Question about acquiring predefined macros in panama-foreign

[Maurizio Cimadamore]

Hi,
dealing with macros is a bit tricky as they do not have corresponding 
symbols in a shared library - a macro is just expanded _in place_ by the 
C compiler. As such, symbol lookup will fail to find e.g. stdin - and 
even for things where lookup succeeds (e.g. errno) the resulting symbol 
is probably _not_ what you want.

The sanest way to deal with these situations is to write a shim C 
library which returns the values you want - and then access this library 
using FFM - we do such an approach to deal with variadic macros in the 
Windows API:

https://github.com/openjdk/jdk/blob/c24c66db97a52371875a63862f85ea5c2010d5a7/src/java.base/windows/native/libsyslookup/syslookup.c

Basically this library creates an array of function pointers, which is 
then accessed from the Java side:

https://github.com/openjdk/jdk/blob/c24c66db97a52371875a63862f85ea5c2010d5a7/src/java.base/share/classes/jdk/internal/foreign/SystemLookup.java#L76

I believe it could be possible (and the FFM API leaves room for it) to 
define _extra_ "synthetic" symbols which could be exposed by the Linker 
lookup. For instance, standard macros like stdin/stdout could be added 
to the set of symbols returned by the default lookup, so that interop 
with some C libraries might become a little easier.

Cheers
Maurizio

On 12/09/2023 06:04, 刘希晨 wrote:
> Hi, I want to know if there is a recomended mechanism to get the 
> predefined macros from C dynamic library or system library.
>
> For example, I want to use stdout and stderr in my program, currently 
> I wrote a method returning it so I can use a MemorySegment to capture 
> it, I wonder if there is any other option that I can directly get it 
> in java side.