Have a #define to squelch -Wunused-result warnings.

In one place, we really *do* legitimately ignore the return value of
pcap_set_datalink(). We cast its return value to void, which appears to
squelch warnings from Clang, but not from at least some versions of GCC,
so provide DIAG_OFF_WARN_UNUSED_RESULT/DIAG_ON_WARN_UNUSED_RESULT and
use them to squelch the warning.

Author: guyharris

diag-control.h

@@ -132,6 +132,22 @@
   #define DIAG_ON_CAST_QUAL \
     DIAG_DO_PRAGMA(GCC diagnostic pop)
 
+  #if ND_IS_AT_LEAST_GNUC_VERSION(4,5)
+    /*
+     * GCC warns about unused return values if a function is marked as
+     * "warn about ignoring this function's return value".
+     *
+     * Clang appears to let you ignore a result without a warning by
+     * casting the function result to void, so we don't appear to
+     * need this for Clang.
+     */
+    #define DIAG_OFF_WARN_UNUSED_RESULT \
+      DIAG_DO_PRAGMA(GCC diagnostic push) \
+      DIAG_DO_PRAGMA(GCC diagnostic ignored "-Wunused-result")
+    #define DIAG_ON_WARN_UNUSED_RESULT \
+      DIAG_DO_PRAGMA(GCC diagnostic pop)
+  #endif
+
   /*
    * Suppress deprecation warnings.
    */
@@ -172,6 +188,12 @@
 #ifndef DIAG_ON_CAST_QUAL
 #define DIAG_ON_CAST_QUAL
 #endif
+#ifndef DIAG_OFF_WARN_UNUSED_RESULT
+#define DIAG_OFF_WARN_UNUSED_RESULT
+#endif
+#ifndef DIAG_ON_WARN_UNUSED_RESULT
+#define DIAG_ON_WARN_UNUSED_RESULT
+#endif
 #ifndef DIAG_OFF_DEPRECATION
 #define DIAG_OFF_DEPRECATION
 #endif

tcpdump.c

@@ -2326,8 +2326,11 @@ main(int argc, char **argv)
 			 * on; this may be a non-Linux "any" device
 			 * that doesn't support DLT_LINUX_SLL2.
 			 */
-			if (strcmp(device, "any") == 0)
+			if (strcmp(device, "any") == 0) {
+DIAG_OFF_WARN_UNUSED_RESULT
 				(void) pcap_set_datalink(pd, DLT_LINUX_SLL2);
+DIAG_ON_WARN_UNUSED_RESULT
+			}
 		}
 #endif
 		i = pcap_snapshot(pd);