Memory Allocation in Zig: Reading stdout

It’s a really good question that comes up a lot when you’re getting into systems programming: why do we need to allocate memory just to read output from a program? This has been especially relevant for me while building cmdtest, my CLI testing tool for Zig. One of the features I’m working on is how to interact with long-lived processes, and that definitely includes reading a line from stdio.

The quick answer is this: you always need somewhere to store the data you read. And because you never know how big that data is going to be, a programs usually have to allocate that memory on the fly. Zig programs deal with this same issue and fix it in similar ways.

The Problem: Unknown Size

When cmdtest runs a command, it has no clue how much stuff that command is going to print to stdout. It could be:

Your program needs a spot to put these bytes as it reads them. That “spot” is a buffer in memory. The big question is: how big should that buffer be?

This problem usually leads to two main ways of doing things in systems languages like Zig.

Using a Fixed-Size Buffer

This is the simplest way. You, as the coder, take a guess at a “reasonable” max size and make a buffer that exact size ahead of time. This memory usually lives on the stack, which is super fast but only for temporary stuff in the current function.

In cmdtest, when handling interactive processes, I use a fixed-size buffer for reading lines from stdout or stderr. This is efficient for predictable, smaller outputs in an “interactive loop.”

Here’s how cmdtest uses fixed-size buffers for interactive processes, specifically for reading lines:

src/root.zig
pub const InteractiveProcess = struct {
    const Self = @This();

    child: Child,
    pid: Child.Id,
    stdout_buffer: [1024]u8 = undefined, // Fixed-size buffer for stdout
    stdin_buffer: [1024]u8 = undefined,  // Fixed-size buffer for stdin
    stderr_buffer: [1024]u8 = undefined, // Fixed-size buffer for stderr

    // ... other fields and methods ...

    /// Reads from the child's stdout until a newline is found or the buffer is full.
    pub fn readLineFromStdout(self: *Self) ![]const u8 {
        const stdout_file = self.child.stdout orelse return error.MissingStdout;
        var stdout_reader = stdout_file.reader(&self.stdout_buffer);
        var stdout = &stdout_reader.interface;
        const line = try stdout.takeDelimiter('\\n') orelse return error.EmptyLine;
        const trimmed = std.mem.trimEnd(u8, line, "\\r");
        return trimmed;
    }
};

The good and bad:

Using a Dynamic Buffer

This way is much more robust and flexible. Instead of guessing a size, the function asks the operating system for memory from a big pool called the heap. If it runs out of room while reading, it asks for a bigger chunk and keeps going.

In cmdtest, when running a command and capturing all its output at once, a dynamic approach is used because the total output size is unknown.

Here’s a look at how cmdtest handles collecting all output, dynamically growing buffers as needed:

src/root.zig
pub fn run(options: RunOptions) !RunResult {
    // ... setup child process ...

    var stdout_buffer: std.ArrayList(u8) = .empty;
    defer stdout_buffer.deinit(options.allocator);

    var stderr_buffer: std.ArrayList(u8) = .empty;
    defer stderr_buffer.deinit(options.allocator);

    try child.collectOutput(
        options.allocator,
        &stdout_buffer,
        &stderr_buffer,
        options.max_output_bytes,
    );
    const term = try child.wait();

    // ... determine exit code ...

    return RunResult{
        .code = code,
        .term = term,
        .stdout = try stdout_buffer.toOwnedSlice(options.allocator),
        .stderr = try stderr_buffer.toOwnedSlice(options.allocator),
        .allocator = options.allocator,
    };
}

Here, std.ArrayList(u8) is used, which is a growable buffer that allocates memory from the provided allocator (typically the heap) as needed. The collectOutput function continuously appends to these ArrayLists until the process finishes or max_output_bytes is reached.

The good and bad:

Tags systems-programming , memory-management , zig , cmdtest