Can "printf" be used as a variable name?

Question

Rules for variable names in C are as follows:

1. A variable name can only have letters (both uppercase and lowercase letters), digits and underscore.
2. The first letter of a variable should be either a letter or an underscore.
3. There is no rule on how long a variable name (identifier) can be. However, you may run into problems in some compilers if the variable name is longer than 31 characters. (source: [https://www.programiz.com/c-programming/c-variables-constants])

I'm wondering about whether, theoretically, if a single underbar(_) or double underbar(__) be used as a variable? and can printf or scanf be used as a variable?

While playing with the c compiler(Dev C++) and linux Ubuntu Vi, even if I used the above as a variable name, there weren't any errors or warnings.

The code I used is as follows:

#include <stdio.h>
int main(void){
   int scanf;
   int printf;
   int _;
   int __;
   return 0;
}

Answer 1

yes, they can be used. see:

The code:

~> cat demo.c
#include <stdio.h>

void show(int i) {
   printf("Just for show: %i\n", i);
}

int main(int argc, char **args) {
   int printf = 42;
   int _ = 11;
   int __ = 22;

   (void)argc;
   (void)args;

   show(printf);
   show(_);
   show(__);

   return 0;
}

The compiling process with no errors:

~>  gcc -std=c99 -Wall -Wextra -pedantic -o demo demo.c

The output:

~>  ./demo
Just for show: 42
Just for show: 11
Just for show: 22