RE: [PATCH 5/9] fs: remove various compat readv/writev helpers

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 23 September 2020 19:46
...
> Regardless of that, another advantage of having the SYSCALL_DECLAREx()
> would be the ability to include that header file from elsewhere with a different
> macro definition to create a machine-readable version of the interface when
> combined with the syscall.tbl files. This could be used to create a user
> space stub for calling into the low-level syscall regardless of the
> libc interfaces,
> or for synchronizing the interfaces with strace, qemu-user, or anything that
> needs to deal with the low-level interface.

A similar 'trick' (that probably won't work here) is to pass
the name of a #define function as a parameter to another define.
Useful for defining constants and error strings together. eg:
#define TRAFFIC_LIGHTS(x) \
	x(RED, 0, "Red") \
	x(YELLOW, 1, "Yellow) \
	x(GREEN, 2, "GREEN)

You can then do thing like:
#define x(token, value, string) token = value,
enum {TRAFFIC_LIGHTS(x) NUM_LIGHTS};
#undef x
#define x(token, value, string) [value] = string,
const char *colours[] = {TRAFFIC_LIGHTS(x)};
#undef x
to initialise constants and a name table that are always in sync.

It is also a good way to generate source lines that are over 1MB.

	David

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