Re: [boost] [system] A non-allocating message() overload

Subject: Re: [boost] [system] A non-allocating message() overload
From: Peter Dimov (lists_at_[hidden])
Date: 2018-09-20 11:51:43

> charleyb123 wrote:
> > Question:  Is there a *guarantee* that 'message()' never return 
> > 'nullptr'? (Is that ever desired?)
>
> The only failure mode would be when the function needs to compose a 
> message and the provided buffer is too small to hold it; I'll have to 
> check what strerror_r(12345, buffer, 1) (for instance) does but in my 
> opinion nullptr should not be a valid return value; in this case something 
> like truncation or returning "Insufficient buffer size for message" would 
> be my preference.

On reflection, I take that back. nullptr will be a valid return value, but 
only in the case where you pass nullptr as the buffer (and, accordingly, 0 
as the length.)

That is, the function should not "fail" with "Insufficient buffer size for 
message", it will just fill the buffer and truncate, even when the buffer 
size is something ridiculously small such as 2, 1, or even 0.

This enables the (apparently idiomatic when using strerror_r) technique of 
using `message( ev, nullptr, 0 )` to detect whether the message is static, 
and do something else if it isn't.