Add flag to support allocation of trampolines within 32-bit offset range of target code on 64-bit Windows

Plus minor formatting fixes and more documentation for flags.

Fixes #9

1 files changed, 34 insertions, 1 deletions

diff --git a/subhook_windows.c b/subhook_windows.c
index 321207e..8c9b477 100644
--- a/subhook_windows.c
+++ b/subhook_windows.c
@@ -27,6 +27,8 @@
 #include <stddef.h>
 #include <windows.h>
 
+typedef __int64 QWORD;
+
 #define SUBHOOK_CODE_PROTECT_FLAGS PAGE_EXECUTE_READWRITE
 
 int subhook_unprotect(void *address, size_t size) {
@@ -38,7 +40,38 @@ int subhook_unprotect(void *address, size_t size) {
   return !result;
 }
 
-void *subhook_alloc_code(size_t size) {
+void *subhook_alloc_code(void *target_address, size_t size) {
+  SYSTEM_INFO sys_info;
+  DWORD page_size = 0x1000;
+  QWORD offset;
+
+#if defined _M_AMD64 || defined __amd64__
+  if (target_address != NULL) {
+    GetSystemInfo(&sys_info);
+    page_size = sys_info.dwPageSize;
+
+    QWORD pivot = (QWORD)target_address & ~((QWORD)page_size - 1);
+    void *result;
+
+    for (offset = page_size; offset <= ((QWORD)1 << 31); offset += page_size) {
+      result = VirtualAlloc((void *)(pivot - offset),
+                            size,
+                            MEM_COMMIT | MEM_RESERVE,
+                            SUBHOOK_CODE_PROTECT_FLAGS);
+      if (result != NULL) {
+        return result;
+      }
+      result = VirtualAlloc((void *)(pivot + offset),
+                            size,
+                            MEM_COMMIT | MEM_RESERVE,
+                            SUBHOOK_CODE_PROTECT_FLAGS);
+      if (result != NULL) {
+        return result;
+      }
+    }
+  }
+#endif
+
   return VirtualAlloc(NULL,
                       size,
                       MEM_COMMIT | MEM_RESERVE,