How can you create a buffer of the same size as a file? - zig

Question

I would like to avoid making a set size buffer because of things like a file being too big, or small enough that there is empty space in the buffer. An ArenaAllocator sounds promising since you can allocate more space as needed. Is there a "proper" way to do this, i.e. load a .json file passed as a command line argument into a buffer?

Answer 1

How can you create a buffer of the same size as a file?
You can request file's size and allocate the same amount of memory:
var file = try std.fs.cwd().openFile("file.txt", .{ open = true }));
const file_size = (try file.stat()).size;
var buffer = try allocator.alloc(u8, file_size);
Then you can read the file by using readNoEof:
try file.reader().readNoEof(buffer);
Alternatively, you can use File's readToEndAlloc function:
const size_limit = std.math.maxInt(u32); // or any other suitable limit
var result = try file.readToEndAlloc(allocator, size_limit);

Answer 2

using GeneralPurposeAllocator. you can learn more about allocators in https://youtu.be/vHWiDx_l4V0
const std = #import("std");
pub fn main() !void {
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
defer _ = gpa.deinit();
const allocator = &gpa.allocator;
const args = try std.process.argsAlloc(allocator);
defer std.process.argsFree(allocator, args);
const file = try std.fs.cwd().openFile(args[1], .{});
const file_content = try file.readToEndAlloc(allocator, 1024 * 1024); // 1MB max read size
defer allocator.free(file_content);
std.debug.print("{s}", .{file_content});
}

Answer 3

On 0.10.0-dev.2345+9747303d1, here's what worked for me:
const std = #import("std");
pub fn main() !void {
var file = try std.fs.cwd().openFile("test.ts", .{ .mode = .read_only });
const file_size = (try file.stat()).size;
const allocator = std.heap.page_allocator;
var buffer = try allocator.alloc(u8, file_size);
try file.reader().readNoEof(buffer);
std.debug.print("{s}\n", .{buffer});
}
OpenFlags for .openFile is defined here:
https://github.com/ziglang/zig/blob/master/lib/std/fs/file.zig#L80

Answer 4

None of the answers worked for me in Zig today sine a few interfaces changed (e.g. allocator -> allocator()). Here is what worked for me (Dec 2022):
const std = #import("std");
pub fn main() !void {
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
defer _ = gpa.deinit();
const allocator = &gpa.allocator();
var file = try std.fs.cwd().openFile("./sample.txt", .{});
const file_content = try file.readToEndAlloc(allocator.*, 10 * 1024 * 1024); // 10MB read
defer allocator.free(file_content);
std.debug.print("{s}", .{file_content});
}