Implementation Notes

On the actual implementation front, several of the subsystems may require simultaneous production. Obviously, that isn’t possible so some form of acceptable substitute will be required for anyone following on themselves. For example, instead of an error handling subsystem that (often!) brings the system back to a sane state it might be required to print a message to stderr, dump the machine state and exit (non-zero, of course).

In one sense, this treatise is going to work in reverse as several key decisions are dependent of the final implementation’s form. So we’ll start by describing the virtual machine then the reader and evaluator which transform the Idio language into something processable by the VM, then a section on garbage collection before the only useful part of the whole thing, the different types of values in the system.

Compiling

I nominally use GCC although FreeBSD and Mac OS X use Clang with no obvious side-effects.

I have defined the C macro _GNU_SOURCE, originally for asprintf(3) on CentOS 6 although, as I wasn’t paying attention, it has some side-effects. Some of which we want!

According to feature_test_macros(7) on Linux, defining _GNU_SOURCE implicitly defines, amongst several others:

  • _POSIX_C_SOURCE with the value 200809L

  • _XOPEN_SOURCE with the value 700

  • _DEFAULT_SOURCE

It is the definition of _POSIX_C_SOURCE that exposes sigsetjmp(3) and sigaction(2), for example, which we use.

Either _XOPEN_SOURCE greater than or equal to 500 or _DEFAULT_SOURCE expose getpwent(3) which we don’t currently use but it is quite likely that we will.

Debugging

Help, in the form of everyone’s favourite debugging tool, printf(3), is at hand. Once we have the ability to construct a string representation of an Idio value it became straightforward to create a C function to print one out to stderr using a simple format string:

void idio_debug(const char *fmt, IDIO o)

which is actually a wrapper to the more generic:

void idio_debug_FILE(FILE *file, const char *fmt, IDIO o)

Here fmt can only contain a single %s specification as we have a single IDIO value. Often I’ll be using something like:

idio_debug ("a=%s ", a);
idio_debug ("b=%s\n", b);

To figure out what I’m doing wrong. Yes, it is crying out for a varargs implementation.

idio_debug() is so useful I created an Idio variant: idio-debug fmt value – although it has been superceded by printf and friends.

There are some slight limits as the depth to which the code will recurse through objects is restricted to minimise issues with circular loops.

void idio_dump(IDIO o, int detail)

(and idio-dump value) are similar except idio_dump() will print out some more developer-friendly details about the internal structure of the value – and then call idio_debug().

Last built at 2024-12-21T07:11:01Z+0000 from 463152b (dev)