Read file from 100th byte - winapi

Question

How can I read a file from 100th byte, using ReadFile function and C++
I used this code but it reads the first 100 bytes of the file
I want to read the second 100 bytes
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
SetFilePointer(hndl, 100, NULL, FILE_BEGIN);
ReadFile(hndl, pbytReadBuffer, 100, NULL, &ol);

Answer 1

The ReadFile API offers two distinct ways to set the starting offset when doing synchronous I/O:
By using the implicitly stored file pointer (which can be manipulated through the SetFilePointer API call).
Or by passing an explicit offset through the OVERLAPPED structure.
Your code fails because you are setting the implicitly stored file pointer, but then (presumably) pass a zero-initialized OVERLAPPED structure, which ignores the file pointer (see Synchronization and File Position for more information).
Either of the following solutions will work. First, by using the implicitly stored file pointer. This is useful, when you want to read chunks of a file in consecutive calls:
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, nullptr,
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
// Move the file pointer to offset 100
SetFilePointer(hndl, 100, NULL, FILE_BEGIN);
// Read contents from the current offset
DWORD dwBytesRead{0};
ReadFile(hndl, pbytReadBuffer, 100, &dwBytesRead, nullptr);
Alternatively, you can pass an OVERLAPPED structure to pass the offset. This ignores the implicitly stored file pointer. It is slightly more efficient, because it doesn't need two calls to the file I/O API.
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, nullptr,
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
OVERLAPPED ol{};
// Set the offset from the start of the file
ol.Offset = 100;
ReadFile(hndl, pbytReadBuffer, 100, nullptr, &ol);
Note that error handling is elided in these samples for brevity. In real code, you always must check for errors.

Answer 2

you need code like this
HANDLE hFile = CreateFile(L"c:\\windows\\notepad.exe", FILE_GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, 0, 0);
if (hFile != INVALID_HANDLE_VALUE)
{
OVERLAPPED ov = {};
ov.Offset = 100;
UCHAR buf[100];
ULONG cb;
if (ReadFile(hFile, buf, sizeof(buf), &cb, &ov))
{
}
else
{
GetLastError();
}
CloseHandle(hFile);
}
you not need call SetFilePointer - much better set offset direct in OVERLAPPED structure
EDIT
for every read/write operation we need to specify the starting byte offset. we can do this directed by setting values in Offset and OffsetHigh from OVERLAPPED.
or indirect only if file opened if synchronous mode - I/O Manager can use the current file position in FILE_OBJECT - so we can not direct set offset - it will be get from FILE_OBJECT.CurrentByteOffset. FILE_OBJECT.CurrentByteOffset we can set with SetFilePointer also every read/write operation update this offset - move forward to count of bytes which readed/ written. of course this is correct only if file used in synchronous mode when all operations with file is sequential
if we use direct offset in OVERLAPPED he and used - so FILE_OBJECT.CurrentByteOffset is ignored - this mean that previous call to SetFilePointer - also lost all effect - will be used offset from OVERLAPPED and after read operation FILE_OBJECT.CurrentByteOffset will be updated based on offset + bytes readed