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Add magic value checking for headers.

--- a/libstd/alloc.myr

+++ b/libstd/alloc.myr

@@ -39,7 +39,9 @@

 	const zbytealloc	: (sz:size	-> byte#)

 	const bytefree	: (m:byte#, sz:size	-> void)

+

 ;;

+extern const put : (b : byte[:], args : ... -> size)

 /* null pointers. only used internally. */

 const Zbyteptr	= 0 castto(byte#)

@@ -57,6 +59,7 @@

 type slheader = struct

 	cap	: size	/* capacity in bytes */

+	magic	: size	/* magic check value */

 ;;

 type bucket = struct

@@ -122,18 +125,30 @@

 	phdr = p castto(slheader#)

 	phdr.cap = allocsz(sz) - align(sizeof(slheader), Align)

+	phdr.magic = (0xdeadbeefbadf00d castto(size))

 	prest = align((p castto(size)) + sizeof(slheader), Align)

 	-> prest castto(byte#)

 }

+const checkhdr = {p

+	var phdr, addr

+

+	addr = p castto(size)

+	addr -= align(sizeof(slheader), Align)

+	phdr = addr castto(slheader#)

+	assert(phdr.magic == (0xdeadbeefbadf00d castto(size)), "corrupt memory\n")

+}

+

 /* Frees a slice */

 generic slfree	 = {sl

 	var head

+

 	if sl.len == 0

 		->

 	;;

+	checkhdr(sl castto(byte#))

 	head = (sl castto(byte#)) castto(size)

 	head -= align(sizeof(slheader), Align)

 	bytefree(head castto(byte#), slcap(sl castto(byte#)))