What exactly is stdin?

Question

I am a bit confused regarding the implementation of stdin. What exactly is it? Is it a pointer? I tried to print the size of stdin using sizeof on a 64 bit machine and got 8. I even de-referenced it using *stdin in the %s specifier and got the not used characters in the input stream( all of them). But if I compare its de referenced value inside if I get an error. I know its an input stream. But how is it implemented?

Here is a sample:

#include<stdio.h>
#include<stdlib.h>

int main(){
    
    char a[10];

    fgets(a,10,stdin);

    printf("a: %s",a);
    
    printf("\nstdin: %s",*stdin);

//if(*stdin=="foo"){
//    printf("True");
//}//Error: invalid operands to binary == (have 'FILE' and 'char *')//and casting to char * doesn't help

    printf("\nSize of stdin: %d\n",sizeof(stdin));

    if(stdin!=NULL){
        printf("This statement is always reached");//always printed even in when input is bigger than 10 chars and otherwise
    }
    

if(!feof(stdin)){
    printf("\nThis statement is also always reached");
}
}

When I input

foobar1234567890

I get the result:

a: foobar123

stdin: 4567890

Size of stdin: 8

This statement is always reached

This statement is also always reached

And when I input

foobar

I get the output

a: foobar

stdin:

Size of stdin: 4

This statement is always reached

This statement is also always reached

I understand that stdin is kind of a FILE pointer, but I don't clearly get it. Could anyone explain the above outputs?

Answer 1

stdin is a pointer of type FILE *. The standard does not restrict the implementation beyond this, the details of what FILE is is entirely up to your compiler. It could even be an incomplete type (opaque).

Of course, trying to print a FILE with %s causes undefined behaviour (unless FILE is a typedef for char * or similar, which it almost certainly is not).