Something funny going on with extern double pointers

[eevee]

0.4.

boolnames is a global array-of-char* that curses provides. (It's actually declared as extern NCURSES_EXPORT_VAR(NCURSES_CONST char * const ) boolnames[];.)

https://gist.github.com/3995350

The C version spits out the entire array, starting with:

0x600aa0 0x7f6eb0dcd831 b bw
0x600aa8 0x7f6eb0dd5703 a am
0x600ab0 0x7f6eb0dd6199 x xsb

The Rust version spits out:

7f7a9ec58831 7378006278007762

...and then segfaults trying to evaluate **p. Cursory inspection reveals that the value given for *p is actually the string bw\0xb\0xs. Somehow the first level of indirection is disappearing.

[ben0x539]

There is no two levels of indirection since it's an array and not a double pointer in curses. In your C code, the implicit array-to-pointer conversion does what you expect, but straight-up telling rustc that the symbol boolnames refers to a double pointer instead of a series of single pointers can't go well.

I don't know how to properly deal with unknown-but-static-length arrays in rust, but if you change the extern declaration to const boolnames: [*c_char * 1]; and then say let mut p: **c_char = ptr::to_unsafe_ptr(&boolnames[0]);, it'll print a pile of stuff and not segfault.

[eevee]

Ah, you're right, I fell into a common C pitfall. (Doesn't help that curses has another definition that is char** but that gets #ifdefed out.)

Definitely not Rust's fault, then. Thanks :)