Thread: Creating a printf function

I guess the first thing to do is start by being able to do
my_printf("hello world");

Then you attempt douxcsp ONE letter at a time, testing as you go.

See also Variadic functions - cppreference.com

I might test this near the beginning.

Code:

my_printf("hello world\n");

It should work by default; but, I would test it because if it does not work then weird things might happen.

Tim S.

I think I might need to brush up on using "va_start, va_arg, va_copy, va_end"; I think this would be a simple project doing that.

Anyone disagree?

I learned C back in K & R days; back then there was two ways that were fighting to be the correct way. And, I forgot which way I learned. Never really wrote variable arguments functions in my embedded projects I worked on. But, my new embedded project will be embedded Linux project that might need that skill. It will need things that I have not used since school.

Tim S.

I am guessing the poster is allowed to use itoa() from stdlib.h or a recent assignment had them write a replacement user defined function for it.

Tim S.

I am guessing the original poster is allowed to use fwrite function.

fwrite - cppreference.com

Originally Posted by stahta01

I am guessing the original poster is allowed to use fwrite function.

fwrite - cppreference.com

Decided fwrite was not needed; it was easier to use putc() standard function instead.

fputc, putc - cppreference.com

Tim S.

Code:

int main (void){
  my_printf("hello world\n");
  my_printf("%d%o%x%s%c", 16, 16, 16, "hello world\n", 'z');
  my_printf("The value is %d\n", 16);
}

The above is my current test code; still have work to do.
I am using printf calls for testing and for some functionality.
I still need to remove that; also, need to support 'u' and 'p' specifications.

'u' should be easy; but, I have no idea about 'p'.

It took me a while to find a good website to lookup the functions.
It has been several months or maybe even a year since I did any C coding other than very minor code correction/addition.

Tim S.

It looks like my complier "itoa" does a poor job of handling unsigned number and "itoa" is not a standard function so the poster likely can not use it.

Tim S.

Yeah, we know what your requirements are.

You've also had 5 days to think about it and make an attempt.

We're not going to do all the work for you, just so you have something to hand in with zero effort.

You wanted guidelines, and you have them - what's stopping you?

If you want to simplify further, then start with

Code:

void my_printf(const char *fmt, ... ) {
    char buffer[1000];  // to be replaced by malloc
    // do stuff
}

Make an effort, and post your code when you're actually stuck.

The others have already said a fair bit themselves of where you're going wrong so I won't repeat what they've said, instead I'll give you my insights from coding my own replacement for printf, wprintf etc. First off when recording the text to be printed as is it's faster to use indices, on the other hand since you're clearly new to C programming & probably programming in general you're better off allocating an additional string of equal length to the format string (plus the additional character for the terminating \0 character) and just clearing & copying to it at the start of each top level loop that searches for %, after which you can just print that then clear & re-use the allocation to stringify the given argument, print it, then return to the start of the top level loop, here's a rough code view, no detailed function etc given since you haven't posted anything to garner help of that detail.

Code:

my_printf(char *format, ...)
{
	// allocate string of equal length or more to format, the more being to allow room for conversion of various % arguments that aren't already strings
	for ( ... )
	{
		// clear contents of allocated string
		for ( ... )
		{
			// if not % then copy into next slot of allocation, otherwise end the loop
		}
		// print copied string
		// add on the length of printed string to total printed
		// clear allocation's contents
		// check what came after the %, generate string and print accordingly
		// again add on length of printed string to total printed
	}
	// return total printed
}

...thinking about it, this might actually be faster than my current method, eh, I 'll try it later when I'm done with unimplemented stuff

Originally Posted by awsdert

The others have already said a fair bit themselves of where you're going wrong so I won't repeat what they've said, instead I'll give you my insights from coding my own replacement for printf, wprintf etc. First off when recording the text to be printed as is it's faster to use indices, on the other hand since you're clearly new to C programming & probably programming in general you're better off allocating an additional string of equal length to the format string (plus the additional character for the terminating \0 character) and just clearing & copying to it at the start of each top level loop that searches for %, after which you can just print that then clear & re-use the allocation to stringify the given argument, print it, then return to the start of the top level loop, here's a rough code view, no detailed function etc given since you haven't posted anything to garner help of that detail.

Code:

my_printf(char *format, ...)
{
    // allocate string of equal length or more to format, the more being to allow room for conversion of various % arguments that aren't already strings
    for ( ... )
    {
        // clear contents of allocated string
        for ( ... )
        {
            // if not % then copy into next slot of allocation, otherwise end the loop
        }
        // print copied string
        // add on the length of printed string to total printed
        // clear allocation's contents
        // check what came after the %, generate string and print accordingly
        // again add on length of printed string to total printed
    }
    // return total printed
}

...thinking about it, this might actually be faster than my current method, eh, I 'll try it later when I'm done with unimplemented stuff

Copying the format string multiple times? That seems to be a complicated and (from a performance standpoint) wasteful approach. Why not just iterate through the format string, character by character? (The format string should be const char *, by the way.) If the character is the null terminator, finish. If it's a percent sign, read and handle the format specifier. Otherwise, it's a literal character, so just print it out.

Code:

const char *s = format;
while (*s != '\0') {
    if (*s == '%') {
        // read and handle a format specifier
    } else {
        // print out a literal character
    }
    ++s;
}