Jochen Kalmbach's WebLog

Confusion about BSS section in DLL/EXEs

Yesterday there was a question in microsoft.public.de.vc about the using of the BSS section in an app with global initialized data. Normally BSS is only used to store uninitialized data. Here is a short example:

  int Val0=0;
  int Val1=1;
  void main() {}

If you compile this with “cl /Fa” you will get the following assembler output (in short):

  _BSS SEGMENT

  ?Val0@@3HA DD01H DUP (?); Val0
  _BSS ENDS
  _DATA SEGMENT
  ?Val1@@3HA DD01H; Val1
  _DATA ENDS

And here you see the problem: Val0 is initialized in the C source code but it seems that it will not be initialized because it is stored in the “uninitialized data” (BSS) section!!! Very confusing, ins´t it?

An other problem is that there is no direct documentation about this behaviour; there are only two references, which leads to the result, that sections with “uninitialized data” (IMAGE_SCN_CNT_UNINITIALIZED_DATA) are always initialized to zero!

First reference: Initializing Scalar Types
If you do not explicitly initialize a global static variable, it is initialized to 0 by default, and every member that has pointer type is assigned a null pointer.

Second reference: Peering Inside the PE: A Tour of the Win32 Portable Executable File Format
The .bss section is where any uninitialized static and global variables are stored.

But I missed some docu about the fact that “uninitialized” only means: The space must not be reserved in the EXE/DLL but the section is always initialized to zero (I also look into the PE-COFF spec but yould not find it either). Maybe someone has a link to this kind of clarification?