Question

Very simple question of where does this code work?

static void *gostruct[] = 
{
    [0 ... 255] = &&l_bad,
    ['\t'] = &&l_loop, [' '] = &&l_loop, ['\r'] = &&l_loop, ['\n'] = &&l_loop,
    ['"'] = &&l_qup,
    [':'] = &&l_loop,[','] = &&l_loop,
    ['['] = &&l_up, [']'] = &&l_down, // tracking [] and {} individually would allow fuller validation but is really messy
    ['{'] = &&l_up, ['}'] = &&l_down,
    ['-'] = &&l_bare, [48 ... 57] = &&l_bare, // 0-9
    ['t'] = &&l_bare, ['f'] = &&l_bare, ['n'] = &&l_bare // true, false, null
};

Reading through it its clear to see that it initializes an array containing 256 entries to the value &&l_bad and then overrides certain indexes with specific values. But this code does not compile in VS2010 which is what I have access to so I am wondering where this is valid C code.

NOTE: This code snippet was taken from a JSON parser on github that, from my understanding, creates jump tables for the processing of JSON strings.

Answer 1

This construct is called Designated Initializers.
Using Range in Designated Initializers is a GNU gcc specific extension.

To initialize a range of elements to the same value, write [first ... last] = value
. This is a GNU extension. For example,

int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

Compiling it with -pedantic shall tell you so.
Note that it is non-portable since it is a compiler specific extension.