AVR Freaks

As printf() is a function in stdio.h I wonder what the documentation for stdio.h in your compiler manual says ;-)

Oh look:

http://www.gnu.org/savannah-chec...

So if you want to know more it looks like your search terms are either fdev_setup_stream or FDEV_SETUP_STREAM (they are different)

The stdiodemo in the compiler examples that John mentioned shows this being used in anger

Cliff

PS I'll move this GCC specific thread to the right forum to appease Bob

Doesn't you copy of the documentation, linked to above, have a section that starts

Lee

Define your term "efficient".

Anyway the redirection through a stream is obviously going to add an "extra layer". If I write:

void uart_init(void) {
    UCSRB= (1 << TXEN);
    UBRR = 37;
}
void uart_putchar(char c) {
    wait_UDRE();
    UDR = c;
}

void uart_puts(char * str) {
    while (*str) {
        uart_putchar(*str++);
    }
}

int main(void) {
    uart_init();
    uart_puts("hello");
}

Then the path to sending characters here is obvious. A pointer to the string "hello" is passed to uart_puts() and it dereferences that pointer and passes each character in turn (until the final 0x00) to the uart_putchar(). It's all very simple.

In the FDEV case I would use something like:

void uart_init(void) {
    UCSRB= (1 << TXEN);
    UBRR = 37;
}
void uart_putchar(char c, FILE * stream) {
    wait_UDRE();
    UDR = c;
    return 0;
}

FILE uart_str = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);

int main(void) {
    uart_init();
    stdout = &uart_str;
    puts("hello");
}

The "hidden bit" here is the system function puts(). It's not that hidden because you can read the source code here:

http://svn.savannah.nongnu.org/v...

So the code is basically this:

ATTRIBUTE_CLIB_SECTION
int
puts(const char *str)
{
	char c;
	int rv = 0;

	if ((stdout->flags & __SWR) == 0)
		return EOF;

	while ((c = *str++) != '\0')
		if (stdout->put(c, stdout) != 0)
			rv = EOF;
	if (stdout->put('\n', stdout) != 0)
		rv = EOF;

	return rv;
}

which actually looks remarkably familiar - this is almost the same as the uart_puts() I wrote above, except that it handles the possibility of the putchar() function returning non 0 (except in my implementation this will never happen as the characters are always reported as sent OK). It also sends a final '\n' at the end. But the key thing is that it's simply calling:

stdout->put(c, stdout)

to send each character. Whereas previously I was using:

uart_putchar(*str++);

so, yes, there is a level of indirection because, instead of just a hard coded CALL to uart_putchar, it is using the "put" member of the FILE structure (that was previously set to point to uart_putchar in the FDEV_SETUP) and will be calling it indirectly using an ICALL.

But other than that these two are almost as "efficient" (whatever that means!).

Of course the advantage of using the stream is that you can now write "normal" C programs that use normal <stdio> functions like puts(), putchar(), printf(). You could even write this on a PC:

int main(void) {
    puts("hello");
    printf("%d / %u = %f", 22, 7, (double)22/7);
    putchar('\n');
}

then just take this to the AVR and add:

int main(void) {
    uart_init();
    stdout = &uart_str;
    puts("hello");
    printf("%d / %u = %f", 22, 7, (double)22/7);
    putchar('\n');
}

and have an expectation that it would work in the same way and produce the same result. The advantage of this is that it only takes about 30 seconds to try:

~$ cat test4.c
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int main(void) {
    puts("hello");
    printf("%d / %u = %f", 22, 7, (double)22/7);
    putchar('\n');
}

~$ gcc -Os test4.c -o test4
~$ ./test4
hello
22 / 7 = 3.142857
~$

so I know what it is going to look like on a PC terminal after I build this for AVR, program it in, connect up the UART and run a terminal program (only I didn't have to do any of that to see what the output might look like!).

And I'll add this for anyone who stumbles on this thread in the future: Why assume that stdio should be tied to the UART? Eg why not an LCD display on PORTB?

a small example of using an LCD display? (a 16x2 standard display that is commonly used)

if it's easy...

it's all, of course, everything "and so it is clear" ... virtually, when in the head and thoughts you imagine ...

a piece of code, as an example, will not be a hindrance for a better understanding