Copy const char* to uint8_t array

Question

My C-API takes an array of uint8_t's as config parameter. I'm arriving at its doorsteps with a const char*. How do I now copy the chars over to the uint8_t array in the most unproblematic way? Here's what I've got (contrived ofc):

Demo

#include <cstdint>
#include <cstring>
#include <cstdio>

struct config
{
    uint8_t ssid_[32];
};


auto set_ssid(const char* ssid) {
    // I know this fn has no sideeffects but assume for demonstration
    // purposes that cfg is only initialized here
    config cfg;

    std::strncpy(static_cast<char*>(&cfg.ssid_), ssid, 32);
}

int main()
{
    set_ssid("ComeOver");
}

But this doesn't work as none of the pointer is of void* type:

<source>:14:18: error: invalid 'static_cast' from type 'uint8_t (*)[32]' {aka 'unsigned char (*)[32]'} to type 'char*'
   14 |     std::strncpy(static_cast<char*>(&cfg.ssid_), ssid, 32);
      |

Is it safe to reinterpret_cast here?

std::strncpy is C API, you should use some idomatic C++ (std::copy?). Note that your set_ssid function doesn't have any observable effects, so it is not clear what you are trying to achieve. Also I do not like design of this API, also use of it looks like indentation for UB. — Marek R
– Marek R, Commented Oct 13, 2022 at 14:36
Possible approach: godbolt.org/z/78WxaWqG8 as you can see compiler sees there is no side effects in set_ssid and removed its code. So please provide code which does something meaningful. — Marek R
– Marek R, Commented Oct 13, 2022 at 14:43
@MarekR Thank you, copy_n didn't come to my mind. I know that the function has no sideffects but i zoomed in onto the problematic matter for demonstration purposes and left all baggage out. — glades
– glades, Commented Oct 13, 2022 at 14:46

relent95 · Accepted Answer · 2022-10-13 16:46:43Z

0

You should use cfg.ssid_ for the destination buffer instead of &cfg.ssid_. Also you should use reinterpret_cast instead of static_cast, because char * and unsigned char *(uint8_t *) are not pointer-interconvertible.(It's a rule refining various casts. See static_cast conversion.)

So this is recommended.

std::strncpy(reinterpret_cast<char*>(cfg.ssid_), ssid, 32);
// Or simply
std::strncpy((char *)(cfg.ssid_), ssid, 32);

edited Oct 13, 2022 at 16:46

answered Oct 13, 2022 at 16:35

relent95

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2 Comments

relent95 Over a year ago

I mean, out of possible conversions between pointers to a POD, there are pointer-interconvertible cases and not pointer-interconvertible cases.

user17732522 Over a year ago

Yes, but all of these conversions go through void* anyway. reinterpret_cast between pointers is also just a sequence of static_casts through void*. The objects here are not pointer-interconvertible, true, but in order for this to work it is important whether the pointer may alias the object (which it may).

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