Re: gcc-4 troubleshooting

[Duilio Protti <dprotti () flowgate net>]

Fyodor wrote:

> While this works for now, a better approach might be to find all the
> lines that apparently violate C99 alias rules and fix them.  Here is
> an example that SoC student Paul Tarjan found (from tcpip.cc):
>
> buf = (u8 *) safe_zalloc(bufsz);
> [...]
> ifr = (struct ifreq *) buf;
> [...]
>   for(; ifr && *((u8 *)ifr) && ((u8 *)ifr) < buf + ifc.ifc_len;
>        ((*(char **)&ifr) += len ))
>
> I don't know if it is technically legal or not, but gcc doesn't seem
> to like it with -O2.  It is pretty gross anyway, and deserves to be
> changed for that reason if no other.
>
>  

An example similar to the code above can be found on a post by Marcus 
Brinkmann here:

http://lists.gnu.org/archive/html/l4-hurd/2005-01/msg00027.html

According to Marcus, the following code is good (for aliasing rules):

 struct a;

struct a_d { struct a a_m; int etc; };

struct a_d a_i;
struct a *ap = &a_i.a_m;

But this other one doesn't respect well the mentioned rules:

struct a *ap = malloc (sizeof (struct a_d));

struct a_d *adp = (struct adp *) ap;


With this in mind, the tcpip.cc fragment above can be changed in the 
form described on the attached patch.


Bye,
Duilio.

--- tcpip.cc.orig       2005-08-29 15:40:32.000000000 -0300
+++ tcpip.cc    2005-08-29 16:38:22.000000000 -0300
@@ -2159,10 +2159,10 @@
   struct ifconf ifc;
   struct ifreq *ifr;
   struct ifreq tmpifr;
+  struct ifreq *buf;
 #endif
   int len, rc;
   char *p;
-  u8 *buf;
   int bufsz;
   struct sockaddr_in *sin;
   u16 ifflags;
@@ -2194,13 +2194,13 @@
     sd = socket(AF_INET, SOCK_DGRAM, 0);
     if (sd < 0) pfatal("socket in getinterfaces");
     bufsz = 20480;
-    buf = (u8 *) safe_zalloc(bufsz);
+    buf = (struct ifreq *) safe_zalloc(bufsz);
     ifc.ifc_len = bufsz;
     ifc.ifc_buf = (char *) buf;
     if (ioctl(sd, SIOCGIFCONF, &ifc) < 0) {
       fatal("Failed to determine your configured interfaces!\n");
     }
-    ifr = (struct ifreq *) buf;
+    ifr = buf;
     if (ifc.ifc_len == 0) 
       fatal("getinterfaces: SIOCGIFCONF claims you have no network interfaces!\n");
 #if HAVE_SOCKADDR_SA_LEN
@@ -2219,7 +2219,7 @@
     printf("Size of struct ifreq: %d\n", sizeof(struct ifreq));
 #endif
 
-    for(; ifr && *((u8 *)ifr) && ((u8 *)ifr) < buf + ifc.ifc_len; 
+    for(; ifr && *((u8 *)ifr) && ((u8 *)ifr) < ((u8 *)ifr) + ifc.ifc_len; 
        ((*(char **)&ifr) += len )) {
 #if TCPIP_DEBUGGING
       printf("ifr_name size = %d\n", sizeof(ifr->ifr_name));



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