Read last X bytes of a file [closed] - c++

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Could anyone tell me a simple way, how to read the last X bytes of a specific file?
If I'm right, I should use ifstream, but I'm not sure how to use it. Currently I'm learning C++ ( at least I'm trying to learn :) ).

Answer 1

Input file streams have the seekg() method that repositions the current position to an absolute position or a relative position. One overload takes a positon type that represents an absolute value. The other takes an offset type and direction mask that determines the relative position to move to. Negating the offset allows you to move backward. Specifying the end constant moves the indicator relative to the end.
file.seekg(-x, std::ios_base::end);

Answer 2

This is a C solution, but works and handles errors. The trick is to use a negative index in fseek to "seek from EOF" (ie: seek from the "right").
#include <stdio.h>
#define BUF_SIZE (4096)
int main(void) {
int i;
const char* fileName = "test.raw";
char buf[BUF_SIZE] = { 0 };
int bytesRead = 0;
FILE* fp; /* handle for the input file */
size_t fileSize; /* size of the input file */
int lastXBytes = 100; /* number of bytes at the end-of-file to read */
/* open file as a binary file in read-only mode */
if ((fp = fopen("./test.txt", "rb")) == NULL) {
printf("Could not open input file; Aborting\n");
return 1;
}
/* find out the size of the file; reset pointer to beginning of file */
fseek(fp, 0L, SEEK_END);
fileSize = ftell(fp);
fseek(fp, 0L, SEEK_SET);
/* make sure the file is big enough to read lastXBytes of data */
if (fileSize < lastXBytes) {
printf("File too small; Aborting\n");
fclose(fp);
return 1;
} else {
/* read lastXBytes of file */
fseek(fp, -lastXBytes, SEEK_END);
bytesRead = fread(buf, sizeof(char), lastXBytes, fp);
printf("Read %d bytes from %s, expected %d\n", bytesRead, fileName, lastXBytes);
if (bytesRead > 0) {
for (i=0; i<bytesRead; i++) {
printf("%c", buf[i]);
}
}
}
fclose(fp);
return 0;
}

Answer 3

You need to use he seekg function and pass a negative offset from the end of the stream.
std::ifstream is("file.txt");
if (is)
{
is.seekg(-x, is.end); // x is the number of bytes to read before the end
}

Answer 4

#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, char* argv)
{
ifstream ifs("F:\\test.data", ifstream::binary);
if(ifs.fail())
{
cout << "Error:fail to open file" << endl;
return -1;
}
//read the last 10 bits of file
const int X = 10;
char* buf = new char[X];
ifs.seekg(-X, SEEK_END);
ifs.read(buf, X);
ifs.close();
delete buf;
return 0;
}

Answer 5

Use seekg() for relative positionning from end of file, then use read():
ifstream ifs("test.txt");
int x=10;
char buffer[11]={};
ifs.seekg(-x, ios_base::end);
if (!ifs.read(buffer, x))
cerr << "There's a problem !\n";
else cout <<buffer<<endl;
Note that read() just takes the x bytes from the file and puts them in the buffer, without adding a '\0' at the end. So if you expect a C string, you have to make sure that your buffer ends with a 0.