how to change single chars by overwriting

[nichtswissender]

hi

i have a problem by writing into a file in binary mode...

i want to overwrite single characters in given positions...

so far i´m using that code:

FILE *out;
out = fopen(<filename>, "ab"); // a for append, b for binary
fseek (out, <intPosition>, SEEK_CUR);
fputc(<char>, out);

the problem with this technique is, that the code doesn't overwrite existing characters, but appends everything at the end of the file (although i have set the filepointer to the correct position)...

using "wb" instead of "ab" when opening the file results in overwriting the whole file...

does anybody know a correct (and maybe quick) way to handle this problem ?

thx a lot
nichtswissender (<-- knownothingman ;))

[sjeng]

Why don't you just read the file in to the memory and change it and write it to disk like a new file?
That would be a simple sollution....

[nichtswissender]

because i have to change up to 600 files. inc. database-files which can have up to 80MB. And i only have to change a few bytes per file.

[sjeng]

I don't know which development envirement you are using, but Visual C++ has the CFile class that has a read-write mode...
I don't know what facilities ansi C++ has :(

[nichtswissender]

i'm working on Borland Builder...

the FILE * object works similar to the CFILE * object from visual...

there also exists the read-write mode, but when i open the file in write-mode, the whole file gets overwritten... when i open the file in append-mode, everything is appended at the end of the file...
:(

[Philip Nicoletti]

Maybe this will do what you want




#include <iostream>
#include <fstream>
#include <ios>

int main()
{
std::fstream myfile("myfile.dat",std::ios::in|std::ios::out|std::ios::binary);

myfile.seekg(0); // goto start of file
myfile.put('c'); // and replace it with 'c'

myfile.seekg(8); // goto the ninth character in file
myfile.put('Q'); // and replace it with 'Q'

myfile.seekg(0,std::ios::end); // goto the end of the file
// start appending as usual
myfile.write("append",6);

return 0;
}

[nichtswissender]

it´s working now :D

ThX

[Ungi]

you also could have used your method with the opening mode "r+b"!

[nichtswissender]

so... here is the next problem...

when i have edited the files i have to cut off a few bytes at the end.

how can i delete the last few bytes? at the moment i only can overwrite them and I can´t find a way to delete them after i have set the EOF byte '\0'.