Zero Initialization

Memory-safe programming languages typically initialize their variables and fields to zero, so that there is no way to read an uninitialized value. By default, Wuffs does so too, but in Wuffs' C/C++ form, there is the option to leave some struct fields uninitialized (although local variables are always zero-initialized), for performance reasons:

uint32_t flags = 0;

// Uncomment this line to take the option.

wuffs_foo__bar__initialize(etc, flags);

With or without this flag bit set, the Wuffs compiler still enforces bounds and arithmetic overflow checks. It's just that for potentially-uninitialized struct fields, the compiler has weaker starting assumptions: their numeric types cannot be refined.

Even with this flag bit set, the Wuffs standard library also considers reading from an uninitialized buffer to be a bug, and strives to never do so, but unlike buffer out-of-bounds reads or writes, it is not a bug class that the Wuffs compiler eliminates.

For those paranoid about security, leave this flag bit unset, so that wuffs_foo__bar__initialize will zero-initialize the entire struct (unless the WUFFS_INITIALIZE__ALREADY_ZEROED flag bit is also set).

Setting this flag bit (avoiding a fixed-size cost) gives a small absolute improvement on micro-benchmarks, mostly noticable (in relative terms) only when the actual work to do (the input) is also small. Look for WUFFS_INITIALIZE__LEAVE_INTERNAL_BUFFERS_UNINITIALIZED in the benchmarks for performance numbers.

In Wuffs code, a struct definition has two parts, although the second part's () parentheses may be omitted if empty:

pub struct bar?(
    // Fields in the first part are always zero-initialized.
    x : etc,
    y : etc,
    // Fields in the second part are optionally uninitialized, but are still
    // zero-initialized by default.
    // Valid types for these fields are either unrefined numerical types, or
    // arrays of a valid type.
    //  - "base.u8" is ok.
    //  - "array[123] base.u8" is ok.
    //  - "array[123] base.u8[0 ..= 99]" is not, due to the refinement.
    z : etc,