CODEWITHSUNDEEP
c++windowsfstream
olaf
olaf

Reputation: 73

fstream gives me wrong file size

I wrote simple function that read whole file into a buffer.

#include <iostream>
#include <fstream>
int main()
{
    std::ios_base::sync_with_stdio(0);
    std::ifstream t;
    t.open("C:\\Users\\sufal\\Desktop\\test.txt");
    t.seekg(0, std::ios::end);    
    long length = t.tellg();           
    t.seekg(0, std::ios::beg);  
    std::cout << "file size: " << length << std::endl;
    char* buffer = new char[length+1];    
    t.read(buffer, length);       
    t.close();
    buffer[length] = 0;
    std::cout << buffer << std::endl;

    
    return 0; 
}

And this is test.txt:

1
2
3

The output that the program produces looks like this: enter image description here

The file size should be 5 bytes. Why my program shows wrong file size? Windows Explorer also seems to show wrong file size of 7 bytes.

Upvotes: 1

Views: 1389

Answers (3)

Remy Lebeau
Remy Lebeau

Reputation: 595981

Your file is 7 bytes in size, because it uses CRLF line breaks.

1[cr][lf]
2[cr][lf]
3

But, you are opening the file in text mode, which on Windows will normalize CRLF line breaks to LF. You are allocating 7 chars for your buffer, but read() is outputting only 5 chars:

1[lf]
2[lf]
3

That is why you see the extra 2 = on the end of the print output, because you didn’t zero out the unused buffer space, so you are seeing random garbage from uninitialized memory.

To do what you are attempting, open the file in binary mode instead.

t.open("C:\\Users\\sufal\\Desktop\\test.txt", std::ios_base::binary);

See Binary and text modes on cppreference.com for more details.

Upvotes: 2

BartoszKP
BartoszKP

Reputation: 35891

On Windows the newline character is "\r\n", which consists of two bytes. So, if your file does not end with a newline, 7 is indeed its size:

1     <-- 1 byte for '1', 2 bytes for CRLF
2     <-- 1 byte for '2', 2 bytes for CRLF
3     <-- 1 byte for '3'

To read the file correctly on a byte level you need to open it in binary mode:

t.open("C:\\Users\\sufal\\Desktop\\test.txt", ios_base::binary);

(you can read about the details of this behavior in the documentation).

You can also see other options to read the whole file into a string in C++:

Upvotes: 4

loa_in_
loa_in_

Reputation: 1011

On Windows this file is indeed 7 bytes: 1 \r\n 2 \r\n 3

Windows encodes new line in two bytes - CR + LF (or \r + \n in other notation).

All is correct.

Upvotes: 1

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