While you can actually write to the error buffer, conceptually it's read-only.

include/SDL_error.h

@@ -41,7 +41,7 @@ extern "C" {
 
 /* Public functions */
 extern DECLSPEC void SDLCALL SDL_SetError(const char *fmt, ...);
-extern DECLSPEC char *SDLCALL SDL_GetError(void);
+extern DECLSPEC const char *SDLCALL SDL_GetError(void);
 extern DECLSPEC void SDLCALL SDL_ClearError(void);
 
 /**

src/SDL_error.c

@@ -198,12 +198,12 @@ SDL_GetErrorMsg(char *errstr, unsigned int maxlen)
 }
 
 /* Available for backwards compatibility */
-char *
+const char *
 SDL_GetError(void)
 {
     static char errmsg[SDL_ERRBUFIZE];
 
-    return ((char *) SDL_GetErrorMsg(errmsg, SDL_ERRBUFIZE));
+    return SDL_GetErrorMsg(errmsg, SDL_ERRBUFIZE);
 }
 
 void