Re: [PATCH v5] printk: Userspace format enumeration support

[Rasmus Villemoes]

On 18/03/2021 11.46, Petr Mladek wrote:

> BTW: Is the trick with int (printk)(const char *s, ...) documented
> somewhere? Is it portable?

It is completely standard and portable C, explicitly spelled out in the
C standard itself. C99:

===
6.10.3 Macro replacement

10 [...] Each subsequent instance of the
function-like macro name followed by a ( as the next preprocessing token
introduces the
sequence of preprocessing tokens that is replaced by the replacement
list in the definition
(an invocation of the macro). [...]
===

and later

===
7.1.4 Use of library functions

1 [...] one
of the techniques shown below can be used to ensure the declaration is
not affected by
such a macro. Any macro definition of a function can be suppressed
locally by enclosing
the name of the function in parentheses, because the name is then not
followed by the left
parenthesis that indicates expansion of a macro function name. For the
same syntactic
reason, it is permitted to take the address of a library function even
if it is also defined as
a macro.
===

Also, the use of printk() inside the definition of a printk()
function-like macro does not lead to infinite recursion, by

===
6.10.3.4 Rescanning and further replacement

2 If the name of the macro being replaced is found during this scan of
the replacement list
(not including the rest of the source file’s preprocessing tokens), it
is not replaced.
===

Rasmus