Should the binary output be the same as the ASCII input?

Question

I'm writing a program that reads an ASCII file and then converts it to a Binary file, as I see it's not such a hard task, but understanding what's happening behind is ...

As I understand, an ASCII file is just human readable text, so if we want to create a new file full of ASCII's, a simple loop with a fputc() would be enough and for a binary file fwrite() will do the job right?

So my question here is, once that the ASCII to Binary conversion is done, what should I see in my .bin file? It should be filled with exactly the same symbols <88><88><88><88><88>?

Code:

/*
*  From "Practical C Programming 2nd Edition"
*  Exercise 14-4: Write a program that reads an ASCII file containing a list of numbers
*  and writes a binary file containing the same list. Write a program that goes the
*  other way so that you can check your work.
*
*/

#include <stdio.h>
#include <stdlib.h>

const char *in_filename = "bigfile.txt";
const char *out_filename = "out_file.bin";

int main()
{

    int ch = 0;

    /* ASCII */
    FILE *in_file = NULL;

    in_file = fopen(in_filename, "r");

    if(!in_file)
    {
         fprintf(stderr, "ERROR: Could not open file %s ... ", in_filename);
         exit(EXIT_FAILURE);
    }

    /* Binary */
    FILE *out_file = NULL;

    out_file = fopen(out_filename, "w+b");

    if(!out_file)
    {
         fprintf(stderr, "ERROR: New file %s, could not be created ... ", out_filename);
         exit(EXIT_FAILURE);

    }

    while(1)
    {
        ch = fgetc(in_file);
            if(ch == EOF)
                break;
            else
               fwrite(in_file, sizeof(char), 1, out_file);
    }

        fclose(in_file);
        fclose(out_file);

    return 0;

}

I'm generating the input file with this shell script:

tr -dc "0-9" < /dev/urandom | fold -w100|head -n 100000 > bigfile.txt

Any help would be very appreciate it.

Thanks.

Answer 1

fwrite(in_file, sizeof(char), 1, out_file);

is wrong because an integer is given where a pointer is expected.

You can use fputc to write one byte like

fputc(in_file, out_file);

If you still want to use fwrite for some reason, prepare a data to write and write that like

{
    unsigned char in_file_byte = in_file;
    fwrite(&in_file_byte, sizeof(in_file_byte), 1, out_file);
}

Now the contents of the output file will be the same as the input file. Some system may perform conversion of newline characters and it may make the contents differ because the input file is opened in text mode.

Answer 2

Opening a file in text mode or binary mode has nothing to do with ASCII/binary conversion. It has to do with how the operating system deals with some special characters (such as new line characters), line size limit or file extensions.

In the fopen Linux man page:

The mode string can also include the letter 'b' either as a last character or as a character between the characters in any of the two-character strings de‐ scribed above. This is strictly for compatibility with C89 and has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux. (Other systems may treat text files and binary files differently, and adding the 'b' may be a good idea if you do I/O to a binary file and expect that your program may be ported to non-UNIX environments.)

For more information about opening a file in text or binary mode, see https://stackoverflow.com/a/20863975/6874310

Now, back to the ASCII conversion:

All the data in a computer is stored in bits so in the end everything is binary.

A text file containing ASCII characters is also a binary file, except its contents can be mapped to the ASCII table characters in a meaningful way.

Have a look at the ASCII table. The ASCII character number zero (0) has a binary value of 0x30. This means that the zero you see in a text file is actually a binary number 0x30 in the memory.

Your program is reading data from a file and writing to another file without performing any ASCII/binary conversion.

Also, there is a small error here:

fwrite(in_file, sizeof(char), 1, out_file);

It probably should be:

fwrite(&ch, sizeof(char), 1, out_file);

This writes the byte in variable ch to out_file. With this fix, the program basically reads data from the file bigfile.txt and write the very same data to the file out_file.bin without any conversion.

To convert a single digit ASCII number to binary, read the digit from your input file in a byte (char type) and subtract 0x30 from it:

char ch = fgetc(in_file);

if(ch == EOF)
{
    break;
}
else if (isdigit(ch))
{
   ch = ch - 0x30;
   fwrite(&ch, sizeof(char), 1, out_file);
}

Now, your output file will be actually binary. Use isdigitto ensure the byte is an ASCII number. Add #include <ctype.h> in the beginning of your file to use it.

So, for a small input file with the following text:

123

Its binary representation will be:

0x313233

And, after the ASCII numbers are converted to binary, the binary contents will be:

0x010203

To convert it back to ASCII, simply reverse the conversion. That is, add 0x30 to each byte of the binary file.

If you're using a Unix-like system, you can use command line tools such as xxd to check binary files. On Windows, any Hex Editor program will do the job.