vt: add ghostty_alloc for buffer allocation

Add a ghostty_alloc function that pairs with the existing
ghostty_free, giving embedders a symmetric malloc/free-style
API for buffer allocation through the libghostty allocator
interface. Returns NULL on allocation failure.

include/ghostty/vt/allocator.h

@@ -44,6 +44,24 @@
  * 2. Create a GhosttyAllocator struct with your vtable and context
  * 3. Pass the allocator to functions that accept one
  *
+ * ## Alloc/Free Helpers
+ *
+ * ghostty_alloc() and ghostty_free() provide a simple malloc/free-style
+ * interface for allocating and freeing byte buffers through the library's
+ * allocator. These are useful when:
+ *
+ * - You need to allocate a buffer to pass into a libghostty-vt function
+ *   (e.g. preparing input data for ghostty_terminal_vt_write()).
+ * - You need to free a buffer returned by a libghostty-vt function
+ *   (e.g. the output of ghostty_formatter_format_alloc()).
+ * - You are on a platform 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), so calling the standard C
+ *   free() on library-allocated memory would be undefined behavior.
+ *
+ * Always use the same allocator (or NULL) for both the allocation and
+ * the corresponding free.
+ *
  * @{
  */
 
@@ -191,6 +209,21 @@ typedef struct GhosttyAllocator {
     const GhosttyAllocatorVtable *vtable;
 } GhosttyAllocator;
 
+/**
+ * Allocate a buffer of `len` bytes.
+ *
+ * Uses the provided allocator, or the default allocator if NULL is passed.
+ * The returned buffer must be freed with ghostty_free() using the same
+ * allocator.
+ *
+ * @param allocator Pointer to the allocator to use, or NULL for the default
+ * @param len Number of bytes to allocate
+ * @return Pointer to the allocated buffer, or NULL if allocation failed
+ *
+ * @ingroup allocator
+ */
+uint8_t* ghostty_alloc(const GhosttyAllocator* allocator, size_t len);
+
 /**
  * Free memory that was allocated by a libghostty-vt function.
  *

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.alloc_alloc, .{ .name = "ghostty_alloc" });
         @export(&c.alloc_free, .{ .name = "ghostty_free" });
 
         // On Wasm we need to export our allocator convenience functions.

src/terminal/c/allocator.zig

@@ -3,6 +3,20 @@ const testing = std.testing;
 const lib_alloc = @import("../../lib/allocator.zig");
 const CAllocator = lib_alloc.Allocator;
 
+/// Allocate a buffer of `len` bytes using the given allocator
+/// (or the default allocator if NULL).
+///
+/// Returns a pointer to the allocated buffer, or NULL if the
+/// allocation failed.
+pub fn alloc(
+    alloc_: ?*const CAllocator,
+    len: usize,
+) callconv(.c) ?[*]u8 {
+    const allocator = lib_alloc.default(alloc_);
+    const buf = allocator.alloc(u8, len) catch return null;
+    return buf.ptr;
+}
+
 /// Free memory that was allocated by a libghostty-vt function.
 ///
 /// This must be used to free buffers returned by functions like
@@ -14,8 +28,25 @@ pub fn free(
     len: usize,
 ) callconv(.c) void {
     const mem = ptr orelse return;
-    const alloc = lib_alloc.default(alloc_);
-    alloc.free(mem[0..len]);
+    const allocator = lib_alloc.default(alloc_);
+    allocator.free(mem[0..len]);
+}
+
+test "alloc returns non-null" {
+    const ptr = alloc(&lib_alloc.test_allocator, 16);
+    try testing.expect(ptr != null);
+    free(&lib_alloc.test_allocator, ptr, 16);
+}
+
+test "alloc with null allocator" {
+    const ptr = alloc(null, 8);
+    try testing.expect(ptr != null);
+    free(null, ptr, 8);
+}
+
+test "alloc zero length" {
+    const ptr = alloc(&lib_alloc.test_allocator, 0);
+    defer free(&lib_alloc.test_allocator, ptr, 0);
 }
 
 test "free null pointer" {
@@ -23,14 +54,14 @@ test "free null pointer" {
 }
 
 test "free allocated memory" {
-    const alloc = lib_alloc.default(&lib_alloc.test_allocator);
-    const mem = try alloc.alloc(u8, 16);
+    const allocator = lib_alloc.default(&lib_alloc.test_allocator);
+    const mem = try allocator.alloc(u8, 16);
     free(&lib_alloc.test_allocator, mem.ptr, mem.len);
 }
 
 test "free with null allocator" {
     // null allocator falls back to the default (test allocator in tests)
-    const alloc = lib_alloc.default(null);
-    const mem = try alloc.alloc(u8, 8);
+    const allocator = lib_alloc.default(null);
+    const mem = try allocator.alloc(u8, 8);
     free(null, mem.ptr, mem.len);
 }

src/terminal/c/main.zig

@@ -116,6 +116,7 @@ pub const mouse_encoder_encode = mouse_encode.encode;
 
 pub const paste_is_safe = paste.is_safe;
 
+pub const alloc_alloc = allocator.alloc;
 pub const alloc_free = allocator.free;
 
 pub const size_report_encode = size_report.encode;