IDA Help: Shifted pointers

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Sometimes in binary code we can encounter a pointer to the middle of a structure. Such pointers usually do not exist in the source code but an optimizing compiler may introduce them to make the code shorter or faster.

Such pointers can be described using shifted pointers. A shifted pointer is a regular pointer with additional information about the name of the parent structure and the offset from its beginning. For example:

        struct mystruct
        {
          char buf[16];
          int dummy;
          int value;            // <- myptr points here
          double fval;
        };
        int *__shifted(mystruct,20) myptr;

The above declaration means that myptr is a pointer to 'int' and if we decrement it by 20 bytes, we will end up at the beginning of 'mystruct'.

Please note that IDA does not limit parents of shifted pointers to structures. A shifted pointer after the adjustment may point to any type except 'void'.

Also, negative offsets are supported too. They mean that the pointer points to the memory before the structure.

When a shifted pointer is used with an adjustment, it will be displayed with the 'ADJ' helper function. For example, if we refer to the memory 4 bytes further, it can be represented like this:

        ADJ(myptr)->fval

Shifted pointers are an improvement compared to the CONTAINING_RECORD macro because expressions with them are shorter and easier to read.