OutOfMemory in simple Zig use case

Question

I'm learning Zig and i'm confused with an OutOfMemoryError.

In theory, the file that i'm trying to read has 9 bytes of size. The max_bytesparameter of readToEndAllocis setted to 9 but the problem i think is the FixedBufferAllocator.

If i don't set the fixed_alloc_buf to [2048]u8 or more then i have the out of memory error. If i try to run this code with [16]u8 then doesn't work:

const std = @import("std");

pub fn main() !void {
    const file = try std.fs.cwd().openFile("./file.txt", .{}); // file of 9 bytes
    defer file.close();

    var fixed_alloc_buf: [16]u8 = undefined; // boom if not [~2048]u8
    var fixed_alloc = std.heap.FixedBufferAllocator.init(&fixed_alloc_buf);

    var arena = std.heap.ArenaAllocator.init(fixed_alloc.allocator());
    defer arena.deinit();

    const allocator = arena.allocator();
    const buffer = try file.readToEndAlloc(allocator, 9);
    defer allocator.free(buffer);
}

Answer 1

Two things:

• ArenaAllocator needs some memory for its own internal state.
• And it also allocates more than you request. As it tries to keep the memory "chunked".

In this case it tries to get 63 bytes from the parent allocator:

const std = @import("std");

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{ .verbose_log = true }){};
    defer std.debug.assert(gpa.deinit() == .ok);
    const allocator = gpa.allocator();

    var arena = std.heap.ArenaAllocator.init(allocator);
    defer arena.deinit();

    _ = try arena.allocator().alloc(u8, 9);
}

This prints:

$ zig build run
info(gpa): small alloc 63 bytes at u8@1e6234e0000
info(gpa): small free 63 bytes at u8@1e6234e0000