fprintf hangs

For the life of me I can't see why the following code hangs in
fprintf():

- (bool) matchesPattern: (NSString *) file
{
unsigned int npatterns = [skipPatterns_ count];
unsigned int i;
for ( i = 0; i < npatterns; i ++ )
{
const char * pattern = [[skipPatterns_ objectAtIndex: i]
cStringUsingEncoding: NSASCIIStringEncoding];
if ( ! pattern )
return FALSE;
// convert file name to c string, making sure it is plain ascii
NSData * fn_data = [file dataUsingEncoding: NSASCIIStringEncoding
allowLossyConversion: YES];
char fn_buf[1000];
unsigned int len = [fn_data length];
len = (len < 999) ? len : 999;
[fn_data getBytes: fn_buf length: len];
fn_buf[len] = 0;
fprintf(stderr, fn_buf );
if ( fnmatch( pattern, fn_buf, 0 /*FNM_LEADING_DIR*/ ) == 0 )
{
// NSLog(@"match %s - %@", pattern, file );
return TRUE;
}
}
return FALSE;
}


The function matchesPattern gets called many times without problems
(apparently).
And when it hangs in fprintf(), I can look at fn_buf in the debugger
and it is a well-formed C-string ...

Does anyone have any idea, why this might occasionally hang? Or what I
can try?

All hints and suggestions will be appreciated.

Best regards,
Gabriel.

>
> That is not sufficient for safely making that call. Consider:
>
>     fprintf( stderr, "%s%d%6f");
>

Thanks a lot for your response!
Yes, you are right.
The fprintf() call was just meant for debugging.
The strings I printed were just path names and didn't contain any
"funny" characters.

Still, I changed it to
fprintf(stderr, "%s\n", fn_buf );
but to no avail. I still hangs ... :-(

Best regards,
Gabriel.