Why do '?' and '\?' give the same output in C?

Question

In C, why do these two pieces of code give the same output?

#include<stdio.h>

int main(void)
{
    const char c='\?';
    printf("%c",c);
}

and

#include<stdio.h>

int main(void)
{
    const char c='?';
    printf("%c",c);
}

I understand that a backslash is used to make quotes (" or ') and a backslash obvious to the compiler when we use printf(), but why does this work for the '?'?

Answer 1

\? is an escape sequence exactly equivalent to ?, and is used to escape trigraphs:

#include <stdio.h>
int main(void) {
    printf("%s %s", "??=", "?\?="); // output is # ??=
}

Answer 2

Quoting C11, chapter §6.4.4.4p4

The double-quote " and question-mark ? are representable either by themselves or by the escape sequences \" and \?, respectively, but ... .

_{Emphasis mine}

So the escape sequence \? is treated the same as ?.

Answer 3

Because '\?' is a valid escape code, and is equal to a question-mark.