Question

I have a FileMapping class that allows me to also lock a file for exclusive use by my process by using the Win32 API function LockFileEx().

bool FileMapping::lockFile(bool wait) {
    if (isFileLocked())
        return true;

    // We want an exclusive lock.
    DWORD flags = LOCKFILE_EXCLUSIVE_LOCK;

    // If we don't want the thread to block, we have to set the appropriate flag.
    if (!wait)
        flags |= LOCKFILE_FAIL_IMMEDIATELY;

    m_isFileLocked = LockFileEx(m_fileDesc, flags, 0, (DWORD) m_mappingLength, (DWORD) (((uint64_t) m_mappingLength) >> 32), NULL);
    return m_isFileLocked;
}

Whenever I get to the LockFileEx() call I get an access violation:

Unhandled exception at 0x7466c2ec in tftpServer.exe: 0xC0000005:
Access violation reading location 0x00000008.

The file handle m_fileDesc is definitely a valid handle (mapping the file into memory with that handle works) and m_mappingLength is just a size_t containing the length of the mapped file portion in bytes.

Does anybody have an idea how to fix this?

Answer 1

Your last argument is NULL, while it should be a pointer to a OVERLAPPED structure. The error about reading location 0x00000008 probably corresponds to the documented requirement that:

You must initialize the hEvent member to a valid handle or zero.

Given the hEvent member comes after two pointers, in 32 bit compilation it’d be 8 bytes from the beginning of the structure. So LockFileEx is probably trying to read the hEvent member, and crashes.