Missing c_float and c_double types

[momumi]

Zig doesn't provide c_float and c_double types. C doesn't fully guarantee how the float and double types are implemented, so shouldn't zig provide types for them instead of relying on f32 and f64? If there is a reason for why they are omitted, it should probably be added to the documentation about floats and C interop.

[andrewrk]

I'd have to double check, but I believe I found in the C spec that it defines float to be IEEE-754-2008 binary32 and double to be IEEE-754-2008 binary64. So the types are not necessary because they are redundant with f32 and f64 in Zig.

I agree that the docs or at the very least FAQ should be updated with an explanation of this.

[momumi]

From K&R The C Programming Language A.4.2:

Any of single precision floating point ( float ), double precision floating point ( double ), and extra precision floating point ( long double ) may be synonymous, but the ones later in the list are at least as precise as those before.

For example in avr-gcc float and double are both 32 bits.

Also according to the rust documentation for c_float:

the standard technically only guarantees that it be a floating-point number, and it may have less precision than f32 or not follow the IEEE-754 standard at all

[shawnl]

Also c_longdouble

Also c_char and c_uchar, however I get the argument that the C environment probably should be sane, which makes all of these suggestions a bit superfluous.

[momumi]

@shawnl there's a related issue for c_char #875. It's still relevant for modern stuff eg: Android NDK

[alexrp]

To make this a bit more concrete: RX microcontrollers (which are still in use and have maintained GCC and QEMU ports) have sizeof(float) = sizeof(double) = 4 by default in GCC because the hardware only supports single-precision. It can be changed to sizeof(double) = 8 but this is non-default and uses soft float for double. I do think Zig needs to be able to cope with this, and introducing c_float and c_double seems the natural way to do so.

Edit: The same is true for RL78 microcontrollers. The GCC port for RL78 was added in 2011.

There are also older architectures like VAX and PDP-11 that have weird, non-IEEE float formats, but I don't know if Zig needs to care about those. These architectures are more or less dead, and it doesn't seem to be popular to create non-IEEE float formats anymore.

[alexrp]

@andrewrk just for the record, I'm volunteering to do the implementation work for this feature if approved.