Don't pass null to memcmp

[rotu]

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /opt/ros/master/src/ros2/rmw_cyclonedds/rmw_cyclonedds_cpp/include/rmw_cyclonedds_cpp/serdes.hpp:135:3 in
/opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:41:15: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:64:33: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:41:15 in
/opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:41:31: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:64:33: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:41:31 in
/opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:45:15: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:64:33: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:45:15 in
/opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:45:30: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:64:33: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /opt/ros/master/src/eclipse-cyclonedds/cyclonedds/src/core/ddsi/src/ddsi_sertopic_default.c:45:30 in

Signed-off-by: Dan Rose dan@digilabs.io

[eboasson]

Oops. (That prohibition on a null pointer for mem* even when the number bytes of is 0 gets me every now and then.) Thanks for checking and doing a PR.

The only thing is that the updated version is safe only because type.m_nkeys == 0 ⟺ type.m_keys == NULL and that means

  if (a->type.m_nkeys != b->type.m_nkeys)
     return false;
  if (
     a->type.m_nkeys > 0 &&
     memcmp (a->type.m_keys, b->type.m_keys, a->type.m_nkeys * sizeof (*a->type.m_keys)) != 0)
     return false;

would do the trick as well while being a little simpler. Self-evidently the same holds for the other one.