Merge pull request #4002 from arnetheduck/malloc-store-size

store block size in when using malloc and nogc

doc/nimc.txt

@@ -243,7 +243,9 @@ Define               Effect
 ``useFork``          Makes ``osproc`` use ``fork`` instead of ``posix_spawn``.
 ``useNimRtl``        Compile and link against ``nimrtl.dll``.
 ``useMalloc``        Makes Nim use C's `malloc`:idx: instead of Nim's
-                     own memory manager. This only works with ``gc:none``.
+                     own memory manager, ableit prefixing each allocation with
+                     its size to support clearing memory on reallocation.
+                     This only works with ``gc:none``.
 ``useRealtimeGC``    Enables support of Nim's GC for *soft* realtime
                      systems. See the documentation of the `gc <gc.html>`_
                      for further information.

lib/system/mmdisp.nim

@@ -389,15 +389,30 @@ elif defined(nogc) and defined(useMalloc):
 
   when not defined(useNimRtl):
     proc alloc(size: Natural): pointer =
-      result = cmalloc(size)
-      if result == nil: raiseOutOfMem()
+      var x = cmalloc(size + sizeof(size))
+      if x == nil: raiseOutOfMem()
+
+      cast[ptr int](x)[] = size
+      result = cast[pointer](cast[int](x) + sizeof(size))
+
     proc alloc0(size: Natural): pointer =
       result = alloc(size)
       zeroMem(result, size)
     proc realloc(p: pointer, newsize: Natural): pointer =
-      result = crealloc(p, newsize)
-      if result == nil: raiseOutOfMem()
-    proc dealloc(p: pointer) = cfree(p)
+      var x = cast[pointer](cast[int](p) - sizeof(newsize))
+      let oldsize = cast[ptr int](x)[]
+
+      x = crealloc(x, newsize + sizeof(newsize))
+
+      if x == nil: raiseOutOfMem()
+
+      cast[ptr int](x)[] = newsize
+      result = cast[pointer](cast[int](x) + sizeof(newsize))
+
+      if newsize > oldsize:
+        zeroMem(cast[pointer](cast[int](result) + oldsize), newsize - oldsize)
+
+    proc dealloc(p: pointer) = cfree(cast[pointer](cast[int](p) - sizeof(int)))
 
     proc allocShared(size: Natural): pointer =
       result = cmalloc(size)

web/news.txt

@@ -32,6 +32,11 @@ Changes affecting backwards compatibility
   raises a ``KeyError`` exception. You can compile with the ``-d:nimJsonGet``
   flag to get a list of usages of ``[]``, as well as to restore the operator's
   previous behaviour.
+- When using ``useMalloc``, an additional header containing the size of the
+  allocation will be allocated, to support zeroing memory on realloc as expected
+  by the language. With this change, ``alloc`` and ``dealloc`` are no longer
+  aliases for ``malloc`` and ``free`` - use ``c_malloc`` and ``c_free`` if
+  you need that.
 
 
 Library Additions