Microsoft KB Archive/73407

= Using PROTO and INVOKE to Call a C Function from MASM =

Article ID: 73407

Article Last Modified on 10/20/2003

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APPLIES TO


 * 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 Q73407



SUMMARY
With the Microsoft Macro Assembler (MASM) version 6.0, the coding for a procedure call may be simplified by the use of the PROTO and INVOKE directives. These directives handle many of the details, such as pushing the parameters on the stack in the correct order, generating the correct external references, coercing arguments to the correct size, and cleaning up the stack (if required) after the function terminates.

The two sample programs below illustrate how a C function is called from MASM, both with and without the PROTO and INVOKE directives. Sample Code 1 calls the C printf function using the conventional method of coding. Sample Code 2 shows the simplified call to the same function through the use of PROTO and INVOKE.



MORE INFORMATION
PROTO defines a prototype for a procedure much the way a function prototype works in C. This is the syntax for PROTO: label PROTO [distance] [langtype] [,[parameter]:tag] The PROTO statement is used by the assembler to check parameter types and quantity along with indicating the naming convention for the function. Arguments for the function are indicated by listing the type, and optionally, a parameter name. For example, myfunc PROTO C arg1:SWORD, arg2:SBYTE This indicates that the function myfunc takes two arguments. The first is a signed word, the second is a signed byte. If you need a variable argument list, you use the type VARARG.

INVOKE actually generates the code to call the function. You must have defined the function previously with either a PROC, an EXTERNDEF, a TYPEDEF, or a PROTO statement. This is the syntax for INVOKE: INVOKE expression [,arguments] Because the assembler knows what the function is expecting in the way of arguments and calling convention, it can take the arguments passed in the INVOKE statement and push them on the stack in the correct order, call the function using the required function name, and clean up the stack afterwards (if required by the calling convention used).

If an argument passed by INVOKE is smaller than the type specified in the PROTO statement, MASM does a type conversion. It widens the argument in the PROTO statement to match that in the INVOKE statement (for example, SBYTE to SWORD). These types of conversions use the AX and DX registers on the 8086 and 8088 and the EAX and EDX registers on the 80386/80486. Because these registers are effectively overwritten, you should take care to avoid using these registers to pass arguments.

The language type for the function determines the naming and calling conventions. In addition to the language type in the PROTO statement, the language type can be set by the .MODEL directive, the OPTION LANGTYPE:, or by the command line switches /Gc (for Pascal) and /Gd (for C). There is a table of the various language conventions provided in Help.

Sample Code 1
; Assemble options needed: /MX

.MODEL small,c            ; The &quot;c&quot; langtype prepends ; labels with an underscore. ;-for OS/2--- ;INCLUDELIB OS2.LIB ;INCLUDE   OS2.INC ;

EXTRN    _acrtused:NEAR EXTRN    printf:NEAR

.DATA fmtlist  db     &quot;%s, %d, %lu&quot;, 0Ah,0 string_1 db     &quot;signed byte and unsigned double word&quot;, 0 data_1   db     -2 data_2   dd     0FFFFFFFFh

.CODE

main     PROC push  word ptr data_2+2   ; push the high word of data_2 push  word ptr data_2     ; push the low word of data_2 mov   al,data_1 cbw                       ; converts data_1 to a word push  ax          mov    ax,offset string_1  ; load the address of string_1 push  ax                  ; push the address on the stack lea   ax,fmtlist          ; load the address of fmtlist push  ax                  ; push the address on the stack call  printf              ; call the C library function add   sp,0Ah              ; adjust the stackpointer main     ENDP ret end

Sample Code 2

 * Assemble options needed: none

.MODEL small,c

;-for OS/2| ;.MODEL small,c,os_os2| ;INCLUDELIB OS2.LIB  <---Not needed if &quot;os_os2&quot; indicated. The ;INCLUDE   OS2.INC   |   assembler knows to look for os2.lib ;-|  in the path set by the lib environment ;                    |   variable.

EXTERNDEF _acrtused:WORD

printf   PROTO arg1:Ptr Byte, printlist: VARARG


 * The first argument is a pointer to a string. The second is a keyword
 * that permits a variable number of arguments.

.STACK 100h .DATA fmtlist  BYTE   &quot;%s, %d, %lu&quot;, 0Ah,0 string_1 BYTE   &quot;signed byte and unsigned double word&quot;, 0 data_1   SBYTE  -2 data_2   DWORD  0FFFFFFFFh

.CODE main     PROC INVOKE   printf, ADDR fmtlist, ADDR string_1, data_1, data_2 main     ENDP ret end

Additional query words: kbinf 6.00 6.00a 6.00b s_c

Keywords: KB73407

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