PRB: C4058 Generated with /ASu, /ASw, /AMu, and /AMw

MS-DOS                 | OS/2       | WINDOWS

   The Microsoft C/C++ Compiler (CL.EXE), included with:
    - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
    - Microsoft C for OS/2, versions 6.0 and 6.0a
    - Microsoft C/C++ for MS-DOS, version 7.0
    - Microsoft Visual C++ for Windows, versions 1.0 and 1.5

SYMPTOMS

An attempt to pass the address of a local variable to a function that expects a pointer fails when the compiler options include /ASu, /ASw, /AMu, or /AMw. C versions 6.x generate the following message:

   C4058: address of automatic (local) variable taken, DS != SS

   C4762: near/far mismatch in argument; conversion supplied

   C4758: address of automatic (local) variable taken, DS != SS
   C4762: near/far mismatch in argument : conversion supplied

   C4762: near/far mismatch in argument : conversion supplied

CAUSE

The segment setup codes "u" and "w" inform the compiler that the stack segment (SS) and the data segment (DS) are not necessarily identical. In the small and medium memory models, data pointers are assumed to be near (in DS) and local variables are stored on the stack (in SS). The warnings occur because DS and SS may not be equal.

RESOLUTION

To eliminate the warning, perform one of the following:

• Declare the local variable with the "static" attribute.
• Declare the function to receive a _far pointer parameter.
• Compile the code with the compact or large memory model. However, this option may have other implications for your code.

MORE INFORMATION

The following sample code illustrates the situation:

Sample Code

/*
 * Compile options needed: /AMw
 */

void inc(int *);

void main(void)

   int p = 666;
   inc(&p);               /* address of stack variable */
}

void inc(int *p)

   (*p)++;