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What is the purpose of having underscored names and then defining a non-underscored alias to it?
In a C implementation in <stdio.h> on Linux I saw something like:
extern FILE *__stdinp;
extern FILE *__stdoutp;
extern FILE *__stderrp;
And then:
#define stdin __stdinp
#define stdout __stdoutp
#define stderr __stderrp
My question is, for what reason would an implementation provide the __name items and then create aliases to them instead of just providing the canonical name to begin with?
1 answer
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.
...
3 The macros areNULL(described in 7.17);
... [[much elided]] ...;
stderr
stdin
stdout
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.
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