使用内存映射读取文件

本示例展示了如何使用内存映射（mmap）来读取文件内容。内存映射文件允许程序将文件内容直接映射到内存地址空间，这样可以像访问内存一样访问文件数据，而无需显式的读写操作。

const std = @import("std");
const fs = std.fs;
const print = std.debug.print;

const filename = "/tmp/zig-cookbook-01-02.txt";

pub fn main() !void {
    if (.windows == @import("builtin").os.tag) {
        std.debug.print("MMap is not supported in Windows\n", .{});
        return;
    }

    const file = try fs.cwd().createFile(filename, .{
        .read = true,
        .truncate = true,
        .exclusive = false, // Set to true will ensure this file is created by us
    });
    defer file.close();
    const content_to_write = "hello zig cookbook";

    // Before mmap, we need to ensure file isn't empty
    try file.setEndPos(content_to_write.len);

    const md = try file.metadata();
    try std.testing.expectEqual(md.size(), content_to_write.len);

    const ptr = try std.posix.mmap(
        null,
        content_to_write.len,
        std.posix.PROT.READ | std.posix.PROT.WRITE,
        .{ .TYPE = .SHARED },
        file.handle,
        0,
    );
    defer std.posix.munmap(ptr);

    // Write file via mmap
    std.mem.copyForwards(u8, ptr, content_to_write);

    // Read file via mmap
    try std.testing.expectEqualStrings(content_to_write, ptr);
}