| pagination_prev |
|---|
language-basics/imports |
import CodeBlock from "@theme/CodeBlock";
import AllocatorsAlloc from "!!raw-loader!./01.allocators-alloc.zig"; import AllocatorsFba from "!!raw-loader!./01.allocators-fba.zig"; import AllocatorsArena from "!!raw-loader!./01.allocators-arena.zig"; import AllocatorsCreate from "!!raw-loader!./01.allocators-create.zig"; import AllocatorsGpa from "!!raw-loader!./01.allocators-gpa.zig";
The Zig standard library provides a pattern for allocating memory, which allows the programmer to choose precisely how memory allocations are done within the standard library - no allocations happen behind your back in the standard library.
The most basic allocator is
std.heap.page_allocator.
Whenever this allocator makes an allocation, it will ask your OS for entire
pages of memory; an allocation of a single byte will likely reserve multiple
kibibytes. As asking the OS for memory requires a system call, this is also
extremely inefficient for speed.
Here, we allocate 100 bytes as a []u8. Notice how defer is used in conjunction
with a free - this is a common pattern for memory management in Zig.
{AllocatorsAlloc}
The
std.heap.FixedBufferAllocator
is an allocator that allocates memory into a fixed buffer and does not make any
heap allocations. This is useful when heap usage is not wanted, for example,
when writing a kernel. It may also be considered for performance reasons. It
will give you the error OutOfMemory if it has run out of bytes.
{AllocatorsFba}
std.heap.ArenaAllocator
takes in a child allocator and allows you to allocate many times and only free
once. Here, .deinit() is called on the arena, which frees all memory. Using
allocator.free in this example would be a no-op (i.e. does nothing).
{AllocatorsArena}
alloc and free are used for slices. For single items, consider using
create and destroy.
{AllocatorsCreate}
The Zig standard library also has a general-purpose allocator. This is a safe allocator that can prevent double-free, use-after-free and can detect leaks. Safety checks and thread safety can be turned off via its configuration struct (left empty below). Zig's GPA is designed for safety over performance, but may still be many times faster than page_allocator.
{AllocatorsGpa}
For high performance (but very few safety features!),
std.heap.c_allocator
may be considered. This, however, has the disadvantage of requiring linking Libc,
which can be done with -lc.
Benjamin Feng's talk What's a Memory Allocator Anyway? goes into more detail on this topic, and covers the implementation of allocators.