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Task-specific data is implemented by keeping such globals in the stack of the main thread.  A single main thread is always present and, hence, provides user-accessible, per-task storage.  Task-specific data is accessed via TLS:  The TLS data of each thread includes a reference to the main thread's task-specific globalslike TLS except that the task-specific data is accessed from the stack of the main thread, rather than the stack stack of the main thread.

Re-Entrant getopt()

One use of task-specific data is for global variables used by the getopt() function.  getopt() is an important C library function used by applications for parsing of task command line parameters.  It is, however, not re-entrant due to the use of global variables:  optarg, opterr, optind, and optopt. There are several, non-standard implementations for a re-entrant version of called getopt_r(), however, these are all wildly different and do not conform to any standard.  Non-standard interfaces are not desirable in NuttX.

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TLS data is always located at the beginning thread's stack.  This is true for both CPUs with push-up stacks and CPUs with push-down stacks.  This location required in order to access the TLS by ANDing the aligned stack pointer address.  The stack memory maps, however, then look very different:differ only in the usage of the available stack

   Push Down          Push Up
+-------------+   +-------------+ <- Stack memory allocation
|  TLS Data   |   |  TLS Data   |
+-------------+   +-------------+
|              Task Data*  |   | Task Data*  |
| Available   | +-------------+   +-------------+
|   Stack       Arguments  |   |  Arguments  |
|             | +-------------+   +-------------+ |
|             |   |             |  v
|             Available   |   | Available   | |
|             Stack     |   |   Stack     | v
|             | ^   |             |
+-------------+ | |             |
|  Arguments  |   |             |
+-------------+   |             |   |             |
| Task Data*  |                | ^ |             |
+-------------+   | +-------------+

* Task data is allocated in the main's thread's stack only

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