CODEWITHSUNDEEP
c++filefile-ioio
ozgeneral
ozgeneral

Reputation: 6779

fprintf always write to the end of the file even when I do rewind(fileptr) before, c++

I want to append a file and update some of its lines at the same time. After appending as I desired, say I want to change only the first line, here is what I tried:

outputptr = fopen(outputName.c_str(), "ar+b");
cout << ftell(outputptr) << " ";
rewind(outputptr);
cout << ftell(outputptr) << "\n";
fprintf(outputptr, "abc");

But that code do not replace the first three letters with abc, instead it also appends the file and writes abc to the end. cout were 60 and 0 for this case, so pointer in fact is moved to the beginning.

How do I go any line of a given file and modify only that line?

Upvotes: 0

Views: 1160

Answers (1)

Mats Petersson
Mats Petersson

Reputation: 129344

The definition of 'a' in the mode field says:

(I've cut out the bits that are relevent for this question - it says some other stuff too)

... Repositioning operations (fseek, fsetpos, rewind) affects the next input operations, but output operations move the position back to the end of file. ...

You probably want "r+b".

http://www.cplusplus.com/reference/cstdio/fopen/

Upvotes: 4

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