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>Exception handling</TITLE
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><H1
><A
NAME="KERNEL-EXCEPTIONS">Exception handling</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN782"
></A
><H2
>Name</H2
>cyg_exception_set_handler, cyg_exception_clear_handler, cyg_exception_call_handler -- Handle processor exceptions</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN787"><H2
>Synopsis</H2
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NAME="AEN788"><P
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/kernel/kapi.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_exception_set_handler</CODE
>(cyg_code_t exception_number, cyg_exception_handler_t* new_handler, cyg_addrword_t new_data, cyg_exception_handler_t** old_handler, cyg_addrword_t* old_data);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_exception_clear_handler</CODE
>(cyg_code_t exception_number);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void cyg_exception_call_handler</CODE
>(cyg_handle_t thread, cyg_code_t exception_number, cyg_addrword_t exception_info);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN817"
></A
><H2
>Description</H2
><P
>Sometimes code attempts operations that are not legal on the current
hardware, for example dividing by zero, or accessing data through a
pointer that is not properly aligned. When this happens the hardware
will raise an exception. This is very similar to an interrupt, but
happens synchronously with code execution rather than asynchronously
and hence can be tied to the thread that is currently running.
</P
><P
>The exceptions that can be raised depend very much on the hardware,
especially the processor. The corresponding documentation should be
consulted for more details. Alternatively the architectural HAL header
file <TT
CLASS="FILENAME"
>hal_intr.h</TT
>, or one of the
variant or platform header files it includes, will contain appropriate
definitions. The details of how to handle exceptions, including
whether or not it is possible to recover from them, also depend on the
hardware.
</P
><P
>Exception handling is optional, and can be disabled through the
configuration option <TT
CLASS="VARNAME"
>CYGPKG_KERNEL_EXCEPTIONS</TT
>. If
an application has been exhaustively tested and is trusted never to
raise a hardware exception then this option can be disabled and code
and data sizes will be reduced somewhat. If exceptions are left
enabled then the system will provide default handlers for the various
exceptions, but these do nothing. Even the specific type of exception
is ignored, so there is no point in attempting to decode this and
distinguish between say a divide-by-zero and an unaligned access.
If the application installs its own handlers and wants details of the
specific exception being raised then the configuration option
<TT
CLASS="VARNAME"
>CYGSEM_KERNEL_EXCEPTIONS_DECODE</TT
> has to be enabled.
</P
><P
>An alternative handler can be installed using
<TT
CLASS="FUNCTION"
>cyg_exception_set_handler</TT
>. This requires a code
for the exception, a function pointer for the new exception handler,
and a parameter to be passed to this handler. Details of the
previously installed exception handler will be returned via the
remaining two arguments, allowing that handler to be reinstated, or
null pointers can be used if this information is of no interest. An
exception handling function should take the following form:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>void
my_exception_handler(cyg_addrword_t data, cyg_code_t exception, cyg_addrword_t info)
{
…
}
</PRE
></TD
></TR
></TABLE
><P
>The data argument corresponds to the <TT
CLASS="PARAMETER"
><I
>new_data</I
></TT
>
parameter supplied to <TT
CLASS="FUNCTION"
>cyg_exception_set_handler</TT
>.
The exception code is provided as well, in case a single handler is
expected to support multiple exceptions. The <TT
CLASS="PARAMETER"
><I
>info</I
></TT
>
argument will depend on the hardware and on the specific exception.
</P
><P
><TT
CLASS="FUNCTION"
>cyg_exception_clear_handler</TT
> can be used to
restore the default handler, if desired. It is also possible for
software to raise an exception and cause the current handler to be
invoked, but generally this is useful only for testing.
</P
><P
>By default the system maintains a single set of global exception
handlers. However, since exceptions occur synchronously it is
sometimes useful to handle them on a per-thread basis, and have a
different set of handlers for each thread. This behaviour can be
obtained by disabling the configuration option
<TT
CLASS="VARNAME"
>CYGSEM_KERNEL_EXCEPTIONS_GLOBAL</TT
>. If per-thread
exception handlers are being used then
<TT
CLASS="FUNCTION"
>cyg_exception_set_handler</TT
> and
<TT
CLASS="FUNCTION"
>cyg_exception_clear_handler</TT
> apply to the current
thread. Otherwise they apply to the global set of handlers.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
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><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
>In the current implementation
<TT
CLASS="FUNCTION"
>cyg_exception_call_handler</TT
> can only be used on
the current thread. There is no support for delivering an exception to
another thread.
</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>Exceptions at the eCos kernel level refer specifically to
hardware-related events such as unaligned accesses to memory or
division by zero. There is no relation with other concepts that are
also known as exceptions, for example the <TT
CLASS="LITERAL"
>throw</TT
> and
<TT
CLASS="LITERAL"
>catch</TT
> facilities associated with C++.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="KERNEL-EXCEPTIONS-CONTEXT"
></A
><H2
>Valid contexts</H2
><P
>If the system is configured with a single set of global exception
handlers then
<TT
CLASS="FUNCTION"
>cyg_exception_set_handler</TT
> and
<TT
CLASS="FUNCTION"
>cyg_exception_clear_handler</TT
> may be called during
initialization or from thread context. If instead per-thread exception
handlers are being used then it is not possible to install new
handlers during initialization because the functions operate
implicitly on the current thread, so they can only be called from
thread context. <TT
CLASS="FUNCTION"
>cyg_exception_call_handler</TT
> should
only be called from thread context.
</P
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