using fwrite and double pointer to output 2D array to file

Question

I've dynamically allocated a 2D array, accessed w/ a double pointer, like so:

  float **heat;
  heat = (float **)malloc(sizeof(float *)*tI); // tI == 50
  int n;
  for(n = 0; n < tI; n++){ // tI == 50
            heat[n] = (float *)malloc(sizeof(float)*sections); // sections == 30
    }

After filling all of its elements with values (I'm sure they are all filled correctly), I'm trying to output it to a file like so:

fwrite(heat, sizeof(heat[tI-1][sections-1]), ((50*30)-1), ofp);

I've also just tried:

fwrite(heat, sizeof(float), ((50*30)-1), ofp);

Am I trying to output this 2D array right?

I know that my file I/O is working (the file pointer ofp is not null, and writing to the file in decimal works), but very strange characters and messages are being outputted to my output file when I try to use fwrite to write in binary.

I know that heat[tI-1][sections-1] is also indeed within the bounds of the 2D array. I was just using it because I know that's the largest element I'm holding.

My output is a lot of NULLs and sometimes strange system paths.

diwatu · Accepted Answer · 2013-05-09 20:59:51Z

7

As i know you should use:

for(int n = 0; n < tI; n++)
{
  fwrite(heat[n], sizeof(float),sections, ofp);
}

answered May 9, 2013 at 20:59

diwatu

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6 Comments

James Holderness Over a year ago

I just edited my answer with example code that looks suspiciously like I copied it directly from you, but I'd like to assure you that wasn't the case. Didn't see your answer until I had finished the edit.

LazerSharks Over a year ago

@JamesHolderness @ Giswin Hmm, I'm trying this code but I'm still getting a lot of strange characters. Can I write floats to binary using fwrite? Or could I possibly be writing to the file succesfully but Notepad++ just cannot read binary?

James Holderness Over a year ago

If you want the output in a form that is human readable, that's a completely different answer. What exactly are you expecting? All the numbers in one line space separated or comma separated? One number per line or one section per line? Basically you're going to need to use something like fprintf.

LazerSharks Over a year ago

I was expecting fwrite to write out to my file in human readable binary, but it's not necessary. Does it write it in non-human readable format? If so that's fine too, as long as it explains these strange characters.

James Holderness Over a year ago

Yeah. It's going to write out exactly what is stored in memory, namely the binary representation of a float, 1500 times (30 * 50).

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James Holderness · Accepted Answer · 2013-05-09 21:04:11Z

5

You can't write it out in one go, because each item in the heat array is allocated a separate segment of memory. You need to loop through the heat array and save one part at a time.

You'd need to do something more like this:

for (n = 0; n < tI; n++)
  fwrite(heat[n], sizeof(float), sections, ofp);

edited May 9, 2013 at 21:04

answered May 9, 2013 at 20:58

James Holderness

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