You can approximate an argument list using compound literals as long as 0/NULL is an acceptable default value. It's a bit noisy, but it works something like this if I remember right:

    result = foo(&(struct foo_args){.arg1 = 4, .arg7 = "2"})

There are libraries for this like GNU ffcall and libffi.

Looked at those. Problem with libffi is it wasn't ported to a 32 bit arm. And the interface is 'clunkie'

I think what I want is llvm's blocks. Which isn't available in gcc.

Like va_list in vprintf ?