UART driver: read function

[Daniel Krüger]

Hello,

what is the right implementation for the UART driver read function 
(console_fns .deviceRead)? It seems to be implemented differently from 
target to target. Some implementations return -1 if the receive FIFO is 
empty and some implementations do a busy waiting for new characters.

I would like to use some Posix application code, which switches stdin 
into non-blocking mode and uses getchar() to read from UART. getchar() 
shall return EOF if no character is available.

This code requires that the UART driver read function does not block, 
otherwise getchar() will block. But a non-blocking UART driver is not 
sufficient. The function __srefill_r() in newlib/libc/stdio/refill.c 
sets the flag __SEOF whenever the driver's read function returns no 
characters. So on subsequent calls of getchar() and hence __srefill_r() 
the UART driver's read function is not called anymore, because __SEOF 
flag is set (see if statement at beginning of __srefill_r()).

My current work-around is to call clearerr(stdin) before calling 
getchar(). clearerr() resets the flag __SEOF, so getchar() will get the 
current character from the UART driver.

There are many other functions and modules (e.g. console*.c, 
rtems_termios*, ...) involved between getchar() and the UART driver's 
read function. So it's a little bit hard to dig into that issue and to 
find a good solution.

Best regards,
   Daniel Krüger

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