INFO: Declaring a Pointer to a Function: C4071, C4001, or C4220

• Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SYMPTOMS

In Microsoft C, the old method of declaring a pointer to a function,

   type (*ptr)();
   ptr = function;

   C4071: 'ptr' : no function prototype given

   C4001: nonstandard extension [...] was used

   C4220: varargs matches remaining parameters

RESOLUTION

Use one of the following methods to avoid the warning message:

1. Prototype the function with a complete argument list and then specify

      type function(parameter_list);
      type (*ptr)(parameter_list);
      ptr = function;

1. Adjust the warning level (i.e., /W1, /W2, etc.).

MORE INFORMATION

The following program will generate a warning when compiled with the warning level set as specified by the "compile options needed" comment. To eliminate the warning use the prototype which is shown in the comment.

Sample Code

  /* Compile options needed: /W3 - Microsoft C version 6.xx
                             /W4 - Microsoft C/C++ version 7.0, Visual C++,
                                   all versions
  */

  #include <stdio.h>

  void main (void)
  {
     /* Declare 'fun_ptr' as a pointer to a function  */
     /* Use the following line to correct the warning */
     /* int (*fun_ptr)(const char *, ... ); */
     int (*fun_ptr)();
     int other_args;

     /* Assign pointer to the specific function, 'printf' */
     fun_ptr = printf;

     /* Standard usage in calling environment */
     fun_ptr("format string goes here", other_args);
  }

Keywords          : CLngIss kbcode kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5; WINDOWS  NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo