selectMachOSlice

fn selectMachOSlice(
    all_mapped_memory: []align(std.heap.page_size_min) const u8,
    arch: std.Target.Cpu.Arch,
) Error![]const u8 {
    // In most cases, the file we just mapped is a Mach-O binary. However, it could be a "universal
    // binary": a simple file format which contains Mach-O binaries for multiple targets. For
    // instance, `/usr/lib/dyld` is currently distributed as a universal binary containing images
    // for both ARM64 macOS and x86_64 macOS.
    if (all_mapped_memory.len < 4) return error.InvalidMachO;
    const magic = std.mem.readInt(u32, all_mapped_memory.ptr[0..4], .little);

    // The contents of a Mach-O file, which may or may not be the whole of `all_mapped_memory`.
    const mapped_macho = switch (magic) {
        macho.MH_MAGIC_64 => all_mapped_memory,

        macho.FAT_CIGAM => mapped_macho: {
            // This is the universal binary format (aka a "fat binary").
            var fat_r: Io.Reader = .fixed(all_mapped_memory);
            const hdr = fat_r.takeStruct(macho.fat_header, .big) catch |err| switch (err) {
                error.ReadFailed => unreachable,
                error.EndOfStream => return error.InvalidMachO,
            };
            const want_cpu_type = switch (arch) {
                .x86_64 => macho.CPU_TYPE_X86_64,
                .aarch64 => macho.CPU_TYPE_ARM64,
                else => unreachable,
            };
            for (0..hdr.nfat_arch) |_| {
                const fat_arch = fat_r.takeStruct(macho.fat_arch, .big) catch |err| switch (err) {
                    error.ReadFailed => unreachable,
                    error.EndOfStream => return error.InvalidMachO,
                };
                if (fat_arch.cputype != want_cpu_type) continue;
                if (fat_arch.offset + fat_arch.size > all_mapped_memory.len) return error.InvalidMachO;
                break :mapped_macho all_mapped_memory[fat_arch.offset..][0..fat_arch.size];
            }
            // `arch` was not present in the fat binary.
            return error.MissingDebugInfo;
        },

        // Even on modern 64-bit targets, this format doesn't seem to be too extensively used. It
        // will be fairly easy to add support here if necessary; it's very similar to above.
        macho.FAT_CIGAM_64 => return error.UnsupportedDebugInfo,

        else => return error.InvalidMachO,
    };
    return mapped_macho;
}