https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80409
Bug ID: 80409
Summary: Document that va_arg(ap, void*) can be used to consume
any pointer argument
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pascal_cuoq at hotmail dot com
Target Milestone: ---
This bug report is vaguely related to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26542 but whereas that report asks
for removing a warning, here there is no warning and this is a request for it
to be documented that the idiom is legal in GCC.
The C11 standard allows to confuse void* and char* when consuming arguments
from a va_list, and only these types of pointers:
http://port70.net/~nsz/c/c11/n1570.html#7.16.1.1p2
As explained by Rich Felker as part of a discussion on musl's scanf
implementation, this makes it very inconvenient to implement POSIX's positional
specifiers for scanf. Clearly the intent of POSIX's “all the leading arguments,
from the first to the (N-1)th, are pointers” is to allow these arguments to be
consumed on the spot when %N$ is encountered:
http://www.openwall.com/lists/musl/2017/04/10/3
Since the open-source world divides the C compilation platform described by the
C standard into C compilers (Clang, GCC) and standard libraries (Glibc, musl,
...), with the implementation of varargs on the compiler side and the
implementation of scanf on the standard library side, it would make sense to
document that on target platforms where the representation of pointers is
uniform, the compilers allow va_arg(ap, void*) to consume any pointer argument.
