add API for integer multiplication with overflow detection

array/array_allocate.c

@@ -0,0 +1,51 @@
+#include "array.h"
+
+#if 0
+      static array x;
+      t *p;
+      int64 pos;
+
+      p = array_allocate(&x,sizeof(t),pos);
+
+   array_allocate makes sure that enough bytes are allocated in x for at
+   least pos+1 objects of type t. (The size of t must be positive;
+   otherwise the effects are undefined.) If not enough bytes are
+   allocated (or x is unallocated), array_allocate allocates more bytes,
+   moving the dynamically allocated region if necessary. array_allocate
+   often allocates somewhat more bytes than necessary, to save time
+   later.
+
+   array_allocate then makes sure that the number of bytes initialized
+   covers at least those pos+1 objects. If not enough bytes are
+   initialized, array_allocate initializes more bytes (setting them to
+   0), up to exactly the end of the pos+1st object.
+
+   array_allocate then returns a pointer to the pos+1st object; i.e.,
+   object number pos, with objects numbered starting at 0. This pointer
+   can be used to change or inspect the object. The pointer can continue
+   to be used through subsequent calls to array_get, array_start,
+   array_length, and array_bytes, but it must not be used after any
+   other operations on this array.
+
+   If something goes wrong, array_allocate returns 0, setting errno
+   appropriately, without touching x. In particular, array_allocate
+   returns 0 if
+
+     * x has failed, or
+     * pos is negative, or
+     * not enough memory is available.
+
+   array_allocate does not change x to have failed; if you want to do
+   that, use array_fail.
+#endif
+
+void* array_allocate(array* x,int64 membersize,int64 pos) {
+  int64 wanted;
+  if (membersize<128)
+    wanted=(pos+127)&(-128ll);	/* round up to multiple of 128 */
+  else
+    wanted=(pos+4095)&(-4096ll); /* round up to 4k pages */
+  /* detect numeric overflow */
+  if (wanted<0) return 0;
+  wanted=membersize*(pos+1);
+}