[iobuf] Add iob_disown() and use it where it simplifies code

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/net/arp.c b/src/net/arp.c
index 011d4fefb..ba9ebf488 100644
--- a/src/net/arp.c
+++ b/src/net/arp.c
@@ -265,8 +265,8 @@ static int arp_rx ( struct io_buffer *iobuf, struct net_device *netdev,
 	memcpy ( arp_sender_ha ( arphdr ), netdev->ll_addr, arphdr->ar_hln );
 
 	/* Send reply */
-	net_tx ( iobuf, netdev, &arp_protocol, arp_target_ha (arphdr ) );
-	iobuf = NULL;
+	net_tx ( iob_disown ( iobuf ), netdev, &arp_protocol,
+		 arp_target_ha ( arphdr ) );
 
  done:
 	free_iob ( iobuf );