jtm111
asked on
Memory Mapping function MapViewOfFile parameters
I am learning how to memory map a large file...
I can map the entire file to a character buffer as follows just fine using the following call:
lpMMFile = (char *) MapViewOfFile (hInMap,
FILE_MAP_READ,
0,
0,
0);
Parameter 3 is the upper DWORD of the file offset
Parameter 4 is the lower DWORD
Parameter 5 is the number of bytes to transfer
When all three are set to 0, the whole file is mapped to lpMMFile. My program works just fine with these arguments.
I don't know how to use parameters 3 through 5 to select parts of the file into RAM.
Isn't a DWORD an 8 bit binary? I can't make it work.
For example, how would I select the contents starting at offset 20 and load 50 bytes into the char buffer?
My program displays (null).
I can map the entire file to a character buffer as follows just fine using the following call:
lpMMFile = (char *) MapViewOfFile (hInMap,
FILE_MAP_READ,
0,
0,
0);
Parameter 3 is the upper DWORD of the file offset
Parameter 4 is the lower DWORD
Parameter 5 is the number of bytes to transfer
When all three are set to 0, the whole file is mapped to lpMMFile. My program works just fine with these arguments.
I don't know how to use parameters 3 through 5 to select parts of the file into RAM.
Isn't a DWORD an 8 bit binary? I can't make it work.
For example, how would I select the contents starting at offset 20 and load 50 bytes into the char buffer?
My program displays (null).
A DWORD stands dor Double WORD and is a 32 bit integer. On Win9x systems the hihg dword of the file offset is supposed to be 0
The maximum length of file is 64 bit number.
Windows used to support only 32 bit variable, so in
order to represent every address in the system, U must use
2 DWORDS (hence, 64 bit variable).
Generally, Parameter 3 - the high-order DWORD of offset is 0.
Parameter 4 - will be the offset of the file
and parameter 5 - will be the size that U wanna map.
Good luck
Windows used to support only 32 bit variable, so in
order to represent every address in the system, U must use
2 DWORDS (hence, 64 bit variable).
Generally, Parameter 3 - the high-order DWORD of offset is 0.
Parameter 4 - will be the offset of the file
and parameter 5 - will be the size that U wanna map.
Good luck
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Thanks for the help everybody. I was using the DWORD correctly after all (casting hex values into DWORD), but I didn't know about the granularity issue so I was just flailing randomly. For benefit of others, I used the following code to get my offset and it seems to work:
LPSYSTEM_INFO si;
si = new SYSTEM_INFO;
GetSystemInfo (si);
DWORD dwOffsetLow = (DWORD) si->dwAllocationGranularit
Windows 98 also uses 65,536.
LPSYSTEM_INFO si;
si = new SYSTEM_INFO;
GetSystemInfo (si);
DWORD dwOffsetLow = (DWORD) si->dwAllocationGranularit
Windows 98 also uses 65,536.
on Windows95/98 and similar systems.