g_alloca() is not very advisable, especially when it might be used to
allocate a big chunk of memory at once. It is better to allocate dynamic
memory with malloc(), or in particular with g_try_malloc() which won't
abort the program on failure.
This might be slightly slower (one of the advantages of memory on the
stack, though not even an absolute truth) but probably not by much, if
at all, and it's better to be safe anyway.