Replacing data in binary file with fstream and seekp

Question

I'm writing a method to replace a specified string from a binary file and it writes NULLs before the position I set with seekp, then writes the string and closes the stream. I only want to replace some bytes in the file. Before this piece of code I tried out with ofstream with ios::binary and ios::out flags. What's wrong to destroy all data in the file? Before this piece of code, I open the file with an instance of ifstream to read the same position verifyng the first byte in the string. I only comment this for information.

Thank you all!

The code:

fstream ofs();
ofs.open(nomArchBin,ios::in | ios::out | ios::binary);

if (!ofs.good()) {
    cout << "...";
    return;
}

ofs.seekp(despEnArchivo,ios::beg);
char* registroChar = registroACadena(reg);
ofs.write(registroChar,cabecera.tamanioReg);

This piece of code destroys the data from the binary file. I only wants to replace some bytes. What have I done wrongly? — Jonás
– Jonás, Commented Apr 1, 2011 at 10:53

James Kanze · Accepted Answer · 2011-04-01 16:15:56Z

4

I know this sounds silly, but the only way to open a file for writing and not to truncate it is to open it for reading as well: if you're really doing ios::in | ios::out | ios::binary, it should work. (But since you obviously reentered the code, and didn't copy/paste it, I'm not sure if this is really what you did.)

Other points you have to pay attention to when trying to seek:

The file must be open in binary mode, and imbued with the "C" locale. (IMHO, a file opened in binary mode should ignore the locale, but this isn't what the standard says.)
Both `seekg` and `seekp` have the same effect; using either changes the position of the other.
The only function which allows seeking to an arbitrary location is the two argument seek; the one argument form can only be used to seek to a position previously obtained by a tell.

answered Apr 1, 2011 at 16:15

James Kanze

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