On Wed, 11 Dec 2024 at 11:28, John Reiser <[email protected]> wrote:
> There's a bug in this program. Can you spot it?
> =====
> #include <stdio.h>
>
> int
> main(int argc, char *argv[])
> {
> printf("Hello, world!\n");
> return 0;
> }
> =====
>
> The bug is that EDQUOT (Disk Quota exceeded:
> /usr/include/asm-generic/errno.h) is not detected. If a shell
> re-directs stdout into a file,
> then the data might never be captured, and the problem would go
> undetected until another program tries to read the file,
> but does not find the expected data.
>
> It is a very common programming error to omit the call to close() of an
> output file, *AND/OR* not to check the return value of close().
> ["What could possibly go wrong?" Answer: EDQUOT or ENOSPC.]
> Upon exit(), then the Linux kernel implicitly closes all open file
> descriptors, but *IGNORES ALL ERRORS* in doing so. It's even documented
> on the manpage: "Failing to check the return value when closing a file
> may lead to silent loss of data. This can especially be observed with
> NFS and with disk quota" [or when the output goes into a VM container.]
>
> The app should call close() and check the return value. Before calling
> close(), then a paranoid programmer will call fsync() (and check for
> errors) in order to increase the reliability and integrity of data.
>
>
I have seen this listed as NOT A BUG even with realtime programmers because
the application can be run in all kinds of ways which could induce failures
that are 'environment' versus 'application'. I expect it depends on the
exact environment but what is the correct code? Something like the
following?
===
#include <stdio.h>
#include <stdlib.h>
// This would be better with subroutines but this is supposed to be
// hello world.
int main(int argc, char *argv[]){
int err=0;
err=printf("Hello, world!\n");
if (err < 0){ // we got an error somewhere
err=fclose(stdout);
if (err != 0){ // we got a bigger error
return err;
}
err=fclose(stderr);
if (err != 0){ // we got a bigger error
return err;
}
return 1;
} else {
err=fclose(stdout);
if (err != 0){ // we got a bigger error
return err;
}
err=fclose(stderr);
if (err != 0){ // we got a bigger error
return err;
}
return 0;
}
}
===
--
Stephen Smoogen, Red Hat Automotive
Let us be kind to one another, for most of us are fighting a hard battle.
-- Ian MacClaren
--
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