_GTSEG - allocate a SEGID.

(GCOS8 only)


   #include <climb.h>
   ptr = _gtseg(svec);


const struct _vector *svec;
points to a shrink vector framing an area of memory.
void *ptr;
is the pointer value resulting from the shrink operation, pointing to the beginning of the segment. The generated SEGID can be extracted from the lowest 12 bits of this pointer.


"_gtseg" allocates a SEGID for the descriptor generated by a shrink operation on "svec". In order to allocate the SEGID, "_gtseg" follows these steps:

  1. First, it looks to see if there are any unused slots in the Linkage Segment. Slots are unused if they were originally allocated through LKED as free slots (discussed below) or if the corresponding SEGID has been freed by a call to "_frseg".
  2. If there is no space left in the Linkage Segment, "_gtseg" next looks for open slots in the Argument Stack. Open slots can arise in two ways: gaps left when previously allocated SEGIDs are freed via "_frseg"; and slots marked unused after a "_restore_arg_stack" operation. If there is an open slot, "_gtseg" uses it.
  3. If there are no open slots in the Argument Stack, "_gtseg" allocates a SEGID by using an SDR instruction to push a new descriptor onto the Argument Stack. If the Argument Stack is already full, the SDR instruction causes a fault.

In other words, "_gtseg" uses the Linkage Segment until it is full, then switches to the Argument Stack. Programs don't have to do their own checking to determine whether they should use the Linkage Segment or the Argument Stack.

If the descriptor resulting from the shrink operation doesn't have save permission, "_gtseg" always uses (c) above; in other words, it always allocates the slot on the Argument Stack.

Restricting Allocation to the Linkage Segment:

You can set up "_gtseg" so that it only allocates from the Linkage Segment, without switching over to the Argument Stack. If you are linking your program with LD, specify


in the LD input. If you are linking your program with LKED, specify


This gives you a version of "_gtseg" that allocates from the Linkage Segment only, not from the Argument Stack.

Argument Stack Map:

The software underlying "_gtseg" maintains a map describing all of the slots allocated in the Argument Stack. Each slot has one of four possible statuses:

The file "expl nsc lib _save_arg_stack" explains the effect of each of these statuses when the Argument Stack is saved and restored over an OCLIMB.

Free Slots in the Linkage Segment:

If you are linking your program with LD, all unused Linkage Segment slots are available for allocation by "_gtseg".

If you are linking your program with LKED, you can make a Linkage Segment slot available by using the -Managed_Slots option of the Define_Domain directive. See your LKED documentation for further details.

See Also:

expl nsc lib _frseg
for freeing SEGIDs allocated via "_gtseg".
expl nsc lib _gtlseg
for allocating a SEGID that will only be used during the current invocation of the program (one that disappears over an OCLIMB).
expl nsc lib _gtsseg
for marking a specific SEGID as busy.
expl nsc lib _save_arg_stack
for saving the Argument Stack before an OCLIMB.
expl nsc lib _restore_arg_stack
for restoring the Argument Stack after an OCLIMB.

Copyright © 1996, Thinkage Ltd.