Header_Os_Specific :: struct {
    magic:   u32;
}

Read_Channel_Os_Specific :: struct {
    shm: s32;
}

Write_Channel_Os_Specific :: struct {
    shm: s32;
}

//
// Reader stuff:
//

// O_EXCL: the caller unlinks any stale object first, so an existing one belongs to a live reader.
reader_create_channel_os_specific :: (name: string, bytes: s64) -> (success: bool, channel: Read_Channel, already_exists: bool = false) {
    P :: Posix;

    total_bytes := bytes + OFFSET_TO_MESSAGE_DATA;
    c_name := Basic.temp_c_string(name);

    shm := P.shm_open(c_name, P.O_CREAT | P.O_RDWR | P.O_EXCL, CHANNEL_FILE_MODE);
    if shm == -1 {
        error_code, error_string := System.get_error_value_and_string();
        if error_code == P.EEXIST  return false, .{}, true;
        Basic.log_error("Unable to map a view of '%' for % bytes (which is the user-requested % bytes, plus overhead). Error code %, %", name, total_bytes, bytes, error_code, error_string);
        return false, .{};
    }

    do_truncate := true;
    #if OS == .MACOS {
        // On macOS, ftruncate can only be called once after creation of the shared memory object.
        // Every subsequent call fails with EINVAL. So let’s check the size and only call ftruncate if it doesn’t match.
        //  -rluba, 2023-05-15
        mem_stat: P.stat_t;
        stat_result := P.fstat(shm, *mem_stat);
        if stat_result == -1 {
            error_code, error_string := System.get_error_value_and_string();
            Basic.log_error("Unable to stat '%'. Error code %, %. The file descriptor was %.", name, error_code, error_string, shm);
            P.close(shm);
            return false, .{};
        }

        if mem_stat.st_size != 0 {
            if mem_stat.st_size >= total_bytes {
                // Already has the right size, don’t try to ftruncate again. It would fail on macOS.
                // We’re comparing with ">=" instead of "=" because the actual size seems to always be slightly bigger than the one requested via ftruncate.
                do_truncate = false;
            } else {
                // We need to remove the old memory object and re-create it to change its size.
                P.close(shm);
                unlink_result := P.shm_unlink(c_name);
                if unlink_result != 0 {
                    error_code, error_string := System.get_error_value_and_string();
                    Basic.log_error("Unable to remove old '%' to re-create it with the right size. Error code %, %.", name, error_code, error_string);
                    return false, .{};
                }

                shm = P.shm_open(c_name, P.O_CREAT | P.O_RDWR | P.O_EXCL, CHANNEL_FILE_MODE);
                if shm == -1 {
                    error_code, error_string := System.get_error_value_and_string();
                    Basic.log_error("Unable to re-create a view of '%' for % bytes (which is the user-requested % bytes, plus overhead). Error code %, %", name, total_bytes, bytes, error_code, error_string);
                    return false, .{};
                }
            }
        }
    }


    if do_truncate {
        truncate_result := P.ftruncate(shm, total_bytes);
        if truncate_result == -1 {
            error_code, error_string := System.get_error_value_and_string();
            Basic.log_error("Unable to ftruncate '%' for % bytes (which is the user-requested % bytes, plus overhead). Error code %, %. The file descriptor was %.\n", name, total_bytes, bytes, error_code, error_string, shm);
            return false, .{};
        }
    }

    // Remove the name now, object stays alive while fd/mmap refs exist
    // if (P.shm_unlink(c_name) < 0) {
    //     Basic.log_error("shm_unlink failed.");
    //     return false, .{};
    // }

    flags: s32 = P.MAP_SHARED;
    #if OS == .LINUX {
        flags |= P.MAP_POPULATE; // Not available on macOS.
    }
    pointer := P.mmap(null, cast(u64) total_bytes, P.PROT_READ | P.PROT_WRITE, flags, shm, 0);
    if pointer == P.MAP_FAILED {
        error_code, error_string := System.get_error_value_and_string();
        Basic.log_error("Unable to mmap for the channel's memory segment, name '%' bytes '%'. Error code %, %. The file descriptor was %.\n", name, bytes, error_code, error_string, shm);
        P.close(shm);
        return false, .{};
    }

    data := cast(*u8) pointer;
    header_os := cast(*Header_Os_Specific)(data + OFFSET_TO_HEADER_OS_SPECIFIC);
    header_os.magic = (.*) cast(*u32) HEADER_OS_MAGIC.data;  // @Endian
    Basic.Initialize(header_os);

    channel: Read_Channel;
    channel.header          = cast(*Header) pointer;
    channel.os_specific.shm = shm;

    return true, channel;
}

reader_release_channel :: (channel: *Read_Channel, channel_name: string) -> bool {
    Posix.shm_unlink(Basic.temp_c_string(channel_name)); // closes kernel object.
    
    data := cast(*u8) channel.header;
    if !data return false;

    Posix.munmap(data, cast(u64) channel.header.full_memory_region_size_in_bytes);

    os := *channel.os_specific;
    if os.shm  Posix.close(os.shm);

    return true;  // We currently don't check for errors.
}


//
// Writer stuff:
//
writer_connect :: (name: string, name_not_found_is_quiet := false) -> (success: bool, channel: Write_Channel) {
    P :: Posix;

    c_name := Basic.temp_c_string(name);
    shm := P.shm_open(c_name, P.O_RDWR, 0);
    if shm == -1 {
        error_code, error_string := System.get_error_value_and_string();
        Basic.log_error("In writer_connect(), unable to map a view of name '%'. Error code %, %\n", name, error_code, error_string);
        return false, .{};
    }

    header_pointer := P.mmap(null, size_of(Header), P.PROT_READ | P.PROT_WRITE, P.MAP_SHARED, shm, 0);
    if header_pointer == P.MAP_FAILED {
        error_code, error_string := System.get_error_value_and_string();
        Basic.log_error("In writer_connect(), unable to mmap for the channel's memory segment, name '%'. Error code %, %. The file descriptor was %.\n", name, error_code, error_string, shm);
        P.close(shm);
        return false, .{};
    }

    // Look inside the header to see how big our view should be.
    header_success, actual_bytes := get_size_of_full_mapping_from_stub_header(header_pointer);

    P.munmap(header_pointer, size_of(Header));

    if !header_success {
        P.close(shm);
        return false, .{};
    }

    // Now make a new pointer that is the full size.
    header_pointer = P.mmap(null, cast(u64) actual_bytes, P.PROT_READ | P.PROT_WRITE, P.MAP_SHARED, shm, 0);
    if header_pointer == P.MAP_FAILED {
        error_code, error_string := System.get_error_value_and_string();
        Basic.log_error("In writer_connect(), unable to mmap for the channel's memory segment, name '%'. Error code %, %\n", name, error_code, error_string);
        P.close(shm);
        return false, .{};
    }

    data := cast(*u8) header_pointer;
    header_os := cast(*Header_Os_Specific)(data + OFFSET_TO_HEADER_OS_SPECIFIC);

    Basic.Initialize(header_os);
    header_os.magic   = (.*) cast(*u32) HEADER_OS_MAGIC.data;  // @Endian

    channel: Write_Channel;
    channel.header = cast(*Header) header_pointer;
    channel.os_specific.shm = shm;

    return true, channel;
}

writer_disconnect :: (channel: *Write_Channel) -> bool {
    Posix.munmap(channel.header, cast(u64) channel.header.full_memory_region_size_in_bytes);
    Posix.close(channel.os_specific.shm);
    return true;
}

#scope_module
Posix   :: #import "POSIX";
System  :: #import "System";

HEADER_OS_MAGIC :: "ShmP";
CHANNEL_FILE_MODE :: 0x180;   // 0600: the owner only; the name already carries the uid