CODEWITHSUNDEEP
c
Vladimir Scofild
Vladimir Scofild

Reputation: 65

read specific address in memory using C

could anybody explain me how to read DWORD ,WORD ,BYTE in C from specific address in memory?
for example, base address that is returned using MapViewOfFile() function,how can I read a consecutive BYTE,WORD, or DWORD using C ?
Thanks.

Upvotes: 0

Views: 3681

Answers (4)

Evan Teran
Evan Teran

Reputation: 90422

Well your question has basically two parts:

  1. could anybody explain me how to read DWORD ,WORD ,BYTE in C from specific address in memory?

    That's easy, but probably not what you want, you simply cast the address to a pointer of the desired type.

    DWORD x = *((DWORD *)address);

  2. for example, base address that is returned using MapViewOfFile() function,how can I read a consecutive BYTE,WORD, or DWORD using C ?

    That's also not too bad. You are getting a void* from MapViewOfFile, simply cast it or assign it to an appropriate pointer type and then you can use it like an array:

    DWORD *p = MapViewOfFile(...);
DWORD x = p[1]; // second DWORD of the mapped file

Upvotes: 1

MattD
MattD

Reputation: 380

MapViewOfFile() returns LPVOID which is a typedef of void*. You'll need a cast.

The easiest thing to do would be to read a byte at a time. You don't specify if there's any kind of performance requirement here (nor do you specify your platform, arch, etc), so I'll assume a "byte at a time" is ok.

Note: WORD is defined as short and should be 16-bits in Win32. DWORD is an int and should be 32-bits in Win32.

LPVOID pvAddr= MapViewOfFile(...);

BYTE* pBytes= (BYTE*)pvAddr;

BYTE firstByte= pBytes[0]; /* copy first byte */

WORD w;
memcpy(&w, pBytes+1, 2); /* copy the next two bytes */

DWORD dw;
memcpy(&dw, pBytes+3, 4); /* copy the next 4 bytes */

Hope that helps.

Upvotes: 1

Mats Petersson
Mats Petersson

Reputation: 129314

Assuming that the address is aligned suitably for the datatype you are using - this is important as in some architectures, it's invalid to access an "unaligned" address!

In C:

void *address = MapViewOfFile(...);
DWORD *dword_ptr = (DWORD *)address; 
WORD  *word_ptr = (WORD *)address;
BYTE  *byte_ptr = (BYTE *)address;

In C++, the pattern is similar, but instead of a basic C style cast, you should use reinterpret_cast<type*>(address), where type is DWORD, WORD, BYTE, etc. E.g.:

DWORD *dword_ptr = reinterpret_cast<DWORD *>(address);

Upvotes: 1

Kninnug
Kninnug

Reputation: 8053

MapViewOfFile returns an LPVOID (which is equivalent to a void *) so it can be read like any other pointer.

Upvotes: 0

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