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