One int to one char* for binary output

c4a · #1 16-Nov-2007, 07:23

Hi!

Still a beginner, I am trying to write binary output in a certain pattern. I have strings and integers to put into the file.

For this I am using a string as buffer, where I just append the data. I assume to save the most storage, I have to put one integer into one byte. So I am using a chars as containers.

This was already solved with java-code. Now I am trying to do the same with c++.

Below an example, how I have implemented this:

CPP / C++ / C Code:


   string str_buffer = ""; 

   string version = "10";
   string creator_name = "xy";
   string data_name = "Data";
   string cc = "DE";
  
   int foo= 12;
   int faa = 52;
   //...(some other integers)...

   str_buffer.push_back(creator_name.size());
   str_buffer += creator_name;
   // ...(same appends for other strings) ...

   int onebyte   = 0x000000ff;
   int twobyte   = 0x0000ff00;
   int threebyte = 0x00ff0000;
   int fourbyte  = 0xff000000;


   char outbyte = foo & onebyte;
   str_buffer.push_back(outbyte);

   outbyte = (foo & twobyte) >> 8;
   str_buffer.push_back(outbyte);

   outbyte = (foo & threebyte) >> 16;
   str_buffer.push_back(outbyte);

   outbyte = (foo & fourbyte) >> 24;
   str_buffer.push_back(outbyte);

   ofstream outputFile("test.bin",ofstream::binary);

   char* c_buffer;

   c_buffer = new char[str_buffer.size()];
   strcpy(c_buffer, str_buffer.c_str());
   outputFile.write (c_buffer, str_buffer.size());

   outputFile.close();

I get actually output for this, the strings seem to be ok, but I think the way I tried to solve the "one integer for one byte"- problem doesn't work.

What am I doing wrong, here?

Thanks for any help!

davekw7x · #2 16-Nov-2007, 12:51

Quote:

Originally Posted by c4a

H...the way I tried to solve the "one integer for one byte"- problem doesn't work....

A char is an integer data type.

If your ints are within the range -128 through +127, then converting between the ints and (signed) chars is lossless. That is: A simple assignment statement will convert one to the other, and you can get back the original value by another assignment statement.

If your ints are within the range 0 through +255, then converting between the ints and unsigned chars is lossless. That is: A simple assignment statement will convert one the other, and you can get back the original value by another assignment statement.

Perhaps you can show how you tested your scheme and came up with results that were not what you expected.

Regards,

Dave

c4a · #3 20-Nov-2007, 02:57

Hi Dave,
thanks for the answer. I solved it! :-)

Of course, you are right with the limited size of the char.
But with my implementation I am only looking byte-wise at the integer, I am throwing in. So I'll get my outcome as byte size - fine for my char. :-)

So the real flaw here was lying in the writing of the string, in which I have stuffed all chars.

CPP / C++ / C Code:

   ofstream outputFile("test.bin",ofstream::binary);

   char* c_buffer;

   c_buffer = new char[str_buffer.size()];
   strcpy(c_buffer, str_buffer.c_str());
   outputFile.write (c_buffer, str_buffer.size());

I don't actually know, why this is happening, but with this I got the weird output. A lot of "cd"s instead of real values, different from each other.

Instead with using just

CPP / C++ / C Code:

outputFile << str_buffer;

it seems to work perfectly well.

But still, I don't know what the problem with the other way is..

Cheers,
Daniela

davekw7x · #4 20-Nov-2007, 07:51

Quote:

Originally Posted by c4a

But with my implementation I am only looking byte-wise at the integer,
So the real flaw here was lying in the writing of the string, in which I have stuffed all chars.

Show us exactly what is in the "string". How did you build the string.

In other words show us exactly what you ran that you didn't understand (the entire program). Tell us exactly what you got and what you expected.

Regards,

Dave

Footnote:

CPP / C++ / C Code:

    strcpy(c_buffer, str_buffer.c_str());

1. This copies all of the bytes in the c_str() and also the terminating zero byte, so you should have allocate one more byte for c_buffer.

2. If your "string" has a zero byte in it, nothing after that will be copied by this statement. Using c_str() or any of the C-style string functions (strcpy, strcat, strlen, etc.) for a "string" that contains arbitrary integer values (like zero, for example) is a bug waiting to happen.

c4a · #5 23-Nov-2007, 08:24

Quote:

Originally Posted by davekw7x

Show us exactly what is in the "string". How did you build the string.
In other words show us exactly what you ran that you didn't understand (the entire program). Tell us exactly what you got and what you expected.

Just as summary (all solved):
As you can see from my original post, I casted bytes of integers to chars and put these together with some other strings into another string.

Like this:

CPP / C++ / C Code:


   string str_buffer = "";

   //int := 1Byte:
     str_buffer.push_back(creator_name.size());
   
  // strings are just appended: 
    str_buffer += creator_name;
   
  // integer > 1Byte - using masks for only 1 Byte output:
   //used as masks: 
     int onebyte   = 0x000000ff;
     int twobyte   = 0x0000ff00;
     int threebyte = 0x00ff0000;
     int fourbyte  = 0xff000000;
  
    //masking and casting with this only one byte into char
    //effect: integer will be "cut into four bytes" and later 
    //with when interpreting file put together, again.
   
    char outbyte = foo & onebyte;
     str_buffer.push_back(outbyte);

     outbyte = (foo & twobyte) >> 8;
     str_buffer.push_back(outbyte);

     outbyte = (foo & threebyte) >> 16;
     str_buffer.push_back(outbyte);

     outbyte = (foo & fourbyte) >> 24;
     str_buffer.push_back(outbyte);

Quote:

Originally Posted by davekw7x

CPP / C++ / C Code:

    strcpy(c_buffer, str_buffer.c_str());

2. If your "string" has a zero byte in it, nothing after that will be copied by this statement. Using c_str() or any of the C-style string functions (strcpy, strcat, strlen, etc.) for a "string" that contains arbitrary integer values (like zero, for example) is a bug waiting to happen.

Thanks a lot for this clear explanation! :-)

I finally managed to get my binary ofstream right.
Sorry, for this delayed answer, but there was another stupid bug somewhere else within my code, which had produced some nonsense in my output, too.

Cheers,
Daniela

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One int to one char* for binary output

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Re: One int to one char* for binary output