According to the function manual page truncatein R, on some platforms, including Windows:
... it will not work for large (> 2Gb) files
After some experiments, I managed to make a toy example showing that this can be done for large files (quite easily) with visual C ++:
#include "stdafx.h"
#include <windows.h>
#include <tlhelp32.h>
#include <tchar.h>
#include <iostream>
#include <string>
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;
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);
}
This will add "helloWorld" to the file "C: /kaiyin/kybig.out" and then truncate the "World". In the console, it should print “World” (tail before truncation), then “hello” (tail after truncation).
There seems to be no problem trimming the tail of a file larger than 2 GB - in fact, I tested a file with a size of 4 bytes and the program still behaves correctly.
- , , truncate ( ) Windows?
Update
@hrbrmstr, R bugzilla, R-, , truncate Windows 8.1
filename = "C:/kaiyin/kybig.out"
f = file(filename, "w")
seek(f, 5L, "end")
truncate(f)
file.info(filename)$size
:
> filename = "C:/kaiyin/kybig.out"
> f = file(filename, "w")
> seek(f, 5L, "end")
[1] 0
> truncate(f)
NULL
> file.info(filename)$size
[1] 0
-, truncate , seek ing .