FIX: Port I/O Functions Not in DLL Version of CRT for VC++ 4.0

ID: Q152030

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 4.0

SYMPTOMS

When compiling a program that uses the _inp and _outp instructions and links with the DLL version of the CRT, you will see one or more of the following errors:

test.obj : error LNK2001: unresolved external symbol __inp

test.obj : error LNK2001: unresolved external symbol __inpw

test.obj : error LNK2001: unresolved external symbol __inpd

test.obj : error LNK2001: unresolved external symbol __outp

test.obj : error LNK2001: unresolved external symbol __outpw

test.obj : error LNK2001: unresolved external symbol __outpd

test.exe : fatal error LNK1120: 6 unresolved externals

CAUSE

The _inp and _outp functions are not exported from the DLL versions of the C Run-time Library (CRT), and do not exist in those versions of the CRT. If you try to use these functions with the DLL version of the CRT, you will get LNK2001 errors.

RESOLUTION

There are several ways to get around the LNK2001 errors:

Use a #pragma intrinsic instruction.

Turn on Instrinsic Optimizations.

Link with one of the static versions of the CRT.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual C++, 32-bit Edition, version 4.1.

MORE INFORMATION

You can avoid the linker errors by using a #pragma intrinsic instruction that will replace the _inp and _outp function calls with their assembly equivalents. In each file that refers to the _inp or _outp instruction, add the following line:


   #pragma intrinsic(_inp,_inpw,_inpd,_outp,_outpw,_outpd)

A similar method that can be used to avoid the link errors is to turn on intrinsic function optimizations for all intrinsic functions. If you are using the Command Line compiler, adding the /Oi switch to the compile options will enable intrinsic function optimizations. To turn on this optimization within Developer Studio, perform the following steps:

From the Build menu, choose the Settings option to display the Project Settings dialog box.

In the Settings For pane, select the Project that is getting the link errors.

Choose the C/C++ tab.

Select Optimizations for the Category.

Choose Customize for the optimizations, then check the Generate Intrinsic Functions optimization.

An additional method to avoid the linker errors is to use the static version of the CRT because the _inp and _outp functions are implemented in this version of the CRT. For more information on determining which static library to use, please see the following article in the Microsoft Knowledge Base:

Q140584 How to Link with the Correct C Run-Time (CRT) Library

Additional query words: 4.00

Keywords : kbCRT kbVC
Version : 4.00
Platform : NT WINDOWS
Issue type :