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>Per-thread data</TITLE
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>eCos Reference Manual</TH
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><H1
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><A
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NAME="KERNEL-THREAD-DATA">Per-thread data</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN660"
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></A
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><H2
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>Name</H2
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>cyg_thread_new_data_index, cyg_thread_free_data_index, cyg_thread_get_data, cyg_thread_get_data_ptr, cyg_thread_set_data&nbsp;--&nbsp;Manipulate per-thread data</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN667"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN668"><P
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></P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/kernel/kapi.h&gt;
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        </PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_ucount32 cyg_thread_new_data_index</CODE
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>(void);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_thread_free_data_index</CODE
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>(cyg_ucount32 index);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_addrword_t cyg_thread_get_data</CODE
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>(cyg_ucount32 index);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>cyg_addrword_t* cyg_thread_get_data_ptr</CODE
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>(cyg_ucount32 index);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_thread_set_data</CODE
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>(cyg_ucount32 index, cyg_addrword_t data);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN696"
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></A
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><H2
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>Description</H2
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><P
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>In some applications and libraries it is useful to have some data that
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is specific to each thread. For example, many of the functions in the
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POSIX compatibility package return -1 to indicate an error and store
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additional information in what appears to be a global variable
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<TT
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CLASS="VARNAME"
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>errno</TT
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>. However, if multiple threads make concurrent
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calls into the POSIX library and if <TT
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CLASS="VARNAME"
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>errno</TT
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> were
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really a global variable then a thread would have no way of knowing
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whether the current <TT
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CLASS="VARNAME"
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>errno</TT
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> value really corresponded
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to the last POSIX call it made, or whether some other thread had run
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in the meantime and made a different POSIX call which updated the
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variable. To avoid such confusion <TT
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CLASS="VARNAME"
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>errno</TT
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> is instead
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implemented as a per-thread variable, and each thread has its own
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instance.
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      </P
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><P
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>The support for per-thread data can be disabled via the configuration
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option <TT
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CLASS="VARNAME"
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>CYGVAR_KERNEL_THREADS_DATA</TT
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>. If enabled, each
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<SPAN
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CLASS="STRUCTNAME"
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>cyg_thread</SPAN
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> data structure holds a small array
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of words. The size of this array is determined by the configuration
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option <TT
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CLASS="VARNAME"
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>CYGNUM_KERNEL_THREADS_DATA_MAX</TT
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>. When a
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thread is created the array is filled with zeroes.
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      </P
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><P
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>If an application needs to use per-thread data then it needs an index
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into this array which has not yet been allocated to other code. This
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index can be obtained by calling
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<TT
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CLASS="FUNCTION"
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>cyg_thread_new_data_index</TT
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>, and then used in
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subsequent calls to <TT
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CLASS="FUNCTION"
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>cyg_thread_get_data</TT
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>.
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Typically indices are allocated during system initialization and
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stored in static variables. If for some reason a slot in the array is
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no longer required and can be re-used then it can be released by calling
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<TT
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CLASS="FUNCTION"
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>cyg_thread_free_data_index</TT
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>,
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      </P
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><P
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>The current per-thread data in a given slot can be obtained using
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<TT
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CLASS="FUNCTION"
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>cyg_thread_get_data</TT
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>. This implicitly operates on
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the current thread, and its single argument should be an index as
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returned by <TT
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CLASS="FUNCTION"
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>cyg_thread_new_data_index</TT
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>. The
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per-thread data can be updated using
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<TT
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CLASS="FUNCTION"
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>cyg_thread_set_data</TT
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>. If a particular item of
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per-thread data is needed repeatedly then
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<TT
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CLASS="FUNCTION"
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>cyg_thread_get_data_ptr</TT
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> can be used to obtain the
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address of the data, and indirecting through this pointer allows the
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data to be examined and updated efficiently.
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      </P
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><P
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>Some packages, for example the error and POSIX packages, have
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pre-allocated slots in the array of per-thread data. These slots
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should not normally be used by application code, and instead slots
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should be allocated during initialization by a call to
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<TT
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CLASS="FUNCTION"
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>cyg_thread_new_data_index</TT
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>. If it is known that,
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for example, the configuration will never include the POSIX
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compatibility package then application code may instead decide to
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re-use the slot allocated to that package,
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<TT
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CLASS="VARNAME"
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>CYGNUM_KERNEL_THREADS_DATA_POSIX</TT
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>, but obviously
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this does involve a risk of strange and subtle bugs if the
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application's requirements ever change.
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      </P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-THREAD-DATA-CONTEXT"
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></A
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><H2
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>Valid contexts</H2
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><P
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>Typically <TT
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CLASS="FUNCTION"
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>cyg_thread_new_data_index</TT
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> is only
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called during initialization, but may also be called at any time in
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thread context. <TT
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CLASS="FUNCTION"
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>cyg_thread_free_data_index</TT
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>, if
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used at all, can also be called during initialization or from thread
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context. <TT
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CLASS="FUNCTION"
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>cyg_thread_get_data</TT
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>,
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<TT
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CLASS="FUNCTION"
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>cyg_thread_get_data_ptr</TT
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>, and
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<TT
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CLASS="FUNCTION"
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>cyg_thread_set_data</TT
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> may only be called from
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thread context because they implicitly operate on the current thread.
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      </P
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