I cant merge two files in a third one

Question

My problem is the following when I show the third file, It shows a lot of 0 and not the ints of file 1 and file 2. My file ends up with 4,00 KB (4.096 bytes) for some reason I don't know. Btw, the third file should have first: the first int of file1 and then the first of file2 and so on

Edit: I was testing the problem, and I realize that the size of the third file is correct before I show it. It's 24 and that's the sum of file1 and file 2. But when I read it, then it goes crazy

 #include <iostream>

using namespace std;
int main()
{
  int buffer1, buffer2, buffer3;
  
  //What is written in file 1 and file 2
  int file1_content[] = {1,3,5};
  int file2_content[] = {2,4,6};

  FILE* f3, *f1, *f2;
  f1 = fopen("archivo1", "rb");
  f2 = fopen("archivo2", "rb");
  f3 = fopen("archivo3", "wb+");
  if(!f1)
  {
      cout<<"Error el archivo 1 no se pudo abrir"<<endl;
      return 1;
  }
  if(!f2)
  {
      cout<<"Error el archivo 2 no se pudo abrir"<<endl;
      return 1;
  }
  if(!f3)
  {
      cout<<"Error el archivo 3 no se pudo abrir"<<endl;
      return 1;
  }

   //write file 1 and file two in file 3
   while(fread(&buffer1, sizeof(int), 1, f1) && fread(&buffer2, sizeof(int), 1, f2) )
   {
      fwrite(&buffer1, sizeof(int), 1, f3);
      fwrite(&buffer2, sizeof(int), 1, f3);
   }
   fclose(f1);
   fclose(f2);

   //show file 3
   while(fread(&buffer3, sizeof(int), 1, f3))
   {
       cout<<buffer3<<" ";
   }
   //EXPECTED OUTPUT
   //1 2 3 4 5 6 
   fclose(f3);

    return 0;
}

It is generally considered a poor practice to mix C-style Standard I/O with C++ iostreams, even if one is used exclusively for console output and the other is exclusively used for file I/O. I would recommend switching entirely over to using `` for the entire project. — Schol-R-LEA, Feb 19 '22 at 19:14
Also, this code does not constitute a [minimal, complete and reproducible example](https://stackoverflow.com/help/minimal-reproducible-example), so I would recommend editing the code to provide such. — Schol-R-LEA, Feb 19 '22 at 19:15
@273k I put that c flag because the file method is written in a c way but the rest is c++. That's the idea, write in c++ but with the c way — Facundo Borrás, Feb 19 '22 at 19:16
@FacundoBorrás is the a specific reason why it needs to use STDIO files? — Schol-R-LEA, Feb 19 '22 at 19:17
@Schol-R-LEA I see, however, I have to do it like this. But I will take note of that — Facundo Borrás, Feb 19 '22 at 19:23
Also, is there a reason why you are reading the files into the buffers one integer at a time? It would make more sense and be more efficient to read in a full buffer's worth at a time (making certain to check the return value of `fread()`) and stepping through the buffers in the loop, rather than reading on every pass of the interleaving loop. — Schol-R-LEA, Feb 19 '22 at 19:27
@Schol-R-LEA the reason is that I'm learning about binary files and I don't know how to do that (I will try it anyways, however you do that). Btw, I put new info about my bug. — Facundo Borrás, Feb 19 '22 at 19:32
@AndreasWenzel that was a mistake when I upload it. I was testing stuff so forget to change it. — Facundo Borrás, Feb 19 '22 at 19:56

Andreas Wenzel · Accepted Answer · 2022-02-19T20:43:55.847

1

Your code has undefined behavior.

According to §7.21.5.3 ¶7 of the ISO C11 standard, when a file is opened in update mode (+), the following restrictions apply:

Output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind).
Input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

In your program, you are violating rule #1. The easiest solution would be to add a call to rewind between the calls to fwrite and fread, which you should do anyway, because you want to start reading from the start of the file.

edited Feb 19 '22 at 20:43

answered Feb 19 '22 at 20:08

Andreas Wenzel

22,760
4
24
39

BIG THANKS. I ended up fixing it, but this way is much better and I learned a bunch of things. Btw, how the hell did you know that? Did someone tell you that? Or did you just read all of that? I didn't even know that documentation even exist. Well, I'm a beginner so. Thks – Facundo Borrás Feb 19 '22 at 21:35
@FacundoBorrás: These restrictions are mentioned in [the C documentation of fopen on cppreference.com](https://en.cppreference.com/w/c/io/fopen) (though strangely not in [the corresponding C++ documentation](https://en.cppreference.com/w/cpp/io/c/fopen), although they probably also applies in C++). That is probably how I first learnt about them. – Andreas Wenzel Feb 19 '22 at 22:38
@FacundoBorrás: Also, this is not the first time that I provided this answer. Here are two other answers of mine in which I provided a similar answer: [1](https://stackoverflow.com/a/70570765/12149471) [2](https://stackoverflow.com/a/70312086/12149471) It seems that you are not the only person making this mistake. :-) – Andreas Wenzel Feb 19 '22 at 22:42

I cant merge two files in a third one

1 Answers1