diff --git a/tests/server/util.c b/tests/server/util.c
index 95e0a0ccf5..65a5f476d3 100644
--- a/tests/server/util.c
+++ b/tests/server/util.c
@@ -448,7 +448,7 @@ static BOOL WINAPI ctrl_event_handler(DWORD dwCtrlType)
   static const char msgend[] = "ctrl_event_handler: end\n";
   HANDLE out = GetStdHandle(STD_ERROR_HANDLE);
   DWORD dwWritten;
-  WriteFile(out, msgbegin, sizeof(msgbegin) - 1, &dwWritten, NULL);
+  WriteFile(out, msgbegin, CURL_CSTRLEN(msgbegin), &dwWritten, NULL);
 #endif
   switch(dwCtrlType) {
 #ifdef SIGINT
@@ -473,7 +473,7 @@ static BOOL WINAPI ctrl_event_handler(DWORD dwCtrlType)
 #ifdef DEBUG_WIN32_CALLBACKS
     logmsg("ctrl_event_handler: %lu -> %d", dwCtrlType, signum);
 #else
-    WriteFile(out, msgend, sizeof(msgend) - 1, &dwWritten, NULL);
+    WriteFile(out, msgend, CURL_CSTRLEN(msgend), &dwWritten, NULL);
 #endif
     raise(signum);
   }
@@ -510,7 +510,7 @@ static LRESULT CALLBACK main_window_proc(HWND hwnd, UINT uMsg,
 #else
       static const char str[] = "main_window_proc\n";
       DWORD dwWritten;
-      WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, sizeof(str) - 1,
+      WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, CURL_CSTRLEN(str),
                              &dwWritten, NULL);
 #endif
       raise(signum);
@@ -543,7 +543,7 @@ static DWORD WINAPI main_window_loop(void *lpParameter)
     fprintf(stderr, "RegisterClass failed: %s\n", buffer);
 #else
     static const char str[] = "RegisterClass() failed\n";
-    WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, sizeof(str) - 1,
+    WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, CURL_CSTRLEN(str),
               &dwWritten, NULL);
 #endif
     return (DWORD)-1;
@@ -563,7 +563,7 @@ static DWORD WINAPI main_window_loop(void *lpParameter)
     fprintf(stderr, "CreateWindowEx failed: (0x%08lx) - %s\n", err, buffer);
 #else
     static const char str[] = "CreateWindowEx() failed\n";
-    WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, sizeof(str) - 1,
+    WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, CURL_CSTRLEN(str),
               &dwWritten, NULL);
 #endif
     return (DWORD)-1;
@@ -578,7 +578,7 @@ static DWORD WINAPI main_window_loop(void *lpParameter)
       fprintf(stderr, "GetMessage failed: (0x%08lx) - %s\n", err, buffer);
 #else
       static const char str[] = "GetMessage() failed\n";
-      WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, sizeof(str) - 1,
+      WriteFile(GetStdHandle(STD_ERROR_HANDLE), str, CURL_CSTRLEN(str),
                              &dwWritten, NULL);
 #endif
       return (DWORD)-1;