Explain what happens for malloc(0), or calloc() where one of the arguments is 0.

Add alloca(3) to SEE ALSO.

man3/malloc.3

@@ -47,12 +47,30 @@ elements of
 .I size
 bytes each and returns a pointer to the allocated memory.
 The memory is set to zero.
+If
+.I nmemb
+or
+.I size
+is 0, then 
+.BR calloc ()
+returns either NULL,
+.\" glibc does this:
+or a unique pointer value that can later be successfully passed to
+.BR free ().
 .PP
 .BR malloc ()
 allocates
 .I size
 bytes and returns a pointer to the allocated memory.
 The memory is not cleared.
+If
+.I size
+is 0, then 
+.BR malloc ()
+returns either NULL,
+.\" glibc does this:
+or a unique pointer value that can later be successfully passed to
+.BR free ().
 .PP
 .BR free ()
 frees the memory space pointed to by
@@ -63,7 +81,7 @@ which must have been returned by a previous call to
 or
 .BR realloc ().
 Otherwise, or if
-.BI "free(" "ptr" )
+.IR free(ptr)
 has already been called before, undefined behavior occurs.
 If
 .I ptr
@@ -85,7 +103,7 @@ if
 .I size
 is equal to zero,
 the call is equivalent to
-.BI "free(" "ptr" ) .
+.IR free(ptr) .
 Unless
 .I ptr
 is NULL, it must have been returned by an earlier call to
@@ -94,7 +112,7 @@ is NULL, it must have been returned by an earlier call to
 or
 .BR realloc ().
 If the area pointed to was moved, a
-.BI "free(" "ptr" )
+.IR free(ptr)
 is done.
 .SH "RETURN VALUE"
 For
@@ -204,4 +222,5 @@ and
 .IR sysctl/vm.txt .
 .SH "SEE ALSO"
 .BR brk (2),
+.BR alloca (3),
 .BR posix_memalign (3)