Fix that malloc() doesn't return a null pointer for too large allocations

The _sbrk() function wasn't implemented. It's used by malloc() to request more heap memory. This function must be defined and correctly implemented if we want malloc() to return a null pointer if more space is requested than is available. It already works that way for the Mbed Core (although with a different implementation), but doesn't for the Renesas Core before this addition.

Author: alrvid

cores/arduino/main.cpp

@@ -145,4 +145,36 @@ extern "C" __attribute__((weak)) void __cxa_pure_virtual(void);
 extern "C" __attribute__((weak)) void __cxa_pure_virtual(void)
 {
    exit(1);
-}
+}
+
+// Used by malloc() to request more heap memory. This function
+// must be defined if we want malloc() to return a null pointer
+// if more space is requested than is available. Notice that
+// there's no support for decrement as the very first action,
+// because the result of that is unspecified by the standard,
+// so we can do whatever we want in that case.
+extern "C" void * _sbrk(ptrdiff_t change)
+{
+  extern char __HeapBase;   // From the linker script
+  extern char __HeapLimit;  // From the linker script
+  static char *programBreak = nullptr;
+
+  // If malloc() hasn't been called before, then set the program break
+  // to __HeapBase from the linker script
+  if (nullptr == programBreak)
+  {
+    programBreak = &__HeapBase;
+  }
+  // We must return the old program break if everything is ok, so save it
+  char * const oldProgramBreak = programBreak;
+  // Check that the new program break doesn't pass __HeapLimit from the
+  // linker script
+  if ((programBreak + change) > &__HeapLimit)
+  {
+    return (void *) -1;
+  }
+  // Update the program break according to the requested amount of heap
+  // memory
+  programBreak += change;
+  return oldProgramBreak;
+}