c++ files array messes up my variable - c++

Question

My programme keeps producing a segmentation fault.
I have simplified my code to the following:
#include <stdio.h>
void open_file(FILE** file)
{
*file = fopen("test" , "wb");
return;
}
int main ()
{
int Tracks = 1;
FILE* Files;
int i = 1;
Files = new FILE[Tracks + 1];
printf("i = %d\n", i); //i = 1
open_file(&Files + i);
printf("i = %d\n", i); /i = 0
fclose(*(&Files + i)); //Segmentation fault
return 0;
}
I must be doing some really stupid pointer-error, but to me my pointer-arithmetics operations look fine... The problem is, that for some magic reason, the variable i changes its value to 0. Could someone explain to me why??
Thanks in advance!

Answer 1

The problem is one of operator precedence, where the address-of operator have higher precedence than the addition operator. That means you are doing e.g. (&Files) + i, which is equivalent to (&Files)[i].

Answer 2

fopen return a FILE*, so you may want to use an array of FILE* or a FILE** to store many of it.
Also you have to check the file to be opened before closing it.
#include <stdio.h>
void open_file(FILE** file)
{
*file = fopen("test" , "wb");
return;
}
int main ()
{
int Tracks = 1;
FILE** Files = new FILE*[Tracks + 1];
int i = 1;
printf("i = %d\n", i); //i = 1
open_file(&Files[i]); // Will write at the address of the i-th element of Files.
printf("i = %d\n", i); //i = 1
// This have to be done for each element that correspond to an open file.
// Add a for-loop should be considered.
if (Files[i]) // Avoid segmentation fault
{
fclose(Files[i]);
Files[i] = 0; // So I know it is closed.
}
delete[] Files;
return 0;
}
About the Files[i] = 0; you can take a look here.
About the magic i modification.
Here what happens :
void open_file(FILE** file)
{
// Write file address or 0 at the given address.
*file = fopen("test" , "wb");
return;
}
int main ()
{
int Tracks = 1;
FILE* Files = new FILE[Tracks + 1]; // Files at address a
int i = 1; // i probably at address a + sizeof(FILE*)
printf("i = %d\n", i); // i = 1
// here i = 1 so &Files + i = &Files + 1 = address of i.
open_file(&Files + i); // Call to open_file with the address of i !
// open_file failed and write 0 to the given address.
printf("i = %d\n", i); // i = 0
}