Microsoft KB Archive/102691
PSS ID Number: 102691
Article Last Modified on 11/6/1999
The information in this article applies to:
- Microsoft FORTRAN PowerStation for MS-DOS 1.0
This article was previously published under Q102691
An attempt to link code that contains a COMMON block fails and the linker generates the following message:
where <_XXXX> is the name of the COMMON block.
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.
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.
These problems do not occur with FORTRAN PowerStation 1.0a.
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.
To demonstrate this error, perform the following four steps:
- Compile each of the code examples below.
- Link Sample 1 with Sample 2 or Sample 3. No error occurs at link time but a DX1020 error occurs at run time.
- Link Sample 2 with Sample 1. The link fails and an LNK2133 error occurs.
- 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
Additional query words: 1.00
Technology: kbAudDeveloper kbFORTRANPower100DOS kbFortranSearch kbZNotKeyword3