Programming

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>


int main(void) 
{
    struct stat st;
    char *whichtty;

    if ( !isatty(STDIN_FILENO) ) 
    {
	    (void) fprintf(stderr, "%s\n", "stdin is not connected to a tty device.");
    } 
    else 
    {
	    if ( (whichtty = ttyname(STDIN_FILENO)) == NULL ) 
	    {
	        perror("ttyname");
            return 1;
	    } 
	    else 
	    {
	        (void) fprintf(stderr, "stdin is connected to tty device %s.\n", whichtty);
	    }
    }

    if ( fstat(STDIN_FILENO, &st) == -1 ) 
    {
	    perror("file status");
	    return 1;
    } 
    else 
    {
	    if ( st.st_mode & S_IWUSR ) 
	    {
	        (void) fprintf(stderr, "%s\n", "stdin appears to be writeable by the user.");
	    } 
	    else 
	    {
	        (void) fprintf(stderr, "%s\n", "stdin does not appear to be writeable by the user.");
	    }
    }
    return 0;
}

[c_d@localhost C scratchpad]$ gcc temp.c
[c_d@localhost C scratchpad]$ ./a.out
stdin is connected to tty device /dev/pts/3.
stdin appears to be writeable by the user.
[c_d@localhost C scratchpad]$ ./a.out <temp.c        #this is what you are doing
stdin is not connected to a tty device.
stdin appears to be writeable by the user.
[c_d@localhost C scratchpad]$ chmod -w temp.c  #remove write perm
[c_d@localhost C scratchpad]$ ./a.out <temp.c
stdin is not connected to a tty device.
stdin does not appear to be writeable by the user.
[c_d@localhost C scratchpad]$ chmod +w temp.c

int main()
{
   if( write(STDIN_FILENO,"arghh!",6) == -1 )
  {
      fprintf(stderr,"Error in write\n");
      exit(2);
  }
  return 0;
}

[c_d@localhost cfiles]$ cat temp.c
#include<unistd.h>
#include<fcntl.h>

int main()
{

    if((write(STDIN_FILENO,"arrgh!",6))==-1)
    {
        perror("error writing to file");
    }    

}


[c_d@localhost cfiles]$ touch file     #create empty file
[c_d@localhost cfiles]$ cat file        #you can see, that the file is empty
[c_d@localhost cfiles]$ gcc temp.c  #compile temp.c
[c_d@localhost cfiles]$ ./a.out 0>file #redirection notice no "arrgh!" prints
[c_d@localhost cfiles]$ cat file         #because "arrgh!" is redirected to file
arrgh![c_d@localhost cfiles]$ rm file ; touch file  
[c_d@localhost cfiles]$ ./a.out <file      # :(
error writing to file: Bad file descriptor
[c_d@localhost cfiles]$