Write to a file in binary in C isn't working

Question

I have an array of floats that I want to output to a file in binary. My relevant code is as follows:

    FILE *binFile;

    binFile = fopen(fileName, "wb");

    if (binFile)
    {
          fwrite(completeList, sizeof(float), size, binFile);
    }

Now, completeList is a pointer to an array of floats I was talking about, and size is another parameter I'm passing it indicating how many elements there are.

When this is outputted to a binary file, opening that file shows a bunch of random ASCII characters. I know this is what I should expect, but when I put it through a hex editor it shows random crap.

What am I doing wrong?

EDIT:

My parsing code:

    FILE *bofFile = fopen(file, "rb");

    if(bofFile)
    {
        float *tempArray;

        fseek(bofFile, 0, SEEK_END);

        unsigned long int size = ftell(bofFile) / sizeof(float);

        tempArray = (float *)malloc(size);

        fread(tempArray, sizeof(float), size, bofFile);

        std::cout << tempArray[0];
    }

float numbers ARE random ascii crap when you view the raw bytes. they don't look like "numbers" because a standard float number's internal field divisions don't line up on normal 8bit/byte boundaries. — Marc B, Sep 27 '12 at 03:30
I'm putting it through a hex editor and that shows me it's not anything meaningful. — Tassos S, Sep 27 '12 at 03:34
Can you interpret IEEE 1394 floats by eye? Can you tell that `40490fdb` is pi? — nneonneo, Sep 27 '12 at 03:35
@nneonneo, IEEE 1394 is FireWire, not floating point. I think you mean 754. — Carl Norum, Sep 27 '12 at 03:43
Yes, I get my standards mixed up. I should've just stuck with 'IEEE float'. Thanks. — nneonneo, Sep 27 '12 at 03:45
I just tried to parse it back in and it's giving me nothing too. — Tassos S, Sep 27 '12 at 04:13

paddy · Answer 1 · 2012-09-27T22:00:49.047

2

Don't know if this is related, but you have a fairly serious problem here:

tempArray = malloc(size);

You should change that to prevent buffer overrun:

tempArray = malloc(size * sizeof(float));

Oh, and you also forgot to seek back to the start of the file before reading (that would be why it's "giving you nothing"):

fseek(bofFile, 0, SEEK_SET);

edited Sep 27 '12 at 22:00

answered Sep 27 '12 at 04:35

paddy

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You should [not cast the return value of `malloc()`](http://stackoverflow.com/a/605858/28169), in C. – unwind Sep 27 '12 at 07:13
True... I just copied/pasted the user's code. Will fix. – paddy Sep 27 '12 at 22:00

score 0 · Answer 2 · answered Sep 27 '12 at 04:55

How is your input file formatted? Your code assumes that it too will look 'random' when viewed in a text editor. Have you tried displaying the entire array, to ensure you are reading the data from the input file correctly? Also, you have a malloc() problem that someone else pointed out.

The binary representation of the IEEE floating point format is not intuitive, and it will look like random data.

Write to a file in binary in C isn't working

2 Answers2