tiny.sys.heap

1const std = @import("std");2const capabilities = @import("capabilities.zig");3const memory = @import("memory.zig");45pub const required_capabilities = capabilities.noLibc(&.{.memory_mapping});67const malloc_alignment = 16;8const header_size = 32;9const magic: u64 = 0x5153_5953_4845_4150;1011const Header = extern struct {12    magic: u64,13    mapped_len: usize,14    payload_len: usize,15};1617comptime {18    std.debug.assert(@sizeOf(Header) <= header_size);19    std.debug.assert(header_size % malloc_alignment == 0);20}2122pub fn malloc(byte_count: usize) callconv(.c) ?*anyopaque {23    const payload_len = @max(byte_count, 1);24    const requested_len = std.math.add(usize, header_size, payload_len) catch return null;25    const mapping = memory.mapAnonymous(requested_len, .{ .read = true, .write = true }) catch return null;2627    const header: *Header = @ptrCast(@alignCast(mapping.ptr));28    header.* = .{29        .magic = magic,30        .mapped_len = mapping.len,31        .payload_len = payload_len,32    };3334    return @ptrCast(mapping.ptr + header_size);35}3637pub fn calloc(count: usize, byte_count: usize) callconv(.c) ?*anyopaque {38    const total = std.math.mul(usize, count, byte_count) catch return null;39    const raw = malloc(total) orelse return null;40    const bytes: [*]u8 = @ptrCast(raw);41    @memset(bytes[0..total], 0);42    return raw;43}4445pub fn realloc(ptr: ?*anyopaque, byte_count: usize) callconv(.c) ?*anyopaque {46    const raw = ptr orelse return malloc(byte_count);47    if (byte_count == 0) {48        free(raw);49        return null;50    }5152    const header = headerFromRaw(raw);53    const old_len = header.payload_len;54    const result = malloc(byte_count) orelse return null;55    const source: [*]const u8 = @ptrCast(raw);56    const target: [*]u8 = @ptrCast(result);57    const copy_len = @min(old_len, byte_count);58    @memcpy(target[0..copy_len], source[0..copy_len]);59    free(raw);60    return result;61}6263pub fn free(ptr: ?*anyopaque) callconv(.c) void {64    const raw = ptr orelse return;65    const header = headerFromRaw(raw);6667    const mapped_len = header.mapped_len;68    header.magic = 0;69    const user: [*]u8 = @ptrCast(raw);70    const base = user - header_size;71    const mapping: [*]align(std.heap.page_size_min) u8 = @ptrCast(@alignCast(base));72    memory.unmap(mapping[0..mapped_len]);73}7475pub fn freeAddress(address: usize) void {76    free(@ptrFromInt(address));77}7879fn headerFromRaw(raw: *anyopaque) *Header {80    const user: [*]u8 = @ptrCast(raw);81    const base = user - header_size;82    const header: *Header = @ptrCast(@alignCast(base));83    if (header.magic != magic) @trap();84    return header;85}8687test "malloc returns aligned memory and free accepts null" {88    const ptr = malloc(33) orelse return error.OutOfMemory;89    try std.testing.expect(std.mem.isAligned(@intFromPtr(ptr), malloc_alignment));90    free(ptr);91    free(null);92}9394test "calloc zeros memory and realloc preserves prefix" {95    const raw = calloc(4, 2) orelse return error.OutOfMemory;96    const bytes: [*]u8 = @ptrCast(raw);97    try std.testing.expectEqualSlices(u8, &@as([8]u8, @splat(0)), bytes[0..8]);98    bytes[0] = 1;99    bytes[7] = 2;100101    const grown = realloc(raw, 16) orelse return error.OutOfMemory;102    const grown_bytes: [*]u8 = @ptrCast(grown);103    try std.testing.expectEqual(@as(u8, 1), grown_bytes[0]);104    try std.testing.expectEqual(@as(u8, 2), grown_bytes[7]);105106    const shrunk = realloc(grown, 1) orelse return error.OutOfMemory;107    const shrunk_bytes: [*]u8 = @ptrCast(shrunk);108    try std.testing.expectEqual(@as(u8, 1), shrunk_bytes[0]);109    try std.testing.expectEqual(@as(?*anyopaque, null), realloc(shrunk, 0));110}

1pub const heap = @import("heap.zig");