Printf redirect to STDOUT

You have similar issues on a PC - if you use full buffering, line buffering or unbuffered streams.

What exactly does your redirection code look like? Using a USB VCP connection?

http://www.keil.com/support/man/docs/gsac/gsac_retargetcortex.htm

I am not doing any thing special was just following Tutorial in Keil ARM Document saying
"Retarget Input and Output via UART"

There seems to be bug with Keil provided retarget_io.c

I had to modify fputc like below in my C file so problem is now resolved. Now putchar and printf works even for single characters, earlier it used to output only when it reaches 64 bytes.

It was nothing to do with PC buffer.

NEW

__weak int fputc (int c, FILE * stream) {
    return (stdout_putchar(c));
}

OLD Keil Provided code

__weak int fputc (int c, FILE * stream) {

#ifdef RTE_Compiler_IO_STDOUT
  if (stream == &__stdout) {
#if (STDOUT_CR_LF != 0)
    if (c == '\n') stdout_putchar('\r');
#endif
    return (stdout_putchar(c));
  }
#endif

#ifdef RTE_Compiler_IO_STDERR
  if (stream == &__stderr) {
#if (STDERR_CR_LF != 0)
    if (c == '\n') stderr_putchar('\r');
#endif
    return (stderr_putchar(c));
  }
#endif

  return (-1);
}

"It was nothing to do with PC buffer."

Notice that I talk about generic printf() output and buffering. So nothing PC-specific, but something that exists outside of the embedded world. setbuf() and setvbuf() are part of the C standard, and controls buffering of FILE-based stream output which includes printf(), fprintf(), ...

When in full-buffer mode, you get data emitted when the buffer is full.
In line-buffer mode, you get data emitted at end of strings.
In unbuffered mode, every single character gets emitted.

Not sure what bug you think you found in Keils fputc function - it's quite common to have support for auto-translation of '\n' into "\r\n" line breaks, since that's the standard on the Windows platform. While the C language considers the single character '\n' to represent a line break.

The only thing is that most implementations who have such logic makes use of a stream-specific flag to check if fopen() did open the stream in binary or text mode. But in this specific case, the logic is hard-coded for stdout and stderr which are quite logical to be operating in text mode.

"at end of strings" should be "at end of lines".

The following code should work:

printf ("Test");
fflush (stdout);