回复: [edk2-devel] How to assign NULL to a VOID* PCD?

Thu Dec 17 00:56:19 UTC 2020

Tim:

 Another way is to UINT64 PCD. PCD value is the pointer address. It can be NULL. But, the pointer address can always be accessed. 

Thanks

Liming

发件人: bounce+27952+69064+4905953+8761045 at groups.io <bounce+27952+69064+4905953+8761045 at groups.io> 代表 Tim Lewis
发送时间: 2020年12月17日 8:07
收件人: devel at edk2.groups.io; michael.d.kinney at intel.com
主题: Re: [edk2-devel] How to assign NULL to a VOID* PCD?

Mike –

I am trying to find a way to have no data assigned to a VOID*. So either having GetPtr returning NULL or returning a pointer to 0 bytes would work (as long as GetSize() returned 0). Either would work for us.  The latter might require pointing to a label in the FixedAtBuild case. 

I don’t think the PI spec prohibits this (in the API), and I don’t see any requirement for distinct pointer values there. But there is no way to enable it today.

We have found that requiring the PCD to be terminated just leads to implementation errors where an engineer forgets to put the terminator in their PCD array.  Catching this requires buffer bounds checking logic in addition to the normal loop logic and static code analysis doesn’t tend to catch this well.  And I just don’t like wasting bytes, especially in PEI.

I can put this in as a BZ request.

Thanks,

Tim

From: devel at edk2.groups.io <mailto:devel at edk2.groups.io>  <devel at edk2.groups.io <mailto:devel at edk2.groups.io> > On Behalf Of Michael D Kinney
Sent: Wednesday, December 16, 2020 3:22 PM
To: devel at edk2.groups.io <mailto:devel at edk2.groups.io> ; tim.lewis at insyde.com <mailto:tim.lewis at insyde.com> ; Kinney, Michael D <michael.d.kinney at intel.com <mailto:michael.d.kinney at intel.com> >
Subject: Re: [edk2-devel] How to assign NULL to a VOID* PCD?

Hi Tim,

With the current design the smallest possible VOID* PCD is 1 byte.  PcdGetSize() can be used the determine the size.  Does that work or do you need some new fetures?

You are welcome to enter some feature requests for this and we can see if this can be supported by the build tools in a backwards compatible manner.

Best regards,

Mike

From: devel at edk2.groups.io <mailto:devel at edk2.groups.io>  <devel at edk2.groups.io <mailto:devel at edk2.groups.io> > On Behalf Of Tim Lewis
Sent: Wednesday, December 16, 2020 1:30 PM
To: Kinney, Michael D <michael.d.kinney at intel.com <mailto:michael.d.kinney at intel.com> >; devel at edk2.groups.io <mailto:devel at edk2.groups.io> 
Subject: Re: [edk2-devel] How to assign NULL to a VOID* PCD?

Mike –

I understand, but this just adds another layer of complexity. For example, we had a list of exception numbers (0x00-0xff) and then had to move away from UINT8 because it couldn’t hold a terminating or illegal value. We prefer to use PcdGetSize. It may be something we will change in our own codebase.

Tim

From: Kinney, Michael D <michael.d.kinney at intel.com <mailto:michael.d.kinney at intel.com> > 
Sent: Wednesday, December 16, 2020 1:28 PM
To: devel at edk2.groups.io <mailto:devel at edk2.groups.io> ; tim.lewis at insyde.com <mailto:tim.lewis at insyde.com> ; Kinney, Michael D <michael.d.kinney at intel.com <mailto:michael.d.kinney at intel.com> >
Subject: RE: [edk2-devel] How to assign NULL to a VOID* PCD?

Hi Tim,

Assigning NULL to a VOID* PCD is not supported.

I recommend you design the structure that the VOID* type PCD points to be self describing.  You can put a count field or if it is an array, use can use a tag in a field to mark the end of the array.

Mike

From: devel at edk2.groups.io <mailto:devel at edk2.groups.io>  <devel at edk2.groups.io <mailto:devel at edk2.groups.io> > On Behalf Of Tim Lewis
Sent: Wednesday, December 16, 2020 11:06 AM
To: devel at edk2.groups.io <mailto:devel at edk2.groups.io> 
Subject: [edk2-devel] How to assign NULL to a VOID* PCD?

I want the PCD database to contain a NULL to a PCD that is of type VOID* but this does not seem to be allowed in the .dsc files.

In .dec files, I can assign an <Expression> for the value, which lets me put a 0. But the syntax for .dsc files only lets me put data. So for array cases where I want 0 entries, I am always forced to put at least 1 dummy entry. Normally, I would like to use PcdGetSize to get the total array size.

Any pointers?

Thanks,

Tim

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