Crash when calling ReadFile after LockFileEx - c++

Question

I have several processes that try to read and write the same file. I want each of them to lock the file so that only one of them accesses it at a time.
I tried this (edit: this is a complete test code this time):
#include "stdafx.h"
#include "Windows.h"
bool test()
{
const char* path = "test.txt";
HANDLE hFile = CreateFileA(path,
GENERIC_READ | GENERIC_WRITE,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL,
OPEN_ALWAYS,
FILE_ATTRIBUTE_NORMAL,
NULL);
if (hFile == INVALID_HANDLE_VALUE)
{
printf("ERROR: Cannot open file %s\n", path);
return false;
}
// Lock the file
{
OVERLAPPED overlapped = {0};
BOOL res = LockFileEx(hFile, LOCKFILE_EXCLUSIVE_LOCK, 0, ~0, ~0, &overlapped);
if (!res)
{
printf("ERROR: Cannot lock file %s\n", path);
return false;
}
}
DWORD fileSize = GetFileSize(hFile, NULL);
if (fileSize > 0)
{
char* content = new char[fileSize+1];
// Read the file
BOOL res = ReadFile(hFile, content, fileSize, NULL, NULL);
if (!res)
{
printf("ERROR: Cannot read file %s\n", path);
}
delete[] content;
}
const char* newContent = "bla";
int newContentSize = 3;
// Write the file
BOOL res = WriteFile(hFile, newContent, newContentSize, NULL, NULL);
if (!res)
{
//int err = GetLastError();
printf("ERROR: Cannot write to file\n");
}
// Unlock the file
{
OVERLAPPED overlapped = {0};
UnlockFileEx(hFile, 0, ~0, ~0, &overlapped);
}
CloseHandle(hFile);
return true;
}
int _tmain(int argc, _TCHAR* argv[])
{
bool res = test();
return 0;
}
This works fine on my computer, which has Windows 8. But on my colleague's computer, which has Windows 7, it crashes. Specifically, the calls to ReadFile and WriteFile crash, always.
Note that it never enters the code paths with the error printfs. This code triggers no error except for a write at location 0x00000000 in ReadFile (when run on Windows 7).
We tried to also pass the overlapped struct to the ReadFile and WriteFile calls. It prevents the crash but the lock doesn't work anymore, the file is all scrambled (not with this test code, with the real code).
What am I doing wrong?

Answer 1

Looks like your problem is:
lpNumberOfBytesRead [out, optional] argument is null in your call.
This parameter can be NULL only when the lpOverlapped parameter is not NULL.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365467%28v=vs.85%29.aspx

Answer 2

Heres your problem :
You are missing a necessary struct-member and:
0 and ~0 and {0} are all bad code, constant expressions like these will always produce unepected results -- WINAPI doesnt work like libc, parameters are not always compared against constants, instead they are tested against/via macros and other preprocessor-definitions themselves so passing constant values or initializing WINAPI structs with constants will often lead to errors like these.
After years of experimenting i have found that there is only one surefire way of avoiding them, i will express it in corrected code :
OVERLAPPED overlapped;
overlapped.hEvent = CreateEvent( ........... ); // put valid parameters here!
UnlockFileEx(hFile, 0 /*"reserved"*/, ULONG_MAX, ULONG_MAX, &overlapped);
please read this carefully : http://msdn.microsoft.com/en-us/library/windows/desktop/aa365716%28v=vs.85%29.aspx