Libguestfs - Apr 2019 - [PATCH nbdkit] server: Initialize ‘buf’ to avoid GCC 9.0.1 warning.

GCC 9.0.1 isn't smart enough to work out that buf is initialized on
all the relevant paths that lead to backend->pwrite being called.  By
initializing it we can avoid the warning.

protocol.c: In function ‘protocol_recv_request_send_reply’:
protocol.c:241:9: error: ‘buf’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
  241 |     if (backend->pwrite (backend, conn, buf, count, offset, f,
&err) == -1)
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
protocol.c:614:9: note: ‘buf’ was declared here
  614 |   char *buf;
      |         ^~~
---
 server/protocol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/server/protocol.c b/server/protocol.c
index 6fd3b62..5033fd7 100644
--- a/server/protocol.c
+++ b/server/protocol.c
@@ -611,7 +611,7 @@ protocol_recv_request_send_reply (struct connection *conn)
   uint16_t cmd, flags;
   uint32_t magic, count, error = 0;
   uint64_t offset;
-  char *buf;
+  char *buf = NULL;
   CLEANUP_EXTENTS_FREE struct nbdkit_extents *extents = NULL;
 
   /* Read the request packet. */
-- 
2.20.1

On 4/24/19 3:31 PM, Richard W.M. Jones wrote:
> GCC 9.0.1 isn't smart enough to work out that buf is initialized on
> all the relevant paths that lead to backend->pwrite being called.  By
> initializing it we can avoid the warning.
> 
> protocol.c: In function ‘protocol_recv_request_send_reply’:
> protocol.c:241:9: error: ‘buf’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
>   241 |     if (backend->pwrite (backend, conn, buf, count, offset, f,
&err) == -1)
>       |        
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> protocol.c:614:9: note: ‘buf’ was declared here
>   614 |   char *buf;
>       |         ^~~
Wow - cross-function inlining evident in that error message. I agree
that it's a false positive; without your patch, buf can be passed to
handle_request() uninitialized, but never in the NBD_CMD_WRITE case,
even if the compiler can't spot that.

ACK.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org