std/c: allow overriding c imports for custom OS

[silbinarywolf]

Description

I've been experimenting with getting my app running on the Wii via the DevkitPro toolchain, and I've gotten various things working using the standard library which includes reading/writing files to an SD card.

However to make it work, it required patching upstream with something like this.

How I use it

Used for Wii SDL2 application here:

• https://github.com/silbinarywolf/zig-wii-sdk/blob/7d5088ad5906c4d17077e9894f5aea0cf100de1f/examples/sdl-app/src/main.zig#L18

Example C OS zig files:

• https://github.com/silbinarywolf/zig-wii-sdk/tree/7d5088ad5906c4d17077e9894f5aea0cf100de1f/src/c

main.zig

pubconstos=struct {
pubconstc=@import("c/os.zig");
};

c/os.zig

conststd=@import("std");
constc=@import("c.zig");
/// os is the C operating system definitions, ie. c/wasi.zigpubconstos=@import("wii.zig");
pubconstpthread_mutex_t=@compileError("pthread not supported by Wii library. Must compile single threaded unless we polyfill pthread functions");
// NOTE(jae): 2024-06-03// AT is seemingly not supported for the Wii as it also doesn't support "openat"// we polyfill "openat" ourselves.pubconstAT=struct {
/// Special value used to indicate openat should use the current working directorypubconstFDCWD=-2;
};
// NOTE(jae): 2024-06-03// Copied from lib/std/os/linux.zig, section: .powerpc, .powerpcle, .powerpc64, .powerpc64lepubconstO=packedstruct(u32) {
ACCMODE: std.posix.ACCMODE=.RDONLY,
_2: u4=0,
CREAT: bool=false,
EXCL: bool=false,
NOCTTY: bool=false,
TRUNC: bool=false,
APPEND: bool=false,
NONBLOCK: bool=false,
DSYNC: bool=false,
ASYNC: bool=false,
DIRECTORY: bool=false,
NOFOLLOW: bool=false,
LARGEFILE: bool=false,
DIRECT: bool=false,
NOATIME: bool=false,
CLOEXEC: bool=false,
SYNC: bool=false,
PATH: bool=false,
TMPFILE: bool=false,
_: u9=0,
};

c/wii.zig

constbuiltin=@import("builtin");
conststd=@import("std");
constc=@import("c.zig");
pubconst_errno=struct {
externfn__errno() *c_int;
}.__errno;
pubconstPATH_MAX=4096;
pubconstmode_t=u32;
pubconsttime_t=i64;
pubconstSTDIN_FILENO=0;
pubconstSTDOUT_FILENO=1;
pubconstSTDERR_FILENO=2;
constclockid_t=i32;
// ... much more things in here, etc ...

c/c.zig

pubusingnamespace@cImport({
// OGC Library@cInclude("ogcsys.h");
@cInclude("gccore.h");
// C library@cInclude("stdio.h");
@cInclude("errno.h");
@cInclude("fcntl.h");
@cInclude("dirent.h");
});

Seperate to this we also have a "runtime" we build to polyfill things.
c/runtime.zig

constroot=@import("root");
conststd=@import("std");
constbuiltin=@import("builtin");
constc=@import("c.zig");
comptime {
@export(openat, .{ .name="openat", .linkage=.strong });
@export(write, .{ .name="__wrap_write", .linkage=.strong });
if (builtin.os.tag==.wasi) {
@export(wasi_errno, .{ .name="errno", .linkage=.strong });
}
}
/// openat is polyfilled as it doesn't have an implementation for devkitPPCfnopenat(dirfd: c_int, pathname: [*c]constu8, flags: c_int) callconv(.C) c_int {
// TODO(jae): 2024-06-02// Make this actually use dirfd_=dirfd; // autofixconstfile=c.open(pathname, flags);
returnfile;
}
/// wrap "write" to get printf debugging in Dolphin emulator, if the "fd" is STDOUT or STDERR we call print./// otherwise fallback to writing to the file descriptorfnwrite(fd: i32, buf: [*]constu8, count: usize) callconv(.C) isize {
const__real_write=struct {
extern"c"fn__real_write(fd: std.c.fd_t, buf: [*]constu8, nbyte: usize) isize;
}.__real_write;
if (fd==std.c.STDOUT_FILENOorfd==std.c.STDERR_FILENO) {
// https://stackoverflow.com/a/3767300return@intCast(c.printf("%.*s", count, buf));
}
return__real_write(fd, buf, count);
}
/// wasi_errno calls the underlying Wii toolchain errno for builds targetting .wasifnwasi_errno() callconv(.C) *c_int {
const_errno=struct {
externfn__errno() *c_int;
}.__errno;
return_errno();
}

[alexrp]

I can't speak to whether this direction is desirable, but in any case, it will need rebasing after #20679.

[silbinarywolf]

mmm, I think a better version of this would be good in the future but for the time-being, I've opted to go with a hacky approach with my zig-wii-sdk and target .wasi, then provide those Wasi functions and call the underlying C library code.