implemented macro OffsetPointer (#7012)

* implemented macro `OffsetPointer`

* consistent naming style

* simpler diff

---------

Co-authored-by: Michael Chirico <[email protected]>

Author: badasahog

src/fread.c

@@ -117,6 +117,8 @@ static void Field(FieldParseContext *ctx);
 #define IGNORE_BUMP(x) abs(x)
 #define TOGGLE_BUMP(x) (-x)
 
+#define OFFSET_POINTER(x, offset) ((typeof(x))((char*)x + (offset)))
+
 //=================================================================================================
 //
 //   Utility functions
@@ -449,7 +451,7 @@ double copyFile(size_t fileSize)  // only called in very very rare cases
     return -1.0; // # nocov
   memcpy(mmp_copy, mmp, fileSize);
   sof = mmp_copy;
-  eof = (char *)mmp_copy + fileSize;
+  eof = (char *)OFFSET_POINTER(mmp_copy, fileSize);
   return wallclock()-tt;
 }
 
@@ -2406,9 +2408,9 @@ int freadMain(freadMainArgs _args) {
               break;
             }
             // shift current buffer positions, since `myBuffX`s were probably moved by realloc
-            fctx.targets[8] = (void*)((char*)ctx.buff8 + myNrow * rowSize8);
-            fctx.targets[4] = (void*)((char*)ctx.buff4 + myNrow * rowSize4);
-            fctx.targets[1] = (void*)((char*)ctx.buff1 + myNrow * rowSize1);
+            fctx.targets[8] = OFFSET_POINTER(ctx.buff8, myNrow * rowSize8);
+            fctx.targets[4] = OFFSET_POINTER(ctx.buff4, myNrow * rowSize4);
+            fctx.targets[1] = OFFSET_POINTER(ctx.buff1, myNrow * rowSize1);
           }
           tLineStart = tch;  // for error message
           const char *fieldStart = tch;