Help understanding this Macro

[Affan Syed]

Hi,
I have done a lot of user space coding in C, which doesnt really need me to
understand the following MACRO:

#define UARTOutput(__x, __y,__args...) do { \
static char __s[] PROGMEM = __y; \
if (debug_out) \
printf_P(__s, ## __args); \
} while (0)

So now what exactly is happening over here?
Does double underscore do something special? Basically i can see that it is
used as outputting a string to the UART like this
UARTOutput(DBG, "%s", buf);

So it just outputs the string in buf to the UART... but how (Assume that the
UART is somehow initialized.. how is it even going to the UART!!!

Thanks..
Affan

[Jogi Kuenstner]

Re: Help understanding this Macro

Affan Syed wrote:
[color=blue]
> [...]
> So it just outputs the string in buf to the UART... but how (Assume that
> the UART is somehow initialized.. how is it even going to the UART!!![/color]

Through the function printf_P ?
The only solution I can imagine...

Jogi

--
The particular mistake will not be repeated. There are plenty of
mistakes left that have not yet been used.          A. Tanenbaum  
JogiKue@kuenstn er.de

[Walter Roberson]

Re: Help understanding this Macro

In article <cvba00$t89$1@g ist.usc.edu>, Affan Syed <asyed@usc.ed u> wrote:
:I have done a lot of user space coding in C, which doesnt really need me to
:understand the following MACRO:

: #define UARTOutput(__x, __y,__args...) do { \
: static char __s[] PROGMEM = __y; \
: if (debug_out) \
: printf_P(__s, ## __args); \
: } while (0)

:So now what exactly is happening over here?

We can't tell -- you haven't told us anything about printf_P(), which
is not a standard C routine.

:Does double underscore do something special?

The standard reserves __ followed by an upper-case letter
for future and implimentation enhancements, but otherwise NO.

:So it just outputs the string in buf to the UART... but how

printf_P is sending it there.
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