Reading using fstream

Question

I am using fstream to read a binary file, but strangely I get different values for the same input file each time I execute the code.

if(fs->is_open())
  {
    while (!fs->eof())
    {
      fs->seekg( pos );
      fs->read( (char *)&mdfHeader, sizeof(mdfHeader_t) );
      pos += mdfHeader.length;
      fs->read( (char *)&eventHeader, sizeof(eventHeader_t) );
      fs->read( (char *)&rawHeader, sizeof(rawHeader_t) );

      fs->read( (char *)&ingressHeader, sizeof(ingressHeader_t) );

      fs->read( (char *)&l1Header_xc0, sizeof(l1Header_xc0_t) );

      fs->read(data, dataLength);
      printf("Data=%#x\n",data);

      std::cout << "counter: " << c << "\n";
      c++;
    }

    fs->close();

  }

As you can see, I print out data, which should be the same each time, but yields a different value. mdfHeader.length is the length of one block of data.

You need to print out a lot more diagnostic data. What's the value in pos; what do you get in mdfHeader; what's in mdfHeader.length; and each of the other variables read? Do you check the I/O status during the operations? How are all the variables created and allocated? — Jonathan Leffler
– Jonathan Leffler, Commented Oct 19, 2012 at 14:36
How does the data you print out correspond to the data that's actually in the file? Can you recognize anything? If so, how far off are the actual results compared to the expected results? If you know how many bytes off you are, then you might recognize that number when it appears in your debugger, and point you to the source of your mistake. — Rob Kennedy
– Rob Kennedy, Commented Oct 19, 2012 at 14:46

Dietmar Kühl · Accepted Answer · 2012-10-19 14:50:24Z

The first things to change are:

The condition eof() is only really useful to determine why reading data failed but it isn't a useful condition for a loop.
You need to check after reading that you successfully read the data you are interested in.

That, the loop would look something like this:

while (*fs) {
    // read data from fs
    if (*fs) {
        // do something with the data
    }
    else if (!fs->eof()) {
        std::cout << "ERROR: failed to read record\n";
    }
}

I'd also guess that you don't need the seeks and it is a good idea to get rid of them: seeking is relatively expensive because it looses any buffer. You didn't show the entire code but the initial value of pos has a fair chance to provide some level of randomness. Also, you assume that the sequence of bytes you are reading matches how the data is laid out in your computer. Typically, that isn't the case and you generally need to adjust the binary format, e.g., to accommodate different sizes of words, different endianess, padding, etc.

BigBoss · Accepted Answer · 2012-10-19 14:51:02Z

Computer is like mathematics, every thing is certain(even for functions like rand if input be the same, the output is also same as before) So if you run a code a hundred time with same input and state you will certainly get same output, unless input or running state changed.

You say that input is same each time you execute the code, so only thing that is changed is running state( for example malloc may return 2 different value each time that you run the program, because it may work in different state, because its state will be indicated by the OS ).

In your code you use printf("Data=%#x\n",data); to output your data, but it actually just print address of data as HEX value, so it is very natural that in multiple runs of the program this address may changed because OS map your executive to different positions or anything else. You should output content of the data and you will see that it will be same as previous run

That's certainly a possibility---probably a likely one; it's fairly common practice for 4malloc to introduce some randomness in the addresses it returns (perhaps using /dev/random, which is truly random). But he doesn't give us enough information to be sure, and there's so much probable undefined behavior in his code that it's hard to be sure. (See Dietmar's answer for many of the other problems.)
@JamesKanze There is a lot of problems, but problems like EOF shall be same on all executions of the program, so other things that will be changed in multiple execution of program are variables that are not initialized correctly and in our case the most possible solution is bad expectation of @mareks about the printf. It is normal that specified printf generate different result! so after solving this it may other errors are it can solve his/her problem completely! that depend on other problems of the code

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