Subj : Mapped view of file read error beyond 4096 Bytes
To   : borland.public.cpp.borlandcpp
From : Douglas Martin
Date : Thu Jan 20 2005 03:40 am


I can only read the first 4096 Bytes on a page using memory mapped file IO:

/**********************************************************/
/* mmfread.c */

#include <windows.h>
#include <stdio.h>

#define BYTE_TO_READ 4096 /* From 0; 4095 OK, 4096 crashes */

VOID main()
{
	HANDLE hFile;                          
	HANDLE hMapFile;                       
	LPVOID lpMapAddress;                   
        LPBYTE lpMapPointer;                  

	// Open the file
	hFile = CreateFile(
	"data.bin",                             
	GENERIC_READ | GENERIC_WRITE,				  
	0,                                   
	NULL,                                 
	OPEN_EXISTING,                         
	FILE_ATTRIBUTE_NORMAL,              
	NULL);                            

        // Create file mapping object
        hMapFile = CreateFileMapping(
        hFile,                                  
        NULL,                                 
        PAGE_READONLY,                          
        0,                                     
        0,                                     
        NULL);                                

        // Map a view of the file for reading
        lpMapAddress = MapViewOfFile(
        hMapFile,
        FILE_MAP_READ,
        0,
        0,
        1);

	lpMapPointer=(LPBYTE)lpMapAddress+BYTE_TO_READ;

        printf("Byte is 0x%02X\n", *lpMapPointer);
        
	UnmapViewOfFile(lpMapAddress);
        CloseHandle(hMapFile);
        CloseHandle(hFile);
        getchar();

        return;
}
/**********************************************************/

"data.bin" must be in the directory and atleast 4096 Bytes.

/**********************************************************/
/* write.c */

#include <windows.h>

#define BYTES_TO_WRITE 5000

void main()
{
	HANDLE hFile;  
	DWORD dwBytesWritten, i;  
	unsigned char *Buffer;

	Buffer=(unsigned char *)malloc(BYTES_TO_WRITE);
	for (i=0; i<BYTES_TO_WRITE; i++) *(Buffer+i)=i;

        hFile = CreateFile(
        "data.bin",
        GENERIC_READ | GENERIC_WRITE,
        0,
        NULL,
        CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL);

	WriteFile(
	hFile,
	Buffer,
	BYTES_TO_WRITE, 
        &dwBytesWritten, 
	NULL); 

	CloseHandle(hFile); 

	return;
}
/**********************************************************/

The following batch file will compile and run the programs:

rem ---------------------- make.bat -----------------------
bcc32 write.c
write
bcc32 mmfread.c
mmfread
rem -------------------------------------------------------

If the BYTE_TO_READ define is 4095 the Byte is read (Byte is 0xFF). If the BYTE_TO_READ define is 4096 and mmfread recompiled, the executable 'crashes'. This occurs after the first 4096 Bytes of any any 65536 Byte page when memory mapping larger files.

.