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Quoting here the 1999 C standard, as being close enough to "this century" while also being hella old.

Source: ISO/IEC 9899:1999

> **7.19 Input/output <stdio.h>**

> **7.19.1 Introduction**

> _1_ The header `<stdio.h>` declares three types, several macros, and many functions for performing input and output.<br/>
> ...<br/>
> _3_ The macros are `NULL` (described in 7.17);

> ... [[much elided]] ...;

> `stderr`<br/>
> `stdin`<br/>
> `stdout`<br/>
> which are expressions of type ‘‘pointer to FILE’’ that point to the FILE objects associated, respectively, with the standard error, input, and output streams.

Note that the C standard here explicitly labels the symbols `stdin, stdout,` and `stderr` as being macros. There is no flexibility, no implementation-dependency, no per-thread or per-CPU optional components. These symbols are macros **because the standard says they are.**

If a library provides you with only `extern FILE * stdin` (&c.) then they are out of compliance, and your C environment is not "standard C" and cannot ever be "standard C." (You may not actually care about this. Many people, including me, do not. But still...)

Note that macros are required not to be recursive on expansion, meaning it is possible to do something like:

    #define stdin stdin
    extern FILE* stdin;

which both conforms to the letter of the standard and also flips off whatever original intent was present. This is why we can't have nice things.