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Comment by peteretep | Hacker News Reader

eg: http://plan9.bell-labs.com/sources/plan9/sys/src/cmd/ramfs.c

Anybody know what the:

  int needfid[] = {
	[Tversion] 0,
	[Tflush] 0,
	[Tauth] 0,

syntax means? Is this some Plan 9 extension, or is it just something I've never encountered?

EDIT: I guess the value in the []'s is the array index to set the value of.

> Is this some Plan 9 extension, or is it just something I've never encountered?

It's C99 (I also only saw it in C99 articles, never in production code). See http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Designated-Inits...

It's heavily used in the Linux kernel and in the Linux world in general. But Visual Studio doesn't support it, so projects that target Windows pretty much have to avoid it.

A variation of this syntax was standardized in C99:

    int needfid[] = {
        [Tversion] = 0,
        [Tflush] = 0,

It works for structs too:

    struct blah needfid = {
        .version = 0,
        .flush = 0,

That's right, it's a designated initializer, except the standard c99 way requires an =. gcc didn't/doesn't but that's nonstandard. So it is an extension, just not a plan9 one - a gcc-ism. clang's error/warning, as usual, is very helpful so it's worth it just to try clang when running into weirdness like that:

    warning: use of GNU 'missing =' extension in designator
          [-Wgnu-designator]

Well, it is a Plan 9 extension. You made it sound like it was only a gcc extension.

You're right, it's described in 3.5 of

http://plan9.bell-labs.com/sys/doc/compiler.html

It seems like it was contemporaneous with the proposed c90 extension and also supported the = except optionally. But it was definitely something in the plan 9 compiler rather than only a gcc thing.

See "3.5 Initialization indexes": http://plan9.bell-labs.com/sys/doc/compiler.html

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