I wanted to know if there was a simple but logical reason for having set the STDIN_FILENO as 0 in the stdio header, I realized about this when I was testing a more complex code, here's the short version of it:

#include <unistd.h>
#include <stdio.h>


void main(char **argv)
{
    char *promt = "Not a terminal";
    int inter = 1;
    int counter = 1;
    isatty(STDIN_FILENO) == 0 ? inter = 0 : inter;
    while (TRUE)
    {
        inter == 1 ? write(1, promt, 16) : inter; 
        fflush(stdout);
        counter  ;
        
    }

}

Also, if possible to know, what's the relation between STDIN_FILENO and Stream?

CodePudding user response：

STDIN_FILENO is POSIX and is required to be 0.

https://man7.org/linux/man-pages/man3/stdout.3.html

CodePudding user response：

Because STDIN_FILENO is defined as zero per POSIX:

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard. [Option End]

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen() function shall create certain descriptive data for a stream and return a pointer to designate the stream in all further transactions. Normally, there are three open streams with constant pointers declared in the <stdio.h> header and associated with the standard open files.

At program start-up, three streams shall be predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

The following symbolic values in <unistd.h> define the file descriptors that shall be associated with the C-language stdin, stdout, and stderr when the application is started:

STDIN_FILENO

Standard input value, stdin. Its value is 0.