Check if a file exists in Zig

Question

What is the easiest way to check if a file exists in zig?

As part of my build process, I need to avoid writing over a file if it already exists.

I have seen std.fs.access but I'm not sure if there is an easier way.

Answer 1

Yes, the std.fs.Dir.access is the one you should use. But for your case, you might want to create the file with the exclusive=true flag and handle the PathAlreadyExists error as recommended in the std.fs.Dir.access documentation:

Be careful of Time-Of-Check-Time-Of-Use race conditions when using this function. For example, instead of testing if a file exists and then opening it, just open it and handle the error for file not found.

For example:

const std = @import("std");

pub fn main() !void {
    try write_cache_file();
}

fn write_cache_file() !void {
    var file = std.fs.cwd().createFile("file.tmp", .{ .exclusive = true }) catch |e|
        switch (e) {
            error.PathAlreadyExists => {
                std.log.info("already exists", .{});
                return;
            },
            else => return e,
        };
    defer file.close();
    std.log.info("write file here", .{});
}

This prints:

$ zig build run
info: write file here

$ zig build run
info: already exists