interaction between *_blocking_section and value

[Samuel Mimram <samuel.mimram@ens-lyon.org>]

Hello,

I'd like to have some clarifications about the interaction between 
threads and the GC in C bindings.
As far as I understand the CAMLparam and CAMLlocal macros prevent values 
from being garbage-collected i.e.
1. being completely removed from memory,
2. being *moved*
during the whole scope of the C function (am I right?).
I have received a mail concerning one of my programs which was telling 
me that this is not true when in blocking sections, and that values can 
be moved by the GC. Is it the case? Consider the following code:

CAMLprim value caml_send(value data)
{
	CAMLparam1(data);
	enter_blocking_section();
	send(0, String_val(data), string_length(data));
	leave_blocking_section();
	CAMLreturn(Val_unit);
}

Is it safe or can the String_val(data) point to an invalid portion of 
memory because data have been move by the GC because of the 
enter_blocking_section()?

Also, it would be nice if the *_blocking_section() could be explained in 
the documentation.

Thanks.

Regards,

Samuel.