Inconsistent output when reading binary file

Question

Adapting some of the code I have seen in SO, I came out with the following solution:

fstream file("sample.bin", ios::binary | ios::in | ios::ate);

unsigned char charsRead[(int)file.tellg()];

file.read((char *) &charsRead, sizeof(char*));
for(int i=0; i<sizeof(charsRead); i++) 
    cout << (int) charsRead[i] << endl;
file.close();

It does compile, but every time is executed, it returns a different output. Does anyone know why this is happening?

unsigned char charsRead[(int)file.tellg()]; is a non standard VLA. If you need to read into a char buffer consider a std::vector<unsigned char>. — NathanOliver
– NathanOliver, Commented Oct 11, 2016 at 11:34
Mmmm not really. But even when that number is a constant, it still returns different outputs when executing different times, so that does not explain the changes. — Alvaro Gomez
– Alvaro Gomez, Commented Oct 11, 2016 at 11:38

max66 · Accepted Answer · 2016-10-11 12:03:03Z

2

I suppose that the first 4 (or 8) bytes are ever equals and that the different output start from 5th or 9th byte.

As pointed by πάντα ῥεῖ, You read sizeof(char*) bytes (usually 4 or 8 bytes) and you print sizeof(charsRead) bytes.

If sizeof(char*) < sizeof(charsRead) (that is: if the dim of the file is bigger that 4 or 8), you write

sizeof(char*) initialized chars
sizeof(charsRead) - sizeof(char*) uninitialized chars (so, casual values).

answered Oct 11, 2016 at 12:03

max66

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2 Comments

Alvaro Gomez Over a year ago

I still do not understand something. Even if it is wrong, why is the output inconsistent given the same input file?

max66 Over a year ago

@AlvaroGomez - I've tryed to explain it in my answer; short answer: you're not reading the input file, you're printing random values

Alvaro Gomez · Accepted Answer · 2016-10-11 13:55:12Z

0

I still do not understand the output, but I found a quite consistent solution for reading a binary file using a buffer. http://www.cplusplus.com/doc/tutorial/files/

answered Oct 11, 2016 at 13:55

Alvaro Gomez

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