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Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi?

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  Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi? xmllmx <xmllmx@gmail.com> - 2012-05-26 13:04 -0700
    Re: Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi? "Don Burn" <burn@windrvr.com> - 2012-05-26 20:19 +0000
      Re: Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi? xmllmx <xmllmx@gmail.com> - 2012-05-26 13:37 -0700
        Re: Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi? Tim Roberts <timr@probo.com> - 2012-05-27 18:52 -0700

#25 — Is memcmp an unsolved symbol in NT kernel driver even if compiled with /Oi?

Code Sample 1:

#include <ntifs.h>

NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
{
	char s1[] = "Hello";
	char s2[] = "World";

	int n = memcmp(s1, s2, 5);

	return 0;
}

WDK compiler reports: error LNK2019: unsolved extern symbol _memcmp referenced in function _DriverEntry@8

However, if I modify a bit the code as follows, then the compiler will accept it.

Code Sample 2:

#include <ntifs.h>

NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
{
	char s1[] = "Hello";
	char s2[] = "World";

	memcmp(s1, s2, 5);

	return 0;
}

Why?

Thanks in advance!

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#26

Use RtlCompareMemory not memcmp.


Don Burn
Windows Filesystem and Driver Consulting
Website: http://www.windrvr.com
Blog: http://msmvps.com/blogs/WinDrvr




"xmllmx" <xmllmx@gmail.com> wrote in message 
news:44201cc4-e49a-49fe-9335-5f642e911afd@googlegroups.com:

> Code Sample 1:
>
> #include <ntifs.h>
>
> NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
> {
> 	char s1[] = "Hello";
> 	char s2[] = "World";
>
> 	int n = memcmp(s1, s2, 5);
>
> 	return 0;
> }
>
> WDK compiler reports: error LNK2019: unsolved extern symbol _memcmp referenced in function _DriverEntry@8
>
> However, if I modify a bit the code as follows, then the compiler will accept it.
>
> Code Sample 2:
>
> #include <ntifs.h>
>
> NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
> {
> 	char s1[] = "Hello";
> 	char s2[] = "World";
>
> 	memcmp(s1, s2, 5);
>
> 	return 0;
> }
>
> Why?
>
> Thanks in advance!

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#27

On Sunday, May 27, 2012 4:19:07 AM UTC+8, Don Burn wrote:
> Use RtlCompareMemory not memcmp.
> 
> 
> Don Burn
> Windows Filesystem and Driver Consulting
> Website: http://www.windrvr.com
> Blog: http://msmvps.com/blogs/WinDrvr
> 
> 
> 
> 
> "xmllmx" <xmllmx@gmail.com> wrote in message 
> news:44201cc4-e49a-49fe-9335-5f642e911afd@googlegroups.com:
> 
> > Code Sample 1:
> >
> > #include <ntifs.h>
> >
> > NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
> > {
> > 	char s1[] = "Hello";
> > 	char s2[] = "World";
> >
> > 	int n = memcmp(s1, s2, 5);
> >
> > 	return 0;
> > }
> >
> > WDK compiler reports: error LNK2019: unsolved extern symbol _memcmp referenced in function _DriverEntry@8
> >
> > However, if I modify a bit the code as follows, then the compiler will accept it.
> >
> > Code Sample 2:
> >
> > #include <ntifs.h>
> >
> > NTSTATUS DriverEntry(PDRIVER_OBJECT param1, PUNICODE_STRING param2)
> > {
> > 	char s1[] = "Hello";
> > 	char s2[] = "World";
> >
> > 	memcmp(s1, s2, 5);
> >
> > 	return 0;
> > }
> >
> > Why?
> >
> > Thanks in advance!

Dear Don,

I know RtlCompareMemory is a better choice. But I just wonder its cause. I have a third-party source code which uses a lot of memcmp, so I have to solve it. 

Thank you very much.

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#28

xmllmx <xmllmx@gmail.com> wrote:
>
>I know RtlCompareMemory is a better choice. But I just wonder its cause. 

You must have changed more than just the "int n =".  The difference here is
the optimization level.

memcmp is an intrinsic.  If you have intrinsics enabled (/Oi), then the
compiler expands the code for memcmp inline, by using a "rep cmpsb"
instruction.  No function call is required.  If you have the instrinsic
switch turned off, then it compiles a call to the external function memcmp,
which must exist.

By the way, "memcmp", like most of the C run-time library, is available for
kernel drivers in the library libcntpr.lib.  Just add

    USE_LIBCNTPR=1

to your "sources" file.
-- 
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

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