Using PROTO and INVOKE to Call a C Function from MASM

Last reviewed: February 21, 1995
Article ID: Q73407

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

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 "c" langtype prepends
                                     ; labels with an underscore.
     ;-----for OS/2-------
     ;INCLUDELIB OS2.LIB
     ;INCLUDE    OS2.INC
     ;--------------------

EXTRN     _acrtused:NEAR
EXTRN     printf:NEAR

          .DATA
fmtlist   db     "%s, %d, %lu", 0Ah,0
string_1  db     "signed byte and unsigned double word", 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 "os_os2" 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   "%s, %d, %lu", 0Ah,0
string_1  BYTE   "signed byte and unsigned double word", 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 reference words: kbinf 6.00 6.00a 6.00b
KBCategory: kbprg
KBSubCategory: MASMLngIss

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Last reviewed: February 21, 1995
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