PBYTE has 16 Bytes of Junk values.

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PBYTE m_pSectorBitmap = new BYTE[512];
memset(m_pSectorBitmap, 0xFF, 512);

For the above code snippet, i am expected to get a BYTE array with FF upto 511 positions, But for some reasons i am getting another 16 Byte that is I am getting 511 BYTES of FF and another 16 Bytes of JUNK values. Can someone explain y it is so. What should i do to get exactly 511 Bytes of FF.

I have tried the same with 128 Bytes, still getting the same problem, Is it so generic, or is it not at all a problem ?

Thanks,
Sajith

Posted 23-Mar-13 7:49am

gssajith87

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H.Brydon · Accepted Answer · 2013-03-23T11:12:00

The way that pointers work is that they point to one memory location. When you allocate memory (returned in a valid, non-NULL pointer), the pointer is set to the first byte in the newly allocated memory. If the allocation is successful, you get to use the exact number of bytes that you requested, and no more. Looking at the data beyond what you requested is called "undefined" behavior, and can result in several behaviors that you can not control (maybe it is "padding" that is really also allocated, it might be overhead for your allocation, invalid memory, another allocation, or something else).

The memory you point to in the example you provide is only valid for the first 512 bytes. If you look beyond that you are breaking the rules. What seems to be junk to you is used by the memory allocation system, and you are not entitled to access it.