Reading a binary file in text or ASCII format Post: 82877

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Top Forums Programming Reading a binary file in text or ASCII format Post 82877 by jim mcnamara on Wednesday 7th of September 2005 11:08:17 AM

09-07-2005

Registered User

This comes up all too often. Windows has text files. Unix does not. Unix is not Windows. In Unix a file is a file is a file. It's a bag of bytes. Period.

Because a standard C string uses ASCII zero (nul character) as the end of string, that data from files that contain nuls (in Windows these are binary files, in Unix they are just files) cannot be parsed as strings because the nuls confuse everything.

To the OP: try

Code:

od -c filename

to find out what is in the file. Then you will know if you can read it using standard string C calls like fgets(). Or if you will have to use fread().

After you've programmed for a while you tend to bypass fread and fgets, especially when you're dealing with large files that may contain interesting stuff.
This reads an entire file containing anything into a buffer:

Code:

#include <stddef.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/stat.h>
#include <errno.h>
#include <assert.h>

#define ck(x) \
if( (x) == (-1) ){ perror("");exit(EXIT_FAILURE);}

/* read a buffer from a file */
ssize_t readall(int fd, void *buf, size_t *bytes){
     ssize_t nread = 0, n=0;
     size_t nbyte = *bytes;

     do {
         if ((n = read(fd, &((char *)buf)[nread], nbyte - nread)) == -1) {
             if (errno == EINTR)
                 continue;
             else
                 return (-1);
         }
         if (n == 0)
             return nread;
         nread += n;
     } while (nread < nbyte);
     return nread;
}

/* read control */
void readfile(char *fname, char *buffer, size_t *size, mode_t *mode)
{
   int fd=0;   
   struct stat st;
   
   ck(fd=open(fname,O_RDONLY) );
   ck(fstat(fd,&st) );
   *size=st.st_size;   
   *mode=st.st_mode;
   buffer=calloc(1,*size+1);
   ck(readall(fd, buffer, size) );    
   ck(close(fd) );
}


int main(int argc, char *argv[])
{
   char *buffer=NULL;
   size_t size;
   mode_t mode;
 
   readfile(argv[1],buffer,&size,&mode);
   /* play with buffer here  */
   free(buffer);
   return 0;  
}

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LEARN ABOUT PLAN9

deb-old

deb-old(5)							    dpkg suite								deb-old(5)

NAME

       deb-old - old style Debian binary package format

SYNOPSIS

       filename.deb

DESCRIPTION

       The  .deb  format  is the Debian binary package file format. This manual page describes the old format, used before Debian 0.93. Please see
       deb(5) for details of the new format.

FORMAT

       The file is two lines of format information as ASCII text, followed by two concatenated gzipped ustar files.

       The first line is the format version number padded to 8 digits, and is 0.939000 for all old-format archives.

       The second line is a decimal string (without leading zeroes) giving the length of the first gzipped tarfile.

       Each of these lines is terminated with a single newline character.

       The first tarfile contains the control information, as a series of ordinary files. The file control must be present,  as  it  contains  the
       core control information.

       In  some  very  old  archives,  the  files  in  the  control  tarfile  may optionally be in a DEBIAN subdirectory. In that case, the DEBIAN
       subdirectory will be in the control tarfile too, and the control tarfile will have only files in that  directory.  Optionally  the  control
       tarfile may contain an entry for '.', that is, the current directory.

       The second gzipped tarfile is the filesystem archive, containing pathnames relative to the root directory of the system to be installed on.
       The pathnames do not have leading slashes.

SEE ALSO

       deb(5), dpkg-deb(1), deb-control(5).

1.19.0.5							    2018-04-16								deb-old(5)

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