GCC Explorer - an interactive take on compilation [LWN.net]

GCC Explorer - an interactive take on compilation

Posted May 29, 2012 0:15 UTC (Tue) by cmccabe (guest, #60281)
In reply to: GCC Explorer - an interactive take on compilation by gb
Parent article: GCC Explorer - an interactive take on compilation

You forgot to make the array const.

It should be const char * const a[].

Likewise, the static version has no const declaration at all, so I can only imagine that it's going to go in the writable portion of the data segment.

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GCC Explorer - an interactive take on compilation

Posted May 29, 2012 9:44 UTC (Tue) by gb (subscriber, #58328) [Link]

> You forgot to make the array const.

It makes no difference, code generated for:
const char* const a[]={"One", "Two", "Three"};

and
const char* a[]={"One", "Two", "Three"};

is same.

> static version has no const declaration at all.

Sure, i just missed it, it would not compile without it, should be:
static const char* a[]={"One", "Two", "Three"};

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 5:18 UTC (Wed) by jzbiciak (✭ supporter ✭, #5246) [Link]

Out of curiosity, in your "cosnt char *" version, are the strings "One", "Two" and "Three" allocated on the stack, or only the array a[]?

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 7:51 UTC (Wed) by jamesh (guest, #1159) [Link]

The array contains pointers in all cases, so the actual strings would be in the read-only data section with any combination of "const".

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 8:18 UTC (Wed) by jzbiciak (✭ supporter ✭, #5246) [Link]

Yep, it does seem to put it in rodata. I've seen enough shenanigans with initialized local arrays that I was prepared to hear it was strcpy()ing everything to local copies on the stack due to some obscure paragraph I missed in the standard. Thankfully, it's not that insane.

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 11:28 UTC (Wed) by jamesh (guest, #1159) [Link]

That kind of thing only really happens when using a static initialiser for a character array. Since this was an array of pointers, that doesn't apply.

This is all part of the C standard, so I doubt you'd see much variation in any compiler made in the last 15 years (probably more).

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 14:03 UTC (Wed) by jzbiciak (✭ supporter ✭, #5246) [Link]

Of course, GCC Explorer is showing us a C++ compiler, not a C compiler. There are many here who will point out that C++ isn't C or even a superset of C. ;-)

GCC Explorer - an interactive take on compilation

Posted May 30, 2012 15:32 UTC (Wed) by jamesh (guest, #1159) [Link]

Right, but you can mostly think of it as a superset of C for things that C compilers agreed on 15+ years ago. And this particular piece of code was just using basic types and syntax common to both languages, so you would expect it to behave similarly.