what would happen if a process wrote to its own stdin? Post: 302302773

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Top Forums Programming what would happen if a process wrote to its own stdin? Post 302302773 by c_d on Wednesday 1st of April 2009 02:06:37 AM

04-01-2009

Registered User

Quote:

Originally Posted by pshaikh

Hi

I know that a.out should be executed as "./a.out" , it was typing mistake.

file.txt is redircted as a.out's standard in put. In my program, I am writing to standard input (as shown in the first post)

yes, so the content of file.txt will be written to stdin of a.out

to demonstrate...

Code:

#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;
}

OP:

Code:

[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

Can someone explain all this to me...or do you think its best I not dig into all this...???

Last edited by c_d; 04-01-2009 at 03:12 AM..

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LEARN ABOUT DEBIAN

bup-margin

bup-margin(1)						      General Commands Manual						     bup-margin(1)

NAME

       bup-margin - figure out your deduplication safety margin

SYNOPSIS

       bup margin [options...]

DESCRIPTION

       bup margin  iterates  through  all  objects  in	your  bup repository, calculating the largest number of prefix bits shared between any two
       entries.  This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.

       For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45.  That  means  a  46-bit
       hash  would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by
       its first 46 bits.

       The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects.  Since SHA-1 hashes have 160 bits,
       that  leaves 115 bits of margin.  Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits
       with far fewer objects.

       If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see	if
       you're getting dangerously close to 160 bits.

OPTIONS

       --predict
	      Guess  the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer
	      from the guess.  This is potentially useful for tuning an interpolation search algorithm.

       --ignore-midx
	      don't use .midx files, use only .idx files.  This is only really useful when used with --predict.

EXAMPLE

	      $ bup margin
	      Reading indexes: 100.00% (1612581/1612581), done.
	      40
	      40 matching prefix bits
	      1.94 bits per doubling
	      120 bits (61.86 doublings) remaining
	      4.19338e+18 times larger is possible

	      Everyone on earth could have 625878182 data sets
	      like yours, all in one repository, and we would
	      expect 1 object collision.

	      $ bup margin --predict
	      PackIdxList: using 1 index.
	      Reading indexes: 100.00% (1612581/1612581), done.
	      915 of 1612581 (0.057%)

SEE ALSO

       bup-midx(1), bup-save(1)

BUP

       Part of the bup(1) suite.

AUTHORS

       Avery Pennarun <apenwarr@gmail.com>.

Bup unknown-															     bup-margin(1)

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