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warning C28138: The constant argument should instead be variable

This warning is reported in a function call that expects a variable or a non-constant expression, but the call includes a constant. For information about the function and its parameter, consult the WDK documentation of the function.

For example, in the following code example, the parameter of the READ_PORT_UCHAR macro must be a pointer to the port address, not the address provided as a constant.

The following code example generates this warning message:


To correct this warning, use a pointer to the port address.


There are a few older devices for which a constant parameter is acceptable with the READ_PORT and WRITE_PORT family of functions. When those devices receive this warning, the warning can be suppressed or ignored. However, any new devices should not assume a constant hardware address.