printf support and extensions

[chqrlie]

Here is an idea to extend printf for custom types and potentially custom conversions:

There is a potential problem with %b and %a conversions in some architectures: clang complains on my macbook that %lb has undefined behavior. For systems that do not support %b or %a, we could convert these formats into %s, passing the return value of a custom format_x type function taking the format flags and the width and precision values:

For example u64 v = ...; printf("value: %#064b\n", v); could be turned into

    uint64_t v = ...;
    printf("value: %s\n", u64_format_b(v, "#0", 64, 1, (char[128]){}, 128);

More generally, this system can be used for other unsupported formats, or for extended formats implemented as type functions:

    module test;
    Point p = { 1, 2 };
    ...
    printf("point: %d\n", p);

Assuming a type function Point.format_d is found with the appropriate prototype, this could generate:

    printf("point: %s\n", test_Point_format_d(v, "", 0, 1, (char[128]){}, 128));

[bvdberg]

%a ? never heard of it. It's not even in lists i find online.. Is that a Mac thing?

We think generating a u64_format_b() for architectures that dont support it is fine . This could be implemented in just the generator. I dont want to solve all problems for everyone, so if they want to print their custom types, they have to solve it. Printf is quite generic, but i think allowing more argument might cause bugs (because people add their type instead of something else)
Does that make sense?

[chqrlie]

%a ? never heard of it. It's not even in lists i find online.. Is that a Mac thing?

%a and %A are for hexadecimal floats, part of Standard C since C99. Much simpler to read and write without rounding issues (what an understatement!)

We think generating a u64_format_b() for architectures that dont support it is fine . This could be implemented in just the generator.

OK.

The format string reconstruction could be performed in the analyser so all generators can take advantage of this feature.

I dont want to solve all problems for everyone, so if they want to print their custom types, they have to solve it. Printf is quite generic, but i think allowing more argument might cause bugs (because people add their type instead of something else) Does that make sense?

The current implementation does not work properly for integer types larger than int if the format string is not a literal. This needs a fix or at least a warning. Definitely more urgent.