Why is it read successfully from the / dev source of the errno data source?

Question

A simple test program in C is called get1:

#include <stdio.h>
#include <string.h>
#include <errno.h>

int main(void) {
    errno = 0;
    int ch = fgetc(stdin);
    printf("ch = %d\n", ch);
    if (errno)
        printf("errno = %d: %s\n", errno, strerror(errno));
    return 0;
}

It simply prints the first byte, read in decimal, and then shows errnothe error message associated with it, if errnonot non-zero.

Some results ( foo- this is a text file, empty- this is a file with zero length):

% ./get1 < foo
ch = 104
% ./get1 < empty
ch = -1

Good, as expected. However:

% ./get1 < /dev/zero 
ch = 0
errno = 25: Inappropriate ioctl for device
% ./get1 < /dev/null 
ch = -1
errno = 25: Inappropriate ioctl for device
% ./get1 < /dev/random 
ch = 196
errno = 25: Inappropriate ioctl for device

Reading works fine, but when I read from any of these devices, it installs errno. Why?

It was on macOS (Darwin). I get the same thing on Linux (Debian) and NetBSD, except for another error only for /dev/random(the errors on other devices were the same as for macOS):

% ./get1 < /dev/random 
ch = 170
errno = 22: Invalid argument

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Answer 1

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Answer 2

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strace ./my program

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