Writing File to User .cache Directory

aldenparker · March 10, 2025, 7:06pm

I am making a simple helper application that changes the brightness of my screen, but I need to cache a value. How do I get the path for the user’s .cache directory so I can write a file there?

dimdin · March 10, 2025, 7:37pm

Getting the cache directory is platform specific.

XDG Base Directory Specification specifies:

$XDG_CACHE_HOME defines the base directory relative to which user-specific non-essential data files should be stored. If $XDG_CACHE_HOME is either not set or empty, a default equal to $HOME/.cache should be used.

Macos uses: ~/Library/Caches
Windows uses: %LOCALAPPDATA%\Temp

You can use the known-folders library to get the .cache folder or other well known folders.

aldenparker · March 10, 2025, 7:42pm

Thank you. I was messing with std.fs.realpath and felt like something was wrong lol.

squeek502 · March 11, 2025, 7:53pm

Here’s what I use to get a path to a subdir within the cache directory FWIW:

github.com/squeek502/resinator

src/auto_includes.zig

10231c38a


      
          pub fn getCachePath(allocator: Allocator, appname: []const u8) error{OutOfMemory}!?[]const u8 {
              switch (builtin.os.tag) {
                  .windows => {
                      const local_app_data = std.process.getEnvVarOwned(allocator, "LOCALAPPDATA") catch |err| switch (err) {
                          error.EnvironmentVariableNotFound => return null,
                          error.InvalidWtf8 => unreachable,
                          else => |e| return e,
                      };
                      defer allocator.free(local_app_data);
                      return try std.fs.path.join(allocator, &.{ local_app_data, "Temp", appname });
                  },
                  .macos => {
                      if (std.posix.getenv("XDG_CACHE_HOME")) |cache| {
                          return try std.fs.path.join(allocator, &.{ cache, appname });
                      }
                      const home = std.posix.getenv("HOME") orelse return null;
                      return try std.fs.path.join(allocator, &.{ home, "Library/Caches", appname });
                  },
                  else => {
                      if (std.posix.getenv("XDG_CACHE_HOME")) |cache| {

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(it’s basically the same as what @dimdin describes and what known-folders does)

AndrewKraevskii · March 12, 2025, 2:35pm

known-folders is probably a better solution, but if you only need the cache directory, you can just yank this function from the compiler.

github.com/ziglang/zig

src/introspect.zig

d0911786c


      
          /// Caller owns returned memory.
          pub fn resolveGlobalCacheDir(allocator: mem.Allocator) ![]u8 {
              if (builtin.os.tag == .wasi)
                  @compileError("on WASI the global cache dir must be resolved with preopens");
          
              if (try std.zig.EnvVar.ZIG_GLOBAL_CACHE_DIR.get(allocator)) |value| return value;
          
              const appname = "zig";
          
              if (builtin.os.tag != .windows) {
                  if (std.zig.EnvVar.XDG_CACHE_HOME.getPosix()) |cache_root| {
                      if (cache_root.len > 0) {
                          return fs.path.join(allocator, &[_][]const u8{ cache_root, appname });
                      }
                  }
                  if (std.zig.EnvVar.HOME.getPosix()) |home| {
                      return fs.path.join(allocator, &[_][]const u8{ home, ".cache", appname });
                  }
              }

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squeek502 · March 12, 2025, 7:32pm

Note that this code is for getting the zig cache directory.

mperillo · March 15, 2025, 10:32am

I’m a bit confused about the preferred path to cache on Windows.

Both the Go std and Zig compiler use %LocalAppData%, but resinator and known-folders use %LocalAppData%/Temp.

This usage is not documented in Environment variable - Wikipedia and, personally, the last path should be Cache, instead of Temp.

dimdin · March 15, 2025, 1:51pm

Saving directly in %LOCALAPPDATA% is wrong. Use %LOCALAPPDATA%\Temp for cache or temporary files.

The related environment variables are:
%LOCALAPPDATA% = %USERPROFILE%\AppData\Local
%TEMP% = %LOCALAPPDATA%\Temp
%TMP% = %LOCALAPPDATA%\Temp

There are a lot of subfolders, such as:

%LOCALAPPDATA%\Programs
%LOCALAPPDATA%\ProgramData
%LOCALAPPDATA%\Desktop
%LOCALAPPDATA%\Documents

mperillo · March 15, 2025, 4:19pm

@dimdin Thanks.

Do you know an official reference from Microsoft?

And does it means that Go and Zig use the %LOCALAPPDATA% incorrectly?

UPDATE:
I just checked on a Windows 12 PC.
Firefox, Chromium and OneDrive (and others) use just %LOCALAPPDATA%.

Temp seems to only used for temporary data, and not cache.

Unofficial reference: Redirecting

dimdin · March 15, 2025, 5:08pm

New API: KNOWNFOLDERID (Knownfolders.h) - Win32 apps | Microsoft Learn
Old (deprecated) API: CSIDL (Shlobj.h) - Win32 apps | Microsoft Learn

Using subfolders in Company\Product format is correct and common usage.
For example: %LOCALAPPDATA%\Microsoft\Windows\Temporary Internet Files

I don’t know if the use a subfolder or not.
Saving files in the %LOCALAPPDATA% folder in incorrect; it is like saving files in C:\.

pachde · March 15, 2025, 6:00pm

As for the Microsoft reference on the usage:
Store and retrieve settings and other app data - Windows apps | Microsoft Learn

squeek502 · March 15, 2025, 6:19pm

Cache data is meant to be temporary data, though (i.e. it can be cleared without causing a problem). From the link that @pachde posted:

Temporary app data

The temporary app data store works like a cache.

mnemnion · March 18, 2025, 7:59pm

I’d suggest checking XDG_* on macOS as well, for the various relevant paths. It’s Unix after all.

I do set those variables, and it’s always nice when programs respect that.

squeek502 · March 18, 2025, 8:33pm

Makes sense. I think I was trying to follow known-folders’ lead, where xdg_on_mac defaults to false.

EDIT: Made that change and updated the link in my previous comment.