c++ files array messes up my variable

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!

You must remove `FILE* Files`. And edit `FILE **Files=new...` — ForceBru, Feb 25 '15 at 14:02
Is there a reason you don't use e.g. [`std::vector`](http://en.cppreference.com/w/cpp/container/vector)? — Some programmer dude, Feb 25 '15 at 14:09
@dfeuer yes, it may still be C++ but it is not idiomatic (i.e. "natural") C++. If one is learning C++, it can be better to learn modern C++ using `std::string`, `std::vector` and not writing it as ["A Better C"](http://www.stroustrup.com/bs_faq.html#difference). — crashmstr, Feb 25 '15 at 14:18

Some programmer dude · Answer 1 · 2015-02-25T14:09:46.800

4

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].

edited Feb 25 '15 at 14:09

answered Feb 25 '15 at 14:03

Some programmer dude

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It is wrong either way. (&Files)[i] is wrong because only i=0 is valid. &Files[i] is wrong because it would cause a type mismatch. This answer is incorrect. The type of (&Files)[i] and type of &Files[i] are different. The problem is because the entire program is screwed up. Why in the world would open_file not just return FILE*... There is also no need to new FILE. fopen does that for you. – thang Feb 25 '15 at 14:06
It does (in an implicit way) answers why the variable i could be changed. – stefaanv Feb 25 '15 at 14:10
@stefaanv, it doesn't. The answer to why i could change is because (&Files)[1] points to the memory address occupied by i. I suspect the original poster knows that &Files + i is the same as (&Files)[i]. It's must be super implicit... – thang Feb 25 '15 at 14:12

score 2 · Accepted Answer · edited May 23 '17 at 11:43

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
}

just one more question, is this how i should delete the pointer-array: delete [] *Files; — Heinrich Heine, Feb 25 '15 at 14:56
@HeinrichHeine, indeed you should delete the **full array**, like this : `delete[] Files;`. Each element of the array have to be closed. — Orace, Feb 25 '15 at 15:00

c++ files array messes up my variable

2 Answers2