Read tails bytes from a binary file in visual c++ - c++

Question

Here is my attempt:
// ConsoleApplication1.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <windows.h>
#include <tlhelp32.h>
#include <tchar.h>
#include <iostream>
#include <string>
// Forward declarations:
void append(LPCTSTR, LPCVOID, DWORD);
void readTail(LPCTSTR, LPVOID, DWORD);
int main()
{
LPCTSTR fn = L"C:/kaiyin/kybig.out";
LPCVOID buf = "eeeee";
append(fn, buf, 5);
LPVOID buf1 = "";
readTail(fn, buf1, 5);
std::cout << (char*) buf1 << std::endl;
printf("hello world\n");
std::string s = "";
std::getline(std::cin, s);
return 0;
}
void append(LPCTSTR filename, LPCVOID buf, DWORD writeSize) {
LARGE_INTEGER size;
size.QuadPart = 0;
HANDLE fh = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
GetFileSizeEx(fh, &size);
SetFilePointerEx(fh, size, NULL, FILE_BEGIN);
WriteFile(fh, buf, writeSize, NULL, NULL);
CloseHandle(fh);
}
void readTail(LPCTSTR filename, LPVOID buf, DWORD readSize) {
LARGE_INTEGER size;
size.QuadPart = 0;
HANDLE fh = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
GetFileSizeEx(fh, &size);
size.QuadPart -= readSize;
SetFilePointerEx(fh, size, NULL, FILE_BEGIN);
ReadFile(fh, buf, readSize, NULL, NULL);
CloseHandle(fh);
}
The append function seems to increase the file size by the correct number (5 bytes), but printing the last 5 byte to in the console doesn't show anything.
What went wrong?

Answer 1

You have not allocated buffer for your result:
LPVOID buf1 = "";
readTail(fn, buf1, 5);
buf1 is only a pointer to empty string, what you want is some buffer where to put your results:
BYTE buf1[10] = { 0 };
readTail(fn, buf1, 5);
Also, you are appending initialy to existing file (due to OPEN_EXISTING),so makesure you have an empty C:/kaiyin/kybig.out file.

Answer 2

Just for the record, here is the modified code according to suggestions from #marcinj :
// ConsoleApplication1.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <windows.h>
#include <tlhelp32.h>
#include <tchar.h>
#include <iostream>
#include <string>
// Forward declarations:
void append(LPCTSTR, LPCVOID, DWORD);
void readTail(LPCTSTR, LPVOID, DWORD);
void truncateTail(LPCTSTR, long);
int main()
{
LPCTSTR fn = L"C:/kaiyin/kybig.out";
char buf[] = "helloWorld";
append(fn, buf, 10);
BYTE buf1[10] = {0};
readTail(fn, buf1, 5);
std::cout << (char*) buf1 << std::endl;
truncateTail(fn, 5);
for (int i = 0; i < 10; i++) {
buf1[i] = 0;
}
readTail(fn, buf1, 5);
std::cout << (char*) buf1 << std::endl;
printf("End of program\n");
std::string s = "";
std::getline(std::cin, s);
return 0;
}
void append(LPCTSTR filename, LPCVOID buf, DWORD writeSize) {
LARGE_INTEGER size;
size.QuadPart = 0;
HANDLE fh = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
GetFileSizeEx(fh, &size);
SetFilePointerEx(fh, size, NULL, FILE_BEGIN);
WriteFile(fh, buf, writeSize, NULL, NULL);
CloseHandle(fh);
}
void readTail(LPCTSTR filename, LPVOID buf, DWORD readSize) {
LARGE_INTEGER size;
size.QuadPart = 0;
HANDLE fh = CreateFile(filename, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
GetFileSizeEx(fh, &size);
size.QuadPart -= readSize;
SetFilePointerEx(fh, size, NULL, FILE_BEGIN);
ReadFile(fh, buf, readSize, NULL, NULL);
CloseHandle(fh);
}
void truncateTail(LPCTSTR filename, long truncateSize) {
LARGE_INTEGER size;
size.QuadPart = 0;
HANDLE fh = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (fh == INVALID_HANDLE_VALUE) {
std::cerr << GetLastError();
return;
}
GetFileSizeEx(fh, &size);
size.QuadPart -= truncateSize;
SetFilePointerEx(fh, size, NULL, FILE_BEGIN);
if (SetEndOfFile(fh) == 0) {
std::cerr << GetLastError();
return;
}
CloseHandle(fh);
}