Peter Wilson

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12 years ago.

how do I make printf use my putc routine ?

Hi,

I'm new to c++ and i'm struggling with the concept of libraries and classes so I'm trying to keep things simple, by avoiding them.

How do i make the built in printf function use my own implementation of a putc routine (not in a library, just a function). I have an LCD screen that my own routines use fine, but I'd liek to uise the more advanced text formatting of printf and I can't make it use my putc routine...

Any help appreciated.

Thanks,

Peter

3 Answers

Richard Ellingworth
12 years ago.

This might not be answering the question but if you want to keep things simple use sprintf to write to a character buffer and then call your custom putc function on each character in the buffer up to the zero character at the end. Like this:

int appleCount = 10;
char buf[ 30 ];
sprintf( buf, "I have %d apples.\r\n", appleCount );
char *ptr = buf;
while( *ptr != (char)0 ) {
    myPutc( *ptr++ );
}

Make sure you use a big enough character array to hold the entire message.

Accepted Answer

Thats exactly how I was solving the 'issue' in the short term and it works fine, but its just a little awkward. I'd hoped for something like the way that the libraries can simply define a putch function and it auto-magically gets used by printf.

posted by Peter Wilson 19 Nov 2012
ceri clatworthy
12 years ago.

if you look through some of the examples in the cookbook and the forums. there are examples but I think you just write your own putc block.

apologies - I only have phone today.

Ceri

Wim Huiskamp
12 years ago.

You can use the printf() in your own code by creating a class thas inherits from 'Stream' and implements putc().

Have a look at the example TextLCD library:

class TextLCD : public Stream {

...
}

The link to .h is here: https://mbed.org/users/simon/code/TextLCD/docs/44f34c09bd37//TextLCD_8h_source.html

The link to .cpp is here: https://mbed.org/users/simon/code/TextLCD/docs/44f34c09bd37//TextLCD_8cpp_source.html

The lib is described here: https://mbed.org/users/simon/code/TextLCD/docs/44f34c09bd37//classTextLCD.html#a98631211a4a8aee62f572375d5b637be

OK... technically I'm clueless about c++ and when I look at the .h and .cpp files I am totally out of my depth, not a clue what is going on there, but I've seen a few implementations that do it this way. I backed away from libraries since I was challenged with using two libraries that both wanted to control the same SPI device with different protocols and simply wouldn't co-exist peacefully, i stripped the code into my main .c and things work fine, but the printf problem then raises its head... for the moment I'm getting by with sprintf and a putch of the resulting strings... its OK, not ideal but it works.

posted by Peter Wilson 19 Nov 2012