fread from c++ binary file returning wrong value

Question

I am trying to read and write some boolean grids to a file using stdio.h. The user inputs a number nx (from 1 to 10, generally) and the program generates a list of nx by ceil(nx / 2) boolean grids (ceil(nx / 2) is ny). The grids themselves are stored in __int64s, so this grid (f is false and T is true):

ffTT
fTfT

would be 172 (10101100).

The end list of grids is outputted to a binary file.

My code:

std::vector<__int64> grids;

...

FILE *cFile;
if (fopen_s(&cFile, ("grid_" + std::to_string(nx) + "_c.bin").c_str(), "wb") != 0) return;

for (int i = 0; i < grids.size(); i++) {
    fwrite(&grids[i], (int) ceil((nx * ny) / 8), 1, cFile);
}

fclose(cFile);

This part works fine.

However, when I try to read from a file, all of the grids are -858993460, regardless of size, although it gets the number of grids correct. My code for reading:

FILE *cFile;
if (fopen_s(&cFile, ("grid_" + std::to_string(nx) + "_c.bin").c_str(), "rb") != 0) return;

fseek(cFile, 0, SEEK_END);
long size = ftell(cFile);

int grids = size / ((nx * ny) / 8);

for (int n = 0; n < shapes; n++) {
    __int64 data;
    fread(&data, (int) ceil((nx * ny) / 8), 1, cFile);
    printf("%i\n", data);
}

fclose(cFile);

What am I doing wrong?

If you need any more information to answer, leave a comment and I'll give it to you.

Thanks in advance!

@RSahu It makes the output an integer number of bytes. Not using ceil would round down, so 603 would end up as 91. — ricky3350
– ricky3350, Commented May 1, 2015 at 0:50

R Sahu · Accepted Answer · 2015-04-29 21:23:07Z

3

Problem

The problem is that you are moving the FILE* to the end of the file using:

fseek(cFile, 0, SEEK_END);

And then, you are trying to read the data without going back to the start of the file.

You are not checking the returned value of:

fread(&data, (int) ceil((nx * ny) / 8), 1, cFile);

to check whether the read was successful or not.

Fix

Add the line

fseek(cFile, 0, SEEK_SET);

to rewind the file.

Always check the return value of reading operations.

if ( fread(&data, (int) ceil((nx * ny) / 8), 1, cFile) == 1 )
{
   // Successful read.
   // Use the data
}

edited Apr 29, 2015 at 21:23

answered Apr 29, 2015 at 20:54

R Sahu

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Comments

R Sahu · Accepted Answer · 2015-04-29 20:56:54Z

0

I think you are using fwrite and fread in strange ways. Please do

 fwrite(&gris[i], sizeof(__int64), 1, cFile);

for dump and

 fread(&data, sizeof(__int64), 1, cFile);

for recovering.

edited Apr 29, 2015 at 20:56

R Sahu

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answered Apr 29, 2015 at 20:49

Mel Viso Martinez

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4 Comments

ricky3350 Over a year ago

sizeof(__int64) would always return 8. I am shooting for the smallest filesize possible, therefore I use (int) ceil((nx * ny) / 8), the smallest possible size.

Mel Viso Martinez Over a year ago

Yes, but for storing a single item you must specify at least 1 byte. The element size is independent of array size.

ricky3350 Over a year ago

Each array is nx by ny

Mel Viso Martinez Over a year ago

yes but you pass &grids[i] (that's a __int64*) when writting and fwrite takes account of the bytes of the element: the second parameter cannot be used for write a single bit, the minimum is 1 byte. cplusplus.com/reference/cstdio/fwrite The same happens for fread.

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