This tutorial covers how to use the LD_PRELOAD trick using with Zig on Linux. mimalloc is used as an example, but any C allocator is possible to use with this trick. Dynamically linking with libc is required for the trick to work.

1. Zig v0.12.0

2. mimalloc

   If mimalloc was successfully installed you can check like so:

   $ ls /usr/local/lib/libmimalloc.so
   /usr/local/lib/libmimalloc.so

1. Make and initialize a Zig project.

   $ mkdir ld-preload-zig
   $ cd ld-preload-zig/
   $ zig init-exe
   info: Created build.zig
   info: Created src/main.zig
   info: Next, try `zig build --help` or `zig build run`

2. Replace the contents of src/main.zig. The C allocator is required for the LD_PRELOAD trick.

   const std = @import("std");
   const c_allocator = std.heap.c_allocator;
   
   pub fn main() !void {
       var x = try c_allocator.create(i64);
       defer c_allocator.destroy(x);
   
       x.* = 100;
   
       std.debug.print("LD_PRELOAD is {}!!\n", .{x.*});
   }

3. Add a line to build.zig which will enable and dynamically link libc.

   (snip)
   const exe = b.addExecutable(.{
       .name = "ld-preload-zig",
       // In this case the main source file is merely a path, however, in more
       // complicated build scripts, this could be a generated file.
       .root_source_file = .{ .path = "src/main.zig" },
       .target = target,
       .optimize = optimize,
   });
   
   // Lets us use the C Allocator during build
   exe.linkLibC();
   (snip)

4. Build the project.

   $ zig build

5. Navigate to the executable and test that the trick works.

   $ cd zig-out/bin
   $ MIMALLOC_VERBOSE=1 LD_PRELOAD=/usr/local/lib/libmimalloc.so ./ld-preload-zig
   mimalloc: option 'show_errors': 0
   mimalloc: option 'show_stats': 0
   mimalloc: option 'verbose': 1
   (snip)

   If the trick is working, there should be verbose output from mimalloc like above.

6. Here is the final result without the verbose output, neat!:

   $ LD_PRELOAD=/usr/local/lib/libmimalloc.so ./ld-preload-zig
   LD_PRELOAD is 100!!