Assembler + WinApi MapViewOfFile - winapi

Question

I ve got a little problem with using MapViewOfFile. This function returns the starting address of the mapped view so as I think it's a sequence of bytes. And this is where I ve stacked:
INVOKE MapViewOfFile, hMapFile, FILE_MAP_READ, 0, 0, 0
mov pMemory, eax
mov edx, DWORD PTR [pMemory]
The pointer is correct cause during saving as a whole block of memory to file, everything is fine. So my question is: how to refer to every single elements(bytes).
Thanks in advance

Answer 1

Cast pMemory to the correct type and move it around from pMemory to pMemory + size of the mapped memory - size of the type to which you refer...
In other words, you have effectively allocated memory and associated the menory with a file that is changed as you change the memory.
In C assuming pMemory is the pointer returned by MapViewOfFile:
int x = (*(int *)pMemory); // Read the first int
char c = (*(char *)pMemory); // Read the first char
typedef struct oddball { int x, int y, int z, char str[256] } oddball; // assume the typedef syntax is right...
oddball *p = (oddball *)pMemory; // point to the base of the mapped memory
p += 14; // p now points to the 15th instance of oddball in the file.
// Or... p = &p[14];
p->x = 0;
p->y = 0;
p->z = 0;
strcpy( p->str( "This is the 0, 0, 0 position" ) );
// You've now changed the memory to which p[14] refers.
// To read every byte... (Again in C, use the compiler to generate asm
// Assumes:
// fileSize is the size of the mapped memory in bytes
// pMemory is the pointer returned by MapViewOfFile
// buffer is a block of memory that will hold n bytes
// pos is the position from which you want to read
// n is the number of bytes to read from position pos and the smallest size in bytes to which buffer can point
void readBytes( unsigned int fileSize, char *pMemory, char *buffer, unsigned int n, unsigned int pos )
{
char *endP = pMemory + fileSize;
char *start = pMemory + pos;
char *end = start + n;
int i = 0;
// Code to stay within your memory boundaries
if( end > endP )
{
n -= (end - endP); // This might be wrong...
end = endP;
}
if( end < start )
return;
// end boundary check
for( ; start < end; start++, i++ )
{
buffer[i] = *start;
}
}
Here's the asm code generated from the code above by the compiler with -O2
.686P
.XMM
.model flat
PUBLIC _readBytes
_TEXT SEGMENT
_fileSize$ = 8 ; size = 4
_pMemory$ = 12 ; size = 4
_buffer$ = 16 ; size = 4
_n$ = 20 ; size = 4
_pos$ = 24 ; size = 4
_readBytes PROC ; COMDAT
mov eax, DWORD PTR _pMemory$[esp-4]
mov edx, DWORD PTR _fileSize$[esp-4]
mov ecx, DWORD PTR _n$[esp-4]
add edx, eax
add eax, DWORD PTR _pos$[esp-4]
add ecx, eax
cmp ecx, edx
jbe SHORT $LN5#readBytes
mov ecx, edx
$LN5#readBytes:
cmp eax, ecx
jae SHORT $LN1#readBytes
push esi
mov esi, DWORD PTR _buffer$[esp]
sub esi, eax
$LL3#readBytes:
mov dl, BYTE PTR [eax]
mov BYTE PTR [esi+eax], dl
inc eax
cmp eax, ecx
jb SHORT $LL3#readBytes
pop esi
$LN1#readBytes:
ret 0
_readBytes ENDP
_TEXT ENDS
END