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

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# 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)
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