Variable padding to alignment boundaries

[Teuxe]

Hi,
I have not found how to have a struct containing a null-terminated char[] and also containing NUL bytes as padding to the next 32-bit boundary (in other file types, this boundary could be every 16 bits, every 64 bits...). Say it otherwise: in a size where you could put 32-bit words (we don't know how many), there is a null-terminated char inside whose size is determined by the first NUL byte. The padding is not necessarily filled with NUL (but usually is).

Examples with 32-bit alignment:

48 65 79 00 DE AD BE EF 55 AA 55 AA
H e y . . . . . 7 . 7 .
^ ^ ^
string...\0 aligned data (no padding)

48 65 6C 6C 6F 00 00 00 DE AD BE EF 55 AA 55 AA
H e l l o . . . . . . . 7 . 7 .
^ x x ^ ^
string... \0 padd. aligned data (2 bytes for padding)

48 65 6C 6C 6F 21 21 00 DE AD BE EF 55 AA 55 AA
H e l l o ! ! . . . . . 7 . 7 .
^ ^ ^
string... \0 aligned data (no padding)

48 65 6C 6C 6F 20 79 6F 75 21 00 00 DE AD BE EF 55 AA 55 AA
H e l l o y o u ! . . . . . . 7 . 7 .
^ x ^ ^
string... \0 p. aligned data (1 byte for padding)

Is there currently a way to do that?
Thanks,

[WerWolv]

Hey
The easiest way is probably to use std::mem::AlignTo.

#include <std/mem.pat>

struct MyString {
    char value[];
    std::mem::AlignTo<8>;
};

This will include one null-byte in the value string and then advance the cursor forward to the next 8-Byte boundary.
If you don't want the null byte be included, you can use

#include <std/mem.pat>

struct MyString {
    char value[while($[$] != 0x00)];
    std::mem::AlignTo<8>;
};

[paxcut]

For any offset and alignment power of two the number of bytes needed to pad to the next aligned address is

-offset & (align - 1)

this assumes offset is unsigned or 2's complement arithmetic is used. Similarly, the next aligned address can be calculated using

(offset + (align - 1)) & -align