Author: Julia Lawall <julia@diku.dk> dtp is derefenced on the lines above the test !dtp, and so it cannot be NULL at this point. A simplified version of the semantic match that finds this problem is as follows: (http://www.emn.fr/x-info/coccinelle/) // @r@ expression x,E,E1; identifier f,l; position p1,p2; @@ *x@p1->f = E1; … when != x = …