PRB: LNK2133 Size Mismatch on Common Block During Link

• Microsoft FORTRAN PowerStation for MS-DOS, version 1.0

SYMPTOMS

An attempt to link code that contains a COMMON block fails and the linker generates the following message:

   error LNK2133: Size mismatch for symbol <_XXXX>

CAUSE

The source code declares an initialized COMMON block with one size in one routine and an uninitialized COMMON block with the same name but with another size in another routine.

RESOLUTION

The best method to resolve this situation involves modifying the source code such that each declaration of the COMMON block has the same size. If this is not possible, modify the LINK command line to list the object module that contains the initialized COMMON block declaration before the other modules. In addition, the initialized COMMON block declaration must be as large or larger than any subsequent COMMON block declaration or memory corruption or other incorrect results may occur.

STATUS

These problems do not occur with FORTRAN PowerStation 1.0a.

MORE INFORMATION

The Common Object File Format (COFF) implements COMMON blocks using two record types: COMDAT and COMDEF. A COMDAT record contains initialized COMMON block data and a COMDEF record contains uninitialized COMMON block data. LINK32 resolves each reference to the COMMON block by associating each COMDEF record with the first COMDAT record it encounters, if any. The first COMDAT record determines the amount of space reserved for the COMMON data storage; if no COMDAT record exists, the largest COMDEF record determines the amount of space reserved. This behavior has the following implications:

   If the data in a COMMON block is not initialized in any routine,
   COMMON block declarations with different sizes should not cause any
   problems and you can link the object files with the varying
   declarations in any order. Each declaration must be in a separate
   source file or the FORTRAN compiler generates an error.

   The application should initialize the COMMON block in only one
   location. LINK32 ignores subsequent COMDAT records and does not
   adjust the amount of storage set aside for the COMMON block. If the
   first COMDAT record in the file is smaller than subsequent COMDAT
   records, LINK32 does not generate an error message. However, when
   you run the application it could corrupt memory. Generating
   debugging information with the /4Yb compiler option switch does not
   catch this condition because each array in a COMMON block is
   correctly dimensioned in each object module.

   If LINK32 encounters a COMDEF record before a COMDAT record, the
   size of the COMDEF record must agree with the size of the COMDAT
   record. Otherwise, LINK32 generates the LNK2133 error and the link
   fails.

1. Compile each of the code examples below.

2. Link Sample 1 with Sample 2 or Sample 3. No error occurs at link time but a DX1020 error occurs at run time.

3. Link Sample 2 with Sample 1. The link fails and an LNK2133 error occurs.

4. Link Sample 3 with Sample 1. No errors occur at link time or at run time. LINK32 ignores the data statement in Sample 1.

Sample Code 1

C Compiler options needed: /c

      program tester
      integer arr
      common /testblk/ arr(1)
      data arr /11/
      call sub1
      print*, arr(1000000)
      end

Sample Code 2

C Compiler options needed: /c

      subroutine sub1
      integer arr
      common /testblk/ arr(1000000)
      arr(1000000) = 99
      end

Sample Code 3

C Compile options needed: /c

      subroutine sub1
      integer arr
      common /testblk/ arr(1000000)
      data arr /1000000*77/
      arr(1000000) = 99
      end