为什么 STDIN_FILENO 总是零？

Question

我想知道在 stdio header 中将 STDIN_FILENO 设置为 0 是否有一个简单但合乎逻辑的原因，我在测试一个更复杂的代码，这里是它的简短版本：

#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++;
        
    }

}

还有，如果可以知道，STDIN_FILENO和Stream有什么关系？

Answer 1

STDIN_FILENO 是 POSIX 并且必须是 0。

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

Answer 2

因为 STDIN_FILENO 定义为零 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.

为什么 STDIN_FILENO 总是零？

Why is STDIN_FILENO always zero?

c

file-descriptor

stdio

tty