Commit c9fa8e46df21cdf66b291fb3dfdcef1b6a1d3cab | zig

author	Carl Åstholm <carl@astholm.se> 2025-11-07 22:15:30+01:00
committer	Andrew Kelley <andrew@ziglang.org> 2026-01-02 16:55:11-08:00
log	c9fa8e46df21cdf66b291fb3dfdcef1b6a1d3cab
tree	be1ef792bffa9d7f6eb6fa186d9100d31181a8db
parent	1bf29757d963b301c1fcfccb9b39b277381de115

Use `mmap` `std.heap.page_allocator` impl when compiling for Wasm + libc

When linking libc, it should be the libc that manages the heap. The main
Wasm memory might have been configured as non-growable, which makes
`WasmAllocator` a poor default and causes the common `DebugAllocator`
use case fail with OOM errors unless the user uses `std_options` to
override the default page allocator. Additionally, on Emscripten,
growing Wasm memory without notifying the JS glue code will cause array
buffers to get detached and lead to spurious crashes.

3 files changed, 14 insertions(+), 12 deletions(-)

lib
- std
  - c.zig
  - heap.zig
  - os
    emscripten.zig

lib/std/c.zig+13-2

 @@ -1885,7 +1885,16 @@ pub const POLL = switch (native_os) {
 /// Basic memory protection flags
 pub const PROT = switch (native_os) {
     .linux => linux.PROT,
-    .emscripten => emscripten.PROT,
+    // https://github.com/emscripten-core/emscripten/blob/08e2de1031913e4ba7963b1c56f35f036a7d4d56/system/lib/libc/musl/include/sys/mman.h#L57-L62
+    // lib/libc/include/wasm-wasi-musl/sys/mman.h
+    .emscripten, .wasi => struct {
+        pub const NONE = 0;
+        pub const READ = 1;
+        pub const WRITE = 2;
+        pub const EXEC = 4;
+        pub const GROWSDOWN = 0x01000000;
+        pub const GROWSUP = 0x02000000;
+    },
     // https://github.com/SerenityOS/serenity/blob/6d59d4d3d9e76e39112842ec487840828f1c9bfe/Kernel/API/POSIX/sys/mman.h#L28-L31
     .openbsd, .haiku, .dragonfly, .netbsd, .illumos, .freebsd, .windows, .serenity => struct {
         /// page can not be accessed
 @@ -8640,7 +8649,9 @@ pub const O = switch (native_os) {
 pub const MAP = switch (native_os) {
     .linux => linux.MAP,
-    .emscripten => packed struct(u32) {
+    // https://github.com/emscripten-core/emscripten/blob/08e2de1031913e4ba7963b1c56f35f036a7d4d56/system/lib/libc/musl/include/sys/mman.h#L21-L39
+    // lib/libc/include/wasm-wasi-musl/sys/mman.h
+    .emscripten, .wasi => packed struct(u32) {
         TYPE: enum(u4) {
             SHARED = 0x01,
             PRIVATE = 0x02,

lib/std/heap.zig+1-1

...	@@ -352,7 +352,7 @@ pub const page_allocator: Allocator = if (@hasDecl(root, "os") and	...	@@ -352,7 +352,7 @@ pub const page_allocator: Allocator = if (@hasDecl(root, "os") and
352	@hasDecl(root.os, "heap") and	352	@hasDecl(root.os, "heap") and
353	@hasDecl(root.os.heap, "page_allocator"))	353	@hasDecl(root.os.heap, "page_allocator"))
354	root.os.heap.page_allocator	354	root.os.heap.page_allocator
355	else if (builtin.target.cpu.arch.isWasm()) .{	355	else if (builtin.target.cpu.arch.isWasm() and !builtin.link_libc) .{
356	.ptr = undefined,	356	.ptr = undefined,
357	.vtable = &WasmAllocator.vtable,	357	.vtable = &WasmAllocator.vtable,
358	} else if (builtin.target.os.tag == .plan9) .{	358	} else if (builtin.target.os.tag == .plan9) .{

lib/std/os/emscripten.zig-9

...	@@ -331,15 +331,6 @@ pub const POLL = struct {	...	@@ -331,15 +331,6 @@ pub const POLL = struct {
331	pub const RDBAND = 0x080;	331	pub const RDBAND = 0x080;
332	};	332	};
333		333
334	pub const PROT = struct {
335	pub const NONE = 0x0;
336	pub const READ = 0x1;
337	pub const WRITE = 0x2;
338	pub const EXEC = 0x4;
339	pub const GROWSDOWN = 0x01000000;
340	pub const GROWSUP = 0x02000000;
341	};
342
343	pub const rlim_t = u64;	334	pub const rlim_t = u64;
344		335
345	pub const RLIM = struct {	336	pub const RLIM = struct {