Re: printf and line buffering

[Jeff Johnston <jjohnstn at redhat dot com>]

Andrew Pamment wrote:
> Hi there,
>
> I have been porting newlib to run on my OS, it runs really well, but I 
> am  having a problem and can't work out what I am doing wrong.
>
> Trying to write a shell, I want to print "Mort$ " without a line break,  
> but printf won't do this. I have been going through the newlib source 
> and  found some comments, the first was about defaulting to line 
> buffered if I  don't supply an fcntl system call, so I implemented one, 
> the second was  that as per ANSI standard ttys are by default line 
> buffered.
>
> I can't work out how to switch line buffering off. I have been tracing 
> the  system calls to my OS, I have isatty being called (to which my OS 
> responds  1 (yes)) but no fcntl - so i guess I am not setting flags 
> incorrectly.  Also, putchar works fine, it puts a char immediatly.
>
> I have tried "setvbuf(stdout, NULL, _IONBF, 0);" but that didn't work,

You have to issue the setvbuf before any operations to stdout.  What 
return code do you get back from setvbuf()?  Have you verified via 
debugging that your write syscall is not being called appropriately 
under the covers?

The following simple test case debugged under both x86-linux and mn10300 
newlib has already issued the line "some line" by the time we get to 
line 7: return 0.  What happens on your system?

#include <stdio.h>

int main()
{
   setvbuf(stdout, NULL, _IONBF, 0);
   printf ("some line");
   return 0;
}

-- Jeff J.