Re: fflush not always flushing

[Jonathan Larmour <jifl at eCosCentric dot com>]

Andrew Lunn wrote:
> fflush() does not always flush. This little program demonstrates the
> problem:
> [snip]
> fdopen on an interactive device sets up the stream to not do
> buffering. ie flags.buffering is false. When fprintf does its write to
> the stream, it passes the output straight down to the next level.
> When fflush is called, it checks if there is anything in the stream
> buffer. If there is, it writes it and then call the flush on the level
> below. Now since the output of fprintf went straight down, the buffer
> is empty and so the flush is not done and the output can get stuck. 
> This patch fixes the problem.
How can it if it also skips the write if len==0? And even if we did the 
write regardless, if we aren't using the fileio library, we'd still 
stumble over a check for len==0 in 
io/common/current/src/iosys.c:cyg_io_write().

Jifl

> Index: language/c/libc//stdio/current/src/common/stream.cxx
> ===================================================================
> RCS file: /cvs/ecos/ecos/packages/language/c/libc/stdio/current/src/common/stream.cxx,v
> retrieving revision 1.5
> diff -u -r1.5 stream.cxx
> --- language/c/libc//stdio/current/src/common/stream.cxx        23 May 2002 23:07:18 -0000      1.5
> +++ language/c/libc//stdio/current/src/common/stream.cxx        6 Sep 2002 17:41:20 -0000
> @@ -543,16 +543,10 @@
>      if ( !flags.opened_for_write )
>          return EINVAL;
>  
> -    // shortcut if nothing to do
> -    if (io_buf.get_buffer_space_used() == 0)
> -        return ENOERR;
> -        
>      len = io_buf.get_buffer_addr_to_read( (cyg_uint8 **)&buffer );
>      
> -    CYG_ASSERT( len > 0, 
> -                "There should be data to read but there isn't!");
> -
> -    write_err = cyg_stdio_write(my_device, buffer, &len);
> +    if ( len )
> +      write_err = cyg_stdio_write(my_device, buffer, &len);
>  
>      // since we're doing a concerted flush, we tell the I/O layer to
>      // flush too, otherwise output may just sit there forever

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