[MLton-user] calling MLton from MLton via FFI

On Wed, Mar 9, 2011 at 6:36 PM, Christopher Cramer <[email protected]
> wrote:

> I need to call some C functions with a pointer to the middle of an array.
> The way I have been doing this is to create a function in ML, and export
> it, so that I can convert the array to a pointer and then add the offset.
>
> Is this safe?


No.

The moment you call ML from C any pointers into the heap you previously held
become invalid. You calling back into MLton via FFI is just a *really short*
path through C. ;)

You cannot assume that the parameter pointer given to gzreadoffset is valid.
The garbage collector might move it at any time.

A good way to achieve what you'd like to do is make the gzreadoffset as a
small piece of C code:

int gzreadoffset(gzFile *f,  unsigned char* buf, int offset, int len) {
  gzread(f, buf+offset, len);
  // buf is still valid because we haven't called out to SML
}

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