realloc((char *)NULL,size) - how standard ?

[Steve Summit]

In article <1991Feb16.010828.2152 at pscnet.UUCP> kean at pscnet.UUCP (Kean Johnston) writes:
>The other day I had occasion to write a little C program which was repeatedly
>increasing an array. Of course, realloc() is perfect for this.
>...Harbison and Steele... state that if you pass as the first parameter to
>realloc a NULL pointer, then realloc() will allocate a block of size bytes
>for you. No need to malloc the very first block.
>Well, under Xenix 2.3.3 with 2.3.0 DevSys, this isn't so. If you don't do
>the initial [m]alloc of the block, then realloc fails. Is using a NULL pointer
>as the first parameter to realloc() a "standard" feature of the function, and
>if so, which of the plethora of standards?

The Teeming Millions (if I may borrow Cecil Adams' term) are
certainly prescient this month!  This is an excellent question,
and calls for another sneak preview of the upcoming, updated
comp.lang.c frequently-asked questions list:

56.  Is it legal to pass a null pointer as the first argument to
     realloc()?  Why would you want to?

A:   Doing so is legal and portable, at least as of ANSI X3.159's
     ratification.  (Not all pre-ANSI libraries accept it, however.)
     Passing an initially-null pointer to realloc can make it very easy
     to write a self-starting incremental allocation algorithm.

To elaborate, BSD (and perhaps V7) have already had the newly-
standardized behavior, but many other historical systems
(including, I am told, pre-ANSI System V) did not.

                                            Steve Summit
                                            scs at adam.mit.edu

P.S. I'm surprised that Harbison & Steele (2nd Ed.) sanction the
usage, since it was not universal in 1987 when the book was
published.  (I have been burned by the historical inconsistency,
as well.)