wait4 maxrss without parent memory

example.c

const std = @import("std");
const assert = std.debug.assert;
const expect = std.testing.expect;

pub fn main() !void {
    var args = std.process.args();
    _ = args.next().?;

    if (args.next()) |_| {
        // child process
        var child = std.ChildProcess.init(&.{"/usr/bin/env"}, std.heap.page_allocator);
        child.stdin_behavior = .Ignore;
        child.stdout_behavior = .Pipe;
        child.stderr_behavior = .Pipe;
        child.request_resource_usage_statistics = true;

        try child.spawn();

        switch (try child.wait()) {
            .Exited => |code| if (code != 0) return error.BadExitCode,
            else => return error.Crashed,
        }
        const max_rss = child.resource_usage_statistics.getMaxRss() orelse 0;
        std.debug.print("child max_rss={d}\n", .{max_rss});
    } else {
        // parent process
        const big = try std.heap.page_allocator.alloc(u8, 2 * 1024 * 1024 * 1024);
        _ = big;

        var self_exe_buf: [std.fs.MAX_PATH_BYTES]u8 = undefined;
        const self_exe = try std.fs.selfExePath(&self_exe_buf);
        var child = std.ChildProcess.init(&.{
            self_exe,
            "--child",
        }, std.heap.page_allocator);
        child.stdin_behavior = .Ignore;
        child.stdout_behavior = .Inherit;
        child.stderr_behavior = .Inherit;
        child.request_resource_usage_statistics = true;

        try child.spawn();

        switch (try child.wait()) {
            .Exited => |code| if (code != 0) return error.BadExitCode,
            else => return error.Crashed,
        }
        const max_rss = child.resource_usage_statistics.getMaxRss() orelse 0;
        std.debug.print("parent max_rss={d}\n", .{max_rss});
    }
}