Reversing a binary file adds bits to it and not reversing

Question

I'm writing a function that gets a binary file with integers and reverses their order. For example, I have this binary file (in hexadecimal):

00 00 00 01 00 00 00 02 00 00 00 03 00 00 00 04

and I want it to be:

00 00 00 04 00 00 00 03 00 00 00 02 00 00 00 01

But with my algorithm I get this:

00 00 00 01 00 00 00 02 00 00 00 03 00 00 00 04 CC CC CC CC 00 00 00 00  CC CC CC CC

And I don't get why... This is my algorithm:

void reverse(FILE * fr)
{
    int i, num1, num2, fileLength;
    fseek(fr, 0, SEEK_END);
    fileLength = ftell(fr) / sizeof(int);
    for(i = 0; i < fileLength / 2; i++)
    {
        fseek(fr, i * sizeof(int), SEEK_SET);
        fread(&num1, sizeof(int), 1, fr);
        fseek(fr, i * sizeof(int), SEEK_END);
        fread(&num2, 4, 1, fr);
        fseek(fr, i * sizeof(int), SEEK_END);
        fwrite(&num2, sizeof(int), 1, fr);
        fseek(fr, i * sizeof(int), SEEK_SET);
        fwrite(&num1, sizeof(int), 1, fr);
    }
}
int main()
{
    FILE * f = fopen("test.bin", "r+b");
    reverse(f);
    fclose(f);
    getchar();
    return 0;
}

What am I doing wrong?

EDIT: I am given a file size that divides by sizeof(int).

EDIT 2:

After changing the code as pts pointed out, it still doesn't work. it outputs:

CC CC CC CC CC CC CC CC 00 00 00 03 00 00 00 04 00 00 00 01 00 00 00 00 00 00 00 02

my edited code is shown above.

(By the way, to all the commenters, I was told not to read the whole file to memory and then reverse it because it's supposed to be an excercise in file manipulation, not in memory manipulation.)

Thanks.

Your solution is incorrect unless the file size is divisible by `sizeof(int)`. — pts, Jun 18 '14 at 12:11
Your solution is very slow, because it seeks too often. You should read at least 8 kB at a time, but preferably much more (8 MB). — pts, Jun 18 '14 at 12:13
This is horrible. Seriously consider reading the whole file in, swapping the bytes in memory, and writing them all out. Doing in-place file modification is really scary. — unwind, Jun 18 '14 at 12:24
The value 0xCC everywhere is most likely due to uninitialized variables http://stackoverflow.com/questions/370195/when-and-why-will-an-os-initialise-memory-to-0xcd-0xdd-etc-on-malloc-free-new — phuclv, Jun 18 '14 at 13:23
and why do you reverse each 4 bytes instead of bytes? Note that int is not always 32 bits — phuclv, Jun 18 '14 at 13:24
@LưuVĩnhPhúc It's not always 32 bits, but it's always sizeof(int). — shoham, Jun 18 '14 at 18:08

BLUEPIXY · Accepted Answer · 2014-06-18T20:45:17.843

try this:

void reverse(FILE * fr)
{
    int i, num1, num2, fileLength;
    fseek(fr, 0, SEEK_END);
    fileLength = ftell(fr) / sizeof(int);
    for(i = 0; i < fileLength / 2; i++){
        fseek(fr, i * sizeof(int), SEEK_SET);
        fread(&num1, sizeof(int), 1, fr);
        fseek(fr, (-1-i) * sizeof(int), SEEK_END);
        fread(&num2, sizeof(int), 1, fr);
        fseek(fr, (-1-i) * sizeof(int), SEEK_END);
        fwrite(&num1, sizeof(int), 1, fr);
        fseek(fr, i * sizeof(int), SEEK_SET);
        fwrite(&num2, sizeof(int), 1, fr);
    }
}

int, int, int, int[EOF]
                  ^fseek(filep, 0, SEEK_END)
               ^
               fseek(filep, -1*sizeof(int), SEEK_END)

This very much works, thanks. But could you add an explanation? — shoham, Jun 18 '14 at 18:16

score 0 · Answer 2 · answered Jun 18 '14 at 12:11

0

In the middle you have to seek again. Also the use of num1 and num2 is wrong when writing.

    fread(&num2, 4, 1, fr);
    fseek(fr, i * sizeof(int), SEEK_END);  // Add this.
    fwrite(&num1, sizeof(int), 1, fr);     // num1.
    fseek(fr, i * sizeof(int), SEEK_SET);
    fwrite(&num2, sizeof(int), 1, fr);     // num2.

answered Jun 18 '14 at 12:11

pts

80,836
20
110
183

Oh, I forgot reading and writing moves the cursor of the file. Thanks. – shoham Jun 18 '14 at 12:19
Not explained here, not explained there either actually, but still, at least there are some words there stating that a seek is necessary while switching between reading and writing: http://stackoverflow.com/a/14878976/2736228 – Utkan Gezer Jun 18 '14 at 12:20

Reversing a binary file adds bits to it and not reversing

2 Answers2