Microsoft KB Archive/39526

= How to Share a Structure Between C and Assembler Modules =

Article ID: 39526

Article Last Modified on 10/17/2003

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APPLIES TO


 * Microsoft Macro Assembler 5.0
 * Microsoft Macro Assembler 5.1 Standard Edition
 * Microsoft Macro Assembler 6.0 Standard Edition
 * Microsoft Macro Assembler 6.0a
 * Microsoft Macro Assembler 6.0b

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This article was previously published under Q39526



SUMMARY
You may declare a type with STRUC directive in your MASM module with the same memory-storage template as the &quot;struct&quot; type declared in your C module. You also can declare the variable as external with WORD type if it is a near data, or DWORD type if it is a far data.

In C, the structure may be packed on a 1, 2, or 4-byte boundary. The 2-byte boundary is the default. As a result, the template constructed in your MASM module may not match the memory storage exactly, unless pad bytes are specifically added to the template. See the example below.

In MASM 5.10 and earlier, any template you may create in with the STRUC directive has to have unique field names throughout the module. The field names represent the offset relative to the beginning of the structure. They do not have to be literally the same field names of the structure defined in the C module.



MORE INFORMATION
The sample program below demonstrates this information. It consists of a C module and an Assembler module.

Sample Code
/* Compile options needed: none

struct sample { char byte_1; unsigned int word_1; char byte_2; unsigned int word_2; } rec = {0x41, 0xffff, 0x42, 0xeeee};

void proc_rec( void );

void main {  proc_rec; }

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 * Assemble options needed: /Mx (or /Cx with 6.0 and later)

.model small,c .data

sample STRUC byte_1 db   ? ; if dw is used, next padding byte is not needed. junk_1 db   ? ; necessary padding, unused byte word_1 dw   ? byte_2 db   ? junk_2 db   ? ; necessary padding, unused byte word_2 dw   ? sample ENDS

EXTRN     rec:word    ; can use EXTRN  rec:sample with 6.0 or later

.code PUBLIC    proc_rec

proc_rec PROC  near MOV ax, rec.word_1 MOV bx, rec.word_2 MOV cl, rec.byte_1 MOV ch, rec.byte_2 RET proc_rec ENDP END

Additional query words: kbinf 5.00 5.10 6.00 6.00a 6.00b

Keywords: KB39526

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