trancate fonction

[manoir]

Hi,

I'm using c++ (2011) with the truncate function. I would like to cut the file fichier.txt after the byte 1000.
So here is my programm

I would like to use the truncate function.
Here is the program

using namespace std;
#include <string>
#include <fstream>
#include <stdio.h>
#include <unistd.h>

 // namespace std;
// {
// include <string>
// }
 int main()
 {

///// creation d'un fichier random avec des a aux bits Ã* supprimer
/// marche pas

fstream lulu;
lulu.open("C:/Users/H2O/Desktop/odoo_exe/fichier.txt",ios::out);

for (int i=1;i<900;i=i+1)
{
lulu << "bonjour le monde"<<i<<"\n";
}
truncate ("C:/Users/H2O/Desktop/odoo_exe/fichier.txt",10000);
lulu.close();

 	return 0;
}

And here is the file

bonjour le monde 1
...../ like that
bonjour le monde478
bonjour le monde479
bonjour le monde480
bonjour le monde481
// the file should stop here but it starts again
bonjour
bonjour le monde890
bonjour le monde891
bonjour le monde892
bonjour le monde893
bonjour le monde894
bonjour le monde895
bonjour le monde896
bonjour le monde897
bonjour le monde898
bonjour le monde899

So i don't understand what is going on

thank you

[wolle]

I'm using c++ (2011) with the truncate function.

The truncate function doesn't seem to belong to the C++ standard but I found a description here,

https://www.ibm.com/support/knowledg...bd00/trunc.htm

It says if truncate returns -1 you can check the errno variable to get an indication what's wrong.

My guess would be that you need to close the file before you call truncate. You can easily check if that's the problem by just swapping two lines like,

lulu.close(); // close file before calling truncate
truncate ("C:/Users/H2O/Desktop/odoo_exe/fichier.txt",10000);

[Marc G]

I suggest to use the portable C++ Standard resize_file() method. It's part of the C++ Standard Library std::filesystem.
See https://en.cppreference.com/w/cpp/fi...em/resize_file

[2kaud]

As a general rule, you should always check return value/error code from a called function for problems arising. Some C++ functions throw exceptions rather than a return value/error code. In these cases these functions should be used within the bounds of a try/catch to check for errors in the called function. The function documentation will describe how issues are reported.

Functions which take a file name as a parameter usually do not succeed if the file is already open. Open files should be first closed before these functions are used.

[Marc G]

Just a note: The C++ Standard resize_file() method I mentioned earlier requires C++17.

[manoir]

Yes i think truncate function do not belong to c++ but linux. When i close the file before truncate it i do not have that problem anymore, it truncate the file.
Thank you for your help, i was stuck with that problem.

[wolle]

i was stuck with that problem.

Or the problem was stuck with you!

You can avoid truncation altogether by only writing exactly what you want on the file in the first place.