memory, truncate it, and write it back out.

scatterbrain · 07-26-2004, 12:46 PM

Renaming a temp would be easier in my case. Thanks for your help! Cheers.