FIX: Using EXTERNDEF w/ a label May Work Incorrectly

MS-DOS           | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

In the Microsoft Macro Assembler (MASM) initializing a data structure with the offset of a label declared with EXTERNDEF or PROTO can cause incorrect results.

RESOLUTION

Use EXTERN to perform the same task as the EXTERNDEF; this method will work properly. Also, instead of using OFFSET to initialize a WORD field in the structure, use the label to initialize a NEAR PTR field.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and 6.0b. This problem was corrected in MASM for MS-DOS version 6.1.

MORE INFORMATION

EXTERNDEF was introduced as a convenient way to declare shared data structures, variables, symbols, and labels inside a header file. The same functionality can be obtained by using EXTERN and PUBLIC. To replace "EXTERNDEF <name>":

• If <name> is used in the current module but is not defined, use "EXTERN <name>:<qualified type>".
• If <name> is defined in the current module, use "PUBLIC <name>".

Sample Code

; Assemble options needed: /c /Zi ; Link options needed: /CO ; If SOURCE1.ASM and SOURCE2.ASM are assembled and linked together, ; The structure fields an_st.func2 and an_st.func3 will be improperly ; initialized with the offset 0x0000.

;===========================<source1.asm>============================= .MODEL small

st_st STRUCT 1b

   func1 WORD ?
   func2 WORD ?
   func3 WORD ?
   func4 WORD ?

; Uncomment the line below to have fn_2a properly marked for external ; linkage. Note that if the EXTERNDEF statement is not commented out, ; the EXTERN statement must come first for this to work properly. ;EXTERN fn_2a:NEAR

EXTERN fn_1a:NEAR EXTERNDEF fn_2a:NEAR fn_3a PROTO NEAR

.DATA an_st st_st { \

   offset fn_1a,
   offset fn_2a, ; This doesn't work.
   offset fn_3a, ; This doesn't work either.
   offset fn_4a}

   .startup
   .exit 0

   ret

;===========================<source2.asm>============================= .MODEL small EXTERNDEF fn_1a:NEAR EXTERNDEF fn_2a:NEAR EXTERNDEF fn_3a:NEAR

.CODE fn_1a:

   ret

   ret

   ret