Take this file: myfile.txt for instance.

How do I read the bits (1s and 0s) of this file and write them somewhere else in C to get the exact myfile.txt

Simply put I want to know how to regenerate files by reading and rewriting their 1s and 0s in C.

CodePudding user response：

You are asking how to read an entire file into memory. This is done by calling read in a loop.

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

#define CHUNK_SIZE ...

// Returns 0 on success.
// Returns -1 and errno set on error.
int read_file( int fd, char **buf_ptr, size_t *buf_size_ptr )
   *buf_ptr      = NULL;
   *buf_size_ptr = 0;

   char *buf = malloc( CHUNK_SIZE );
   if ( !buf )
      goto ERROR1;

   size_t total_bytes_read = 0;
   while ( 1 ) {
      ssize_t bytes_read = read( fd, buf   total_bytes_read, CHUNK_SIZE );
      if ( bytes_read == -1 )
         goto ERROR2;

      if ( bytes_read == 0 )
         break;

      total_bytes_read  = bytes_read;

      char *tmp = realloc( buf, total_bytes_read   CHUNK_SIZE );
      if ( !tmp )
         goto ERROR2;

      buf = tmp;
   }

   {
      char *tmp = realloc( buf, total_bytes_read );
      if ( tmp ) {
         buf = tmp;
   }

   *buf_ptr      = buf;
   *buf_size_ptr = total_bytes_read ;
   return 0;

ERROR2:
   free( buf );
ERROR1:
   return -1;
}


int main( void ) {
   int fd = ...;

   char *buf;
   size_t buf_size;
   if ( read_file( ..., &buf, &buf_size ) == -1 ) {
      perror( NULL );
      exit( EXIT_FAILURE );
   }

   // Do something with buf...

   free( buf );

   return EXIT_SUCCESS;
}

Regenerating the file is simply writing the buffer to disk.

// Returns 0 on success.
// Returns -1 and errno set on error.
int write_file( int fd, const char *buf, size_t buf_size ) {
   while ( buf_size > 0 )  {
      ssize_t bytes_written = write( fd, buf, buf_size );
      if ( bytes_written == -1 )
         return -1

      buf       = byte_written;
      buf_size -= byte_written;
   }

   return 0;
}

CodePudding user response：

All others have clarified that in C (and in any other programming language which is not hardware oriented) you need to cope with bytes as a minimum unit.

If you want to "play with bits", you can read bytes and then extract the single bits from them:

#include <stdio.h>
#include <stdbool.h>

int readbit(FILE *f)
{
    static int buffer;
    static int n = 0;
    
    if (n == 0) {
        buffer = fgetc(f);
        if (buffer == EOF) {
            return EOF;
        }
        n = 8;
    }
    return (buffer >> --n) & 1;
}

int main(void) 
{
    for (int i = 0, bit; (bit = readbit(stdin)) != EOF;   i) {
        printf("%d%c", bit, ((i   1) % 8 == 0)*' ');
    }

    return 0;
}

Here you see a simple function that "reads bits" from a file. How? It reads and buffers a single byte, then it returns a bit of that byte.

The main function uses it to read stdin and send it in textual binary representation on standard output. If you try it with " a0" it sends to stdout "00100000 01100001 00110000 ", i.e. the binary representation of those three bytes (the numerical code for space is 32, for a is 97, for 0 is 48).

Try it online on ideone.com.

DISCLAIMER: I'm not suggesting that this is a production ready function, nor that using static variables in functions is a good idea. But it makes everything simpler here. Don't multithread this.