Z88DK ZX Spectrum "%f" printf not working

Question

I'm attempting to compile a program in C for the ZX Spectrum using Z88DK. However, I'm facing an issue when trying to use printf to print out the value of a float.

The code I'm trying to compile:

#include <stdio.h>
#include <math.h>

int main()
{
    printf("Hello World\n");
    printf("Testing float...\n");
    double pi = 3.141;
    printf("Pi is: %f\n", pi);
    return 1;
}

and I'm compiling with:

zcc +zx -lm -create-app -startup=1 -clib=new  main.c -o floatTest

The program compiles however nothing is printed on the screen where the value of the float should be. Here is a screenshot of the Fuse emulator output of the program above.

I've tried using float and double, adding different formatting options like %06.2f, but nothing seems to work. I can't get anything to display on the screen.

I'm stuck on how to resolve this issue and any help would be appreciated!

Answer 1

Assuming you use the SDCC backend: SDCCs printf does not include float support by default. You need to recompile the z80.lib library with USE_FLOATS=1 set in printf_large.c

Answer 2

With newlib it seems you have to explicitly state that you want to include the floating point formatter with a pragma, just add:

#pragma printf %f

to the source code. That way it will work with both sccz80 and zsdcc as the compiler. By default the integer converters are included.

sccz80 does format detection, but this is only picked up the classic library. So if you want to use zsdcc and classic you'll have to specify them again.

Likewise for scanf the set of converters can be configured using

#pragma scanf converters

There's some documentation on z88dk wiki here: https://github.com/z88dk/z88dk/wiki/Classic--Pragmas#configuring-printf-and-scanf-converters

Answer 3

The z88dk does not differentiate between float or double, at least with its own compiler; the SDCC backend is another beast.

Some years ago I played around with the z88dk, targetting the ZX81. I was kind of disappointed that its implementation of printf() does not support floating point data types. It might have changed in the meantime, but I'm afraid that rather not. A quick look through the Wiki does not reveal anything, even though you might like to check this yourself.

Actually there are different implementations of printf(). The compiler decides by introspection of your source which one to use.

Answer 4

Although the printf implementation z88dk's classic C library doesn't support outputting floats, it does have in its stdlib.h a mostly undocumented function named ftoa that will convert a float to a string:

https://github.com/z88dk/z88dk/blob/master/libsrc/math/genmath/ftoa.c

Answer 5

The zx81 fp support is complete since.. very long time! Printf %f works a well. On a Spectrum there is plenty of fp libraries you can choose, varying in precision and size, including a shortcut to the ROM ones. The same shortcut is available on the zx81.