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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: memcpy can't take null.
Date: Sun, 05 Apr 2020 16:00:28 -0700
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Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
> memcpy() cannot take null as a pointer parametrer, even if the size argument
> is zero.
>
> So what are the implications for passing empty buffers about? For example,
> how would people write
>
> /* struct employee has no embedded pointers */
>
> Employee *duplicateEmployees(Employee *employees, int Nemployees)

C doesn't directly support empty arrays, but since arrays are usually
manipulated via pointers to the initial element, it's easy enough
to treat a pointer as pointing to an empty array.

A null pointer doesn't point to an empty array.  It doesn't point
to *anything*.  Similarly, a null pointer doesn't point to an empty
string (there have been problems in the past with code that assumed
it did, and that happened to "work" on some systems).

Just keep that consistently in mind, and write your code in
whatever way is necessary to maintain that.  This might require
a little special case code for handling the result of malloc(0).
So write that code.

Employee *duplicateEmployees(Employee *employees, size_t Nemployees) {
    const size_t requestedCount = Nemployees + (Nemployees == 0);
    Employee *const new = malloc(requestedCount * sizeof *new);
    if (new != NULL) {
        memcpy(new, employees, Nemployees * sizeof *new);
    }
    return new;
}

An alternative would be to treat a null pointer as a pointer to an
empty array, but then you'd probably need more special case code on
top of that to keep things consistent (to avoid passing null pointers
to memcpy, for example).  By *not* treating a null pointer as a
pointer to an empty array, you can (probably) isolate the special
case code to allocation functions.  You'd also lose the ability to
use a null pointer as a special value that doesn't point to anything.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */