3 ms·

Afaik the only integer type it's safe to cast from and to pointers is intptr_t. Not exactly the type people reach to when writing the first "simple" version. An

by c-cube 6y ago

Afaik the only integer type it's safe to cast from and to pointers is intptr_t. Not exactly the type people reach to when writing the first "simple" version. Another comment also points out that hcreate (which is not posix, only GNU?) and the likes use a global hashmap? I can't see a lot of use cases for such a singleton datastructure.

remexre 6y ago
hcreate is POSIX, hcreate_r (which allows having multiple hashmaps per process) is the GNU extension, iirc. I thought POSIX added a bunch more safe pointer casts, but I don't know off the top of my head...
- therealcamino 6y ago
  I've been programming in C for decades and I'd never heard of hcreate. I'm sitting here wondering how the heck did that ever get standardized, allowing single hash table per process. What a bizarre feature.
  - c-cube 6y ago
    I wonder if it's like insanities like SIGPIPE: it's kind of useful if you write IO tools in a few dozen/hundred lines of C. You don't need to carry around explicit context, or check if writes failed, or anything. Of course it sucks for anything else and we still have to deal with SIGPIPE .
beached_whale 6y ago
At least in C++, it’s any integer type that is large enough. uintmax_t/intmax_t are guaranteed to be large enough
scatters 6y ago
It just means that you need to cast via intptr_t, i.e. double casts `(void*)(intptr_t)i` and `(int)(intptr_t)p`. GLib has macros for it if you feel that's useful.
fanf2 6y ago
If you need to round-trip a pointer through an integer and back to the same pointer, you need to use intptr_t or uintptr_t. (Note that this is true for object pointers but not necessarily function pointers. There’s no integer type guaranteed to be big enough for a function pointer.) If you want to round-trip an integer through a pointer and back to the same integer, you can use any integer type provided it is not bigger than intptr_t or uintptr_t.

by c-cube 6y ago

remexre 6y ago

hcreate is POSIX, hcreate_r (which allows having multiple hashmaps per process) is the GNU extension, iirc. I thought POSIX added a bunch more safe pointer casts, but I don't know off the top of my head...

therealcamino 6y ago
I've been programming in C for decades and I'd never heard of hcreate. I'm sitting here wondering how the heck did that ever get standardized, allowing single hash table per process. What a bizarre feature.
- c-cube 6y ago
  I wonder if it's like insanities like SIGPIPE: it's kind of useful if you write IO tools in a few dozen/hundred lines of C. You don't need to carry around explicit context, or check if writes failed, or anything. Of course it sucks for anything else and we still have to deal with SIGPIPE .

beached_whale 6y ago

At least in C++, it’s any integer type that is large enough. uintmax_t/intmax_t are guaranteed to be large enough

scatters 6y ago

It just means that you need to cast via intptr_t, i.e. double casts `(void*)(intptr_t)i` and `(int)(intptr_t)p`. GLib has macros for it if you feel that's useful.

fanf2 6y ago

If you need to round-trip a pointer through an integer and back to the same pointer, you need to use intptr_t or uintptr_t. (Note that this is true for object pointers but not necessarily function pointers. There’s no integer type guaranteed to be big enough for a function pointer.) If you want to round-trip an integer through a pointer and back to the same integer, you can use any integer type provided it is not bigger than intptr_t or uintptr_t.