C++: Patching a binary file using fstream

Question

I have a huge file that is already created. I need to write some data at the start of the file while retaining the other contents of the file as such. The following code corrupts the existing file. Can anyone help me with the right method.

 ofstream oFile(FileName,ios::out|ios::binary);  
 oFile.seekp(0);  
 oFile.write((char*)&i,sizeof(i));       
 oFile.write((char*)&j,sizeof(i));
 oFile.close();

EDIT: Basically I want to overwrite some bytes of an already existing file at different locations including start. I know the byte address of locations to write. My write will not change the file size.

I need to do something equivalent to the following code that works:

int  mode = O_RDWR;
int myFilDes = open (FileName, mode, S_IRUSR | S_IWUSR);
lseek (myFilDes, 0, SEEK_SET);
write (myFilDes, &i, sizeof (i));
write (myFilDes, &j, sizeof (j));

Are you intending to append new bytes to the front of the file or overwrite it? — GWW
– GWW, Commented Mar 7, 2011 at 0:37
You will need to test in an hexadecimal editor, then when it works, pass it to C++. — Gareve
– Gareve, Commented Mar 7, 2011 at 0:39
@GWW: You don't "append new bytes to the front". That would be prepending. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Mar 7, 2011 at 0:49
@Tomalak: Thanks, I can't edit my my comment. My brain is broken apparently :P. — GWW
– GWW, Commented Mar 7, 2011 at 0:52
Basically I want to overwrite some bytes of an already existing file at different locations including start. I know the offset to write. My write will not change the file size. — Arpit
– Arpit, Commented Mar 7, 2011 at 0:52

Anya Shenanigans · Accepted Answer · 2011-03-07 00:42:40Z

3

you should perform an:

 oFile.seekp(0);

before performing the write. ios::ate implies you're appending to the file.

You also need to use ios::in instead of ios::out. ios::out implies you plan on truncating the file, which may have unintented consequences.

It's not intuitive

answered Mar 7, 2011 at 0:42

Anya Shenanigans

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2 Comments

Arpit Over a year ago

I tried it. But the original file gets overwritten. Could you pls check the edited question.

Lightness Races in Orbit Over a year ago

@Arpit: The answer you later accepted says the same thing as this answer in less detail.

smatter · Accepted Answer · 2011-03-07 05:02:58Z

1

You are missing ios::in

Use:

ofstream oFile(FileName,ios::out|ios::in|ios::binary);

answered Mar 7, 2011 at 5:02

smatter

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Comments

fpointbin · Accepted Answer · 2011-03-07 00:43:33Z

0

If you wanna "insert", you've have to know that C++ see the "file" like a stream of bytes... So if you have got it:

|1|5|10|11|2|3|

And you wanna insert data in the first position( set your position at 0 ), you will have to move the rest of the file...

answered Mar 7, 2011 at 0:43

fpointbin

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