Another type of stack, called the reserve stack, is allocated for each thread and used to handle out-of-storage conditions. Its size is controlled by the 4th subparameter of the STORAGE runtime option, but its usage is neither tracked nor reported in the storage report.
In a single-threaded environment, Language Environment allocates the initial segments for STACK, LIBSTACK and reserve stack using GETMAIN. The LIBSTACK initial segment allocation is deferred until first use, except when STACK(,,BELOW,,) is in effect. The reserve stack is allocated with STACK. In a multi-threaded POSIX(ON) environment, allocation of stack storage for the initial processing thread (IPT) is the same as the single-threaded environment. For threads other than the IPT, the initial STACK (or THREADSTACK) segment and reserve stack is allocated from ANYHEAP or BELOWHEAP, according to the STACK (or THREADSTACK) location. The initial LIBSTACK segment allocation is again deferred until first use, except when STACK(,,BELOW,,) or THREADSTACK(ON,,,BELOW,,) is in effect. When a STACK, THREADSTACK, or LIBSTACK overflow occurs on any thread, Language Environment® obtains the new segment using GETMAIN. The reserve stack does not tolerate overflow.