Linker and Libraries Guide

Thread Creation

For the initial thread, and for each new thread created, the thread library allocates a new TLS block for each loaded dynamic object. Blocks can be allocated separately, or as a single contiguous block.

Each thread t, has an associated thread pointer tpt, which points to the thread control block, TCB. The thread pointer, tp, always contains the value of tpt for the current running thread.

The thread library then creates a vector of pointers, dtvt, for the current thread t. The first element of each vector contains a generation number gent, which is used to determine when the vector needs to be extended. See Deferred Allocation of Thread-Local Storage Blocks.

Each element remaining in the vector dtvt,m, is a pointer to the block that is reserved for the TLS belonging to the dynamic object m.

For dynamically loaded, post-startup objects, the thread library defers the allocation of TLS blocks. Allocation occurs when the first reference is made to a TLS variable within the loaded object. For blocks whose allocation has been deferred, the pointer dtvt,m is set to an implementation-defined special value.

Note –

The runtime linker can group TLS templates for all startup objects so as to share a single element in the vector, dtv t,1. This grouping does not affect the offset calculations described previously or the creation of the list of initialization records. For the following sections, however, the value of M, the total number of objects, start with the value of 1.

The thread library then copies the initialization images to the corresponding locations within the new block of storage.