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


 
Thread Tools Search this Thread
Top Forums Programming what would happen if a process wrote to its own stdin?
# 1  
Old 03-29-2009
what would happen if a process wrote to its own stdin?

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

Code:
#include<unistd.h>
#include<fcntl.h>

int main()
{

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

}

output:
Code:
[c_d@localhost C scratchpad]$ gcc temp.c
[c_d@localhost C scratchpad]$ ./a.out 
arrgh![c_d@localhost C scratchpad]$

apparently it wrote into its stdout...how and why? no asked to process to do so...
# 2  
Old 03-29-2009
It wrote to stdin, aka file descriptor 0. You can add:
close(1);
close(2);
in front of your "if" statement to help prove it.

In this case, the shell opened /dev/tty for reading and writing and made all three initial fd's point to it.

Try:
echo hello > file
./a.out < file
echo who knows | ./a.out
# 3  
Old 03-29-2009
Quote:
Originally Posted by Perderabo
It wrote to stdin, aka file descriptor 0. You can add:
close(1);
close(2);
in front of your "if" statement to help prove it.

In this case, the shell opened /dev/tty for reading and writing and made all three initial fd's point to it.

Try:
echo hello > file
./a.out < file
echo who knows | ./a.out

i tried the exercise you suggested on
Code:
#include<unistd.h>
#include<fcntl.h>

int main()
{

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

}

i get no output or error whatsoever
but when i perform the same exercise on my original code

i get
Code:
[c_d@localhost C scratchpad]$ ./a.out <file
error writing to file: Bad file descriptor
[c_d@localhost C scratchpad]$ echo watup | ./a.out
error writing to file: Bad file descriptor

Quote:
In this case, the shell opened /dev/tty for reading and writing and made all three initial fd's point to it.
i wanted to see that...but i m not able to make fcntl() work...
Code:
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>

int main()
{
    int x=2,file,fd;
    
    if((file=open("fdvals",O_RDWR))==-1)
    {
        perror("error opening fdvals");
    }
    fd=fcntl(file,F_GETFD,0);
    printf("fd=%d",fd);
    fd=fcntl(STDOUT_FILENO,F_GETFD,0);
    printf("fd=%d",fd);
}

it tells fd=0fd=0 Smilie
# 4  
Old 03-31-2009
Code:
#include <stdio.h>

int main(void) {
    if ( fprintf(stdin, "A") < 1 )
        perror("fprintf");
    return 0;
}

OP:
Code:
[c_d@localhost C scratchpad]$ gcc temp.c
[c_d@localhost C scratchpad]$ ./a.out 
fprintf: Bad file descriptor

so...how come now stdin becomes an invalid fd?

now when i do this
Code:
#include <stdio.h>
#include <unistd.h>

int main(void) {
    FILE *fakestdin;
    fakestdin = fdopen(STDIN_FILENO, "a+");
    if(fakestdin!=NULL)
    {
        fprintf(fakestdin, "A\n");
         perror("fakestdin");
    }
    else
          printf("fakestdin is NULL");
    return 0;    
}

OP:
Code:
[c_d@localhost C scratchpad]$ gcc temp.c
[c_d@localhost C scratchpad]$ ./a.out <temp.c
fakestdin is NULL[c_d@localhost C scratchpad]$ ./a.out 
A
fakestdin: Success
[c_d@localhost C scratchpad]$

it opens and writes to /dev/tty...

so what does this mean?

does it mean that (STDIN_FILENO) points to /dev/tty under normal conditions, but stdin does not? then where does it point to?

why did i have to open "stdin"'s fd to write to it? because i never have to open "stdout"'s fd to write to it...

Code:
#include <stdio.h>

int main(void) {
     fprintf(stdout, "HELLO!");
        perror("fprintf");
    return 0;
}

Code:
[c_d@localhost C scratchpad]$ gcc temp.c
[c_d@localhost C scratchpad]$ ./a.out 
fprintf: Success
HELLO![c_d@localhost C scratchpad]$

# 5  
Old 03-31-2009
Interesting!

I obtained file status flags of STDIN_FILENO, and it is open for O_RDWR.

Why then .\a.out < file.txt doesn't write "arghh!" to file.txt?
# 6  
Old 03-31-2009
Quote:
Originally Posted by pshaikh
Interesting!

I obtained file status flags of STDIN_FILENO, and it is open for O_RDWR.

Why then .\a.out < file.txt doesn't write "arghh!" to file.txt?
you cannot execute a file using \ ...it should be./a.out
and
< means you are redirecting the content of file.txt to stdin of a.out

to write "arghh!" to file.txt one would generally do ./a.out >file.txt

but in this case it will not write to file.txt because there is nothing in stdout of a.out process...
# 7  
Old 04-01-2009
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)
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. What is on Your Mind?

Ah, the AMIGA, (another poem I wrote in 2005).

Well I wrote this in 2005 and uploaded to AMINET.as a commemoration of a machine that is still in use today. It is now 29 years since this machne came into being. Phenominal and it is still being supported- WOW! My A1200 is on 24/7 and I use it to test code developed on AMIGA emulators... ... (0 Replies)
Discussion started by: wisecracker
0 Replies

2. Shell Programming and Scripting

Crontab - wrote Simple Script but i cant work out how to play it at a certain time.

Hi everyone. Silly might be silly be I'm still new to bash. I'm trying to make an Alarm Clock for in the morning using my laptop i have wrote this Simple Script but i cant work out how to play it at a certain time. #!/bin/bash cd /home/josh/Music/Bruno_Mars/Hooligans/ cvlc... (8 Replies)
Discussion started by: jtsmith90
8 Replies

3. UNIX for Advanced & Expert Users

Who actually wrote Professional Linux Programming ?

Hello, Who actually wrote Professional Linux Programming of Wrox publication as there are two different sets of writers, one set consists of Jon Masters and Richard Blum and another set of writers is Neil Mathew with lots of other writers. Plz resolve it. I'm really confused. Regards.. (0 Replies)
Discussion started by: vectrum
0 Replies

4. UNIX for Dummies Questions & Answers

What Does Happen During Boot Process? - BIOS and MBR

I'm talking about boot process in multi-boot Linux perceptive. Please tell me whether my explanation is right or wrong? If wrong, please explain. "The BIOS checks the system and loads this initial bootstrapping code into memory. This initial bootstrap code searches for an active partition... (0 Replies)
Discussion started by: f.ben.isaac
0 Replies

5. AIX

How does ITIL processing happen in AIX?

How does ITIL process is implemened in AIX? (6 Replies)
Discussion started by: AIXlearner
6 Replies

6. UNIX for Dummies Questions & Answers

whats happen when we create new user

hi frndz I wanna knw exatly what happen when we create new user... which directories are created ?? which files are modified ?? thanx.... (2 Replies)
Discussion started by: luckypower
2 Replies

7. UNIX for Advanced & Expert Users

Unix ID deleted - What happen to process

I have an unix id (AIX system) which is used to run a couple of processes. They also write some log files into a file system (that is not in the home directory of the user id, but in different location). One bad day, the id was deleted accidentally. But the home directory, files and everything... (1 Reply)
Discussion started by: cmgreat
1 Replies

8. UNIX for Dummies Questions & Answers

What would happen if. . .

Hi, Could someone please tell me what would happen if the following were entered into the command line: rm -i /books/*.* rm /books/* Many thanks! (3 Replies)
Discussion started by: crispy
3 Replies

9. Programming

Any one can tell me how this happen?

The #1 Online Store for Louis Vuitton Replicas is: http://www.opichina.com.cn. We offer Louis Vuitton Replicas and more! Whatever you call it: LV Bags, LV Replicas, Louis Vuitton Fake, Louis Vuitton Knockoffs, Louis Vuitton Bag, Louis Vuitton Purse, Louis Vuitton Wallet, Louis Vuitton Shoes,... (10 Replies)
Discussion started by: jiangyanna
10 Replies

10. UNIX for Dummies Questions & Answers

what happen when changing Hostname?

I 'm using RH 7.2 Genome in the Network Configuration I change therer are two places one for static hostname for my machine and in DNS hostname I don't know what happen when restarting my PC when connecting using dialer I can't browse the Internet also I can't use sendmail .......Server timeout... (2 Replies)
Discussion started by: atiato
2 Replies
Login or Register to Ask a Question