On Fri, Jul 26, 2024 at 09:22:38PM -0700, Tony Ambardar wrote: > On Thu, Jul 25, 2024 at 01:27:03PM -0700, Andrii Nakryiko wrote: > > On Thu, Jul 25, 2024 at 3:39 AM Tony Ambardar <tony.ambardar@xxxxxxxxx> wrote: > > > > > > From: Tony Ambardar <tony.ambardar@xxxxxxxxx> > > > > > > Typically stdin, stdout, stderr are treated as reserved identifiers under > > > ISO/ANSI C, and a libc implementation is free to define these as macros. > > > > Ok, wow that. Do you have a pointer to where in the standard it is > > said that stdin/stdout/stderr is some sort of reserved identifier that > > can't be used as a field name? > > > > I'll need to dig around to share some references. The short answer IIRC > is there's enough potential variation in their definitions that their > use requires care (or better avoidance). > Hi Andrii, Following up on your request for pointers, some excerpts from a quasi-draft C17 ISO doc located here: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf 7.1.2 Standard headers (2) The standard headers are ... <stdio.h> ... (5) Any definition of an object-like macro ... shall expand to code that is fully protected by parentheses ... 7.1.3 Reserved identifiers (1) ... Each macro name in any of the following subclauses ... is reserved for use as specified if any of its associated headers is included ... 7.21.1 Input/output <stdio.h>, Introduction (1) The header <stdio.h> defines several macros ... (3) The macros are ... stderr stdin stdout which are expressions of type "pointer to FILE" ... 7.21.5.4 The freopen function (2) (Footnote 278) The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin, or stdout), as those identifiers need not be modifiable lvalues ... So we have reserved idents (IANALL so not sure of field names), macros, parentheses, and potentially unassignable stdout/stderr that might break the output redirection hack in test_progs.c. More than enough to tread carefully I think... Cheers, Tony
