Real life C programs are often written using C dialects which, for the ISO C standard, have undefined behaviours. In particular, according to the ISO C standard, reading an uninitialised variable has an undefined behaviour and low-level pointer operations are implementation defined. We propose a formal semantics which gives a well-defined meaning to those behaviours for the C dialect of the CompCert compiler. Our semantics builds upon a novel memory model leveraging a notion of symbolic values. Symbolic values are used by the semantics to delay the evaluation of operations and are normalised lazily to genuine values when needed. We show that the most precise normalisation is computable and that a slightly relaxed normalisation can be efficiently implemented using an SMT solver. The semantics is executable and our experiments show that the enhancements of our semantics are mandatory to give a meaning to low-levels idioms such as those found in the allocation functions of a C standard library.