Back to Basics: Reading Files Fully in Zig
Ways to Read a File Fully with the New IO in Zig
(For reading a file line by line, see File Reading with the New IO in Zig.)
A common task in programming is file reading, especially reading the entire content of a file. In Zig, there are a number of ways to read an entire file into memory. Let’s look at four distinct approaches for reading a file fully, using the new IO API in Zig.
Method 1: The Idiomatic One-Liner (std.fs.Dir.readFileAlloc)
For most applications, when you just need the entire content of
a file and don’t care about internal buffer management,
the readFileAlloc function on the Dir struct is the way to go.
It handles opening the file, getting the size, allocating the memory,
and reading all the bytes in one function call.
const std = @import("std");
pub fn read_full1(allocator: std.mem.Allocator, path: []const u8) ![]u8 {
const content = try std.fs.cwd().readFileAlloc(allocator, path, .unlimited);
return content; // The caller is responsible for freeing the content.
}
pub fn main() !void {
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
defer _ = gpa.deinit();
const allocator = gpa.allocator();
const content = try read_full1(alloc, "data.txt");
defer allocator.free(content);
// Now you have the file content as a mutable slice.
std.debug.print("{s}\n", .{content});
}
Method 2: Read File with Pre-allocated Buffer (std.fs.Dir.readFile)
This method is slightly more explicit. It requires getting the file size,
allocating a buffer of that size, and calling std.fs.Dir.readFile to read the whole file.
pub fn read_full2(allocator: std.mem.Allocator, path: []const u8) ![]u8 {
var file = try std.fs.cwd().openFile(path, .{});
defer file.close();
const content = try allocator.alloc(u8, (try file.stat()).size);
_ = try std.fs.Dir.readFile(std.fs.cwd(), path, content);
return content;
}
Method 3: Fill Data with the Reader Interface
This is a very efficient way to read a file. The allocated buffer
matches the size of the file. The buffer is placed directly as the
internal reading buffer of the std.io.Reader interface. The .fill() call
reads the whole file into the buffer in one shot.
pub fn read_full3(allocator: std.mem.Allocator, path: []const u8) ![]u8 {
var file = try std.fs.cwd().openFile(path, .{});
defer file.close();
const file_size = (try file.stat()).size;
const content = try alloc.alloc(u8, file_size);
var file_reader = file.reader(content);
try file_reader.interface.fill(file_size);
return content;
}
Method 4: Read All the Data Without Knowing the Size
Sometimes the file size is not available, e.g. reading from
a network stream. To read all the data, you need a growing buffer.
std.Io.Writer.Allocating is such a thing. It’s a writer with an internal buffer
which grows as needed.
pub fn read_full4(allocator: std.mem.Allocator, path: []const u8) !std.Io.Writer.Allocating {
var file = try std.fs.cwd().openFile(path, .{});
defer file.close();
var read_buf: [4096]u8 = undefined;
var file_reader = file.reader(&read_buf);
var buf = std.Io.Writer.Allocating.init(alloc);
_ = try file_reader.interface.streamRemaining(&buf.writer);
return buf;
}
Call Allocating.written() to access the bytes.
const buf = try read_full4(alloc, "data.txt");
defer buf.deinit();
std.debug.print("{s}\n", .{buf.written()});