what would happen if a process wrote to its own stdin? Post: 302302522

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Top Forums Programming what would happen if a process wrote to its own stdin? Post 302302522 by c_d on Tuesday 31st of March 2009 07:42:18 AM

03-31-2009

Registered User

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]$

c_d

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explain_fprintf_or_die

explain_fprintf_or_die(3)				     Library Functions Manual					 explain_fprintf_or_die(3)

NAME

       explain_fprintf_or_die - formatted output conversion and report errors

SYNOPSIS

       #include <libexplain/fprintf.h>
       int explain_fprintf_or_die(FILE *fp, const char *format, ...);
       int explain_fprintf_on_error(FILE *fp, const char *format, ...);

DESCRIPTION

       The  explain_fprintf_or_die  function  is  used	to  call  the fprintf(3) system call. On failure an explanation will be printed to stderr,
       obtained from the explain_fprintf(3) function, and then the process terminates by calling exit(EXIT_FAILURE).

       The explain_fprintf_on_error function is used to call the fprintf(3) system call. On failure an explanation  will  be  printed  to  stderr,
       obtained from the explain_fprintf(3) function, but still returns to the caller.

       fp      The fp, exactly as to be passed to the fprintf(3) system call.

       format  The format, exactly as to be passed to the fprintf(3) system call.

RETURN VALUE

       The  explain_fprintf_or_die  function  only  returns on success, see fprintf(3) for more information. On failure, prints an explanation and
       exits, it does not return.

       The explain_fprintf_on_error function always returns the value return by the wrapped fprintf(3) system call.

EXAMPLE

       The explain_fprintf_or_die function is intended to be used in a fashion similar to the following example:
	      int result = explain_fprintf_or_die(fp, format, ...);

SEE ALSO

       fprintf(3)
	       formatted output conversion

       explain_fprintf(3)
	       explain fprintf(3) errors

       exit(2) terminate the calling process

COPYRIGHT

       libexplain version 0.52
       Copyright (C) 2010 Peter Miller

															 explain_fprintf_or_die(3)

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explain_fprintf_or_die