support for RS485

[xoac]

Hi,
There exist crate rs485 that define const from libc. The problem is that ioctls change signature depending on system platform (for example it accept c_int when using musl on x86 and c_ulong when use glibc on x86_64). So compilation fill when musl is involved:

rust-musl-builder-armv7 cargo build --examples                                                                                                              18:21:01 
   Compiling rs485 v0.1.0
error[E0308]: mismatched types
  --> /root/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/rs485-0.1.0/src/lib.rs:92:45
   |
92 |         let rval = unsafe { libc::ioctl(fd, TIOCGRS485, &mut conf as *mut SerialRs485) };
   |                                             ^^^^^^^^^^ expected i32, found u32

error[E0308]: mismatched types
   --> /root/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/rs485-0.1.0/src/lib.rs:186:45
    |
186 |         let rval = unsafe { libc::ioctl(fd, TIOCSRS485, self as *const SerialRs485) };
    |                                             ^^^^^^^^^^ expected i32, found u32

It would be nice to add this constants (TIOCGRS485, TIOCSRS485) to libc. I am not sure how to do it properly. For musl target constant can be found here:
http://git.musl-libc.org/cgit/musl/tree/arch

in most cases looks like this:

#define TIOCGRS485	0x542E
#define TIOCSRS485	0x542F

but there are different here:
http://git.musl-libc.org/cgit/musl/tree/arch/mips64/bits/ioctl.h#n61

[alexcrichton]

Looks like you've got a PR for this, which sounds great to me!

[gnzlbg]

This was closed by #1094 right ?

[xoac]

@gnzlbg I think no -- I just added implementation to musl target and it's still missing for others. I don't need more since I use just musl -- but I think rs485 crate will still fail to compile on others platform.

[gnzlbg]

The RS485 crate define the constants itself, so it cannot fail to compile AFAICT.

[xoac]

It can fail c_int and c_ulong are different types. Since libc (this crate) didn't define it. So rs485 failed to compile on musl buf successful for example on glibc.
libc::ioctl on musl required c_int (i32) but got c_long(u32) [as defined in RS485 crate]. Now when libc define TIOCGRS485 and TIOCSRS485 it can be used in rs485 crate - and it will successfully compile on all defined platforms <3