libghostty: add ghostty_free for cross-runtime memory safety

On Windows, Zig's built-in libc and MSVC's CRT maintain separate
heaps, so calling free() on memory allocated by the library causes
undefined behavior. Add ghostty_free() that frees through the same
allocator that performed the allocation, making it safe on all
platforms.

Update format_alloc docs and all examples to use ghostty_free()
instead of free().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

example/c-vt-cmake-static/src/main.c

@@ -45,7 +45,7 @@ int main() {
   fwrite(buf, 1, len, stdout);
   printf("\n");
 
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;

example/c-vt-cmake/src/main.c

@@ -45,7 +45,7 @@ int main() {
   fwrite(buf, 1, len, stdout);
   printf("\n");
 
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;

example/c-vt-formatter/src/main.c

@@ -56,7 +56,7 @@ int main() {
   printf("\n");
 
   // Clean up
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;

include/ghostty/vt/allocator.h

@@ -191,6 +191,31 @@ typedef struct GhosttyAllocator {
     const GhosttyAllocatorVtable *vtable;
 } GhosttyAllocator;
 
+/**
+ * Free memory that was allocated by a libghostty-vt function.
+ *
+ * Use this to free buffers returned by functions such as
+ * ghostty_formatter_format_alloc(). Pass the same allocator that was
+ * used for the allocation, or NULL if the default allocator was used.
+ *
+ * On platforms where the library's internal allocator differs from the
+ * consumer's C runtime (e.g. Windows, where Zig's libc and MSVC's CRT
+ * maintain separate heaps), calling the standard C free() on memory
+ * allocated by the library causes undefined behavior. This function
+ * guarantees the correct allocator is used regardless of platform.
+ *
+ * It is safe to pass a NULL pointer; the call is a no-op in that case.
+ *
+ * @param allocator Pointer to the allocator that was used to allocate the
+ *   memory, or NULL if the default allocator was used
+ * @param ptr Pointer to the memory to free (may be NULL)
+ * @param len Length of the allocation in bytes (must match the original
+ *   allocation size)
+ *
+ * @ingroup allocator
+ */
+void ghostty_free(const GhosttyAllocator* allocator, uint8_t* ptr, size_t len);
+
 /** @} */
 
 #endif /* GHOSTTY_VT_ALLOCATOR_H */

include/ghostty/vt/formatter.h

@@ -186,10 +186,9 @@ GhosttyResult ghostty_formatter_format_buf(GhosttyFormatter formatter,
  *
  * Each call formats the current terminal state. The buffer is allocated
  * using the provided allocator (or the default allocator if NULL).
- * The caller is responsible for freeing the returned buffer. When using
- * the default allocator (NULL), the buffer can be freed with `free()`.
- * When using a custom allocator, the buffer must be freed using the
- * same allocator.
+ * The caller is responsible for freeing the returned buffer with
+ * ghostty_free(), passing the same allocator (or NULL for the default)
+ * that was used for the allocation.
  *
  * @param formatter The formatter handle (must not be NULL)
  * @param allocator Pointer to allocator, or NULL to use the default allocator

src/lib_vt.zig

@@ -218,6 +218,7 @@ comptime {
         @export(&c.grid_ref_graphemes, .{ .name = "ghostty_grid_ref_graphemes" });
         @export(&c.grid_ref_style, .{ .name = "ghostty_grid_ref_style" });
         @export(&c.build_info, .{ .name = "ghostty_build_info" });
+        @export(&c.free_alloc, .{ .name = "ghostty_free" });
 
         // On Wasm we need to export our allocator convenience functions.
         if (builtin.target.cpu.arch.isWasm()) {

src/terminal/c/main.zig

@@ -1,3 +1,6 @@
+const lib_alloc = @import("../../lib/allocator.zig");
+const CAllocator = lib_alloc.Allocator;
+
 const buildpkg = @import("build_info.zig");
 pub const cell = @import("cell.zig");
 pub const color = @import("color.zig");
@@ -112,6 +115,21 @@ pub const mouse_encoder_encode = mouse_encode.encode;
 
 pub const paste_is_safe = paste.is_safe;
 
+/// Free memory that was allocated by a libghostty-vt function.
+///
+/// This must be used to free buffers returned by functions like
+/// `format_alloc`. Pass the same allocator (or NULL for the default)
+/// that was used for the allocation.
+pub fn free_alloc(
+    alloc_: ?*const CAllocator,
+    ptr: ?[*]u8,
+    len: usize,
+) callconv(.c) void {
+    const mem = ptr orelse return;
+    const alloc = lib_alloc.default(alloc_);
+    alloc.free(mem[0..len]);
+}
+
 pub const size_report_encode = size_report.encode;
 
 pub const cell_get = cell.get;