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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Exception handling</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-EXCEPTIONS">Exception handling</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN782"
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></A
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><H2
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>Name</H2
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>cyg_exception_set_handler, cyg_exception_clear_handler, cyg_exception_call_handler&nbsp;--&nbsp;Handle processor exceptions</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN787"><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="AEN788"><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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>void cyg_exception_set_handler</CODE
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>(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
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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_exception_clear_handler</CODE
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>(cyg_code_t exception_number);</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_exception_call_handler</CODE
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>(cyg_handle_t thread, cyg_code_t exception_number, cyg_addrword_t exception_info);</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="AEN817"
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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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>Sometimes code attempts operations that are not legal on the current
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hardware, for example dividing by zero, or accessing data through a
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pointer that is not properly aligned. When this happens the hardware
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will raise an exception. This is very similar to an interrupt, but
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happens synchronously with code execution rather than asynchronously
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and hence can be tied to the thread that is currently running.
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      </P
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><P
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>The exceptions that can be raised depend very much on the hardware,
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especially the processor. The corresponding documentation should be
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consulted for more details. Alternatively the architectural HAL header
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file <TT
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CLASS="FILENAME"
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>hal_intr.h</TT
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>, or one of the
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variant or platform header files it includes, will contain appropriate
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definitions. The details of how to handle exceptions, including
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whether or not it is possible to recover from them, also depend on the
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hardware.
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      </P
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><P
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>Exception handling is optional, and can be disabled through the
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configuration option <TT
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CLASS="VARNAME"
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>CYGPKG_KERNEL_EXCEPTIONS</TT
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>. If
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an application has been exhaustively tested and is trusted never to
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raise a hardware exception then this option can be disabled and code
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and data sizes will be reduced somewhat. If exceptions are left
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enabled then the system will provide default handlers for the various
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exceptions, but these do nothing. Even the specific type of exception
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is ignored, so there is no point in attempting to decode this and
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distinguish between say a divide-by-zero and an unaligned access.
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If the application installs its own handlers and wants details of the
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specific exception being raised then the configuration option
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<TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_EXCEPTIONS_DECODE</TT
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> has to be enabled.
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      </P
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><P
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>An alternative handler can be installed using
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<TT
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CLASS="FUNCTION"
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>cyg_exception_set_handler</TT
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>. This requires a code
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for the exception, a function pointer for the new exception handler,
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and a parameter to be passed to this handler. Details of the
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previously installed exception handler will be returned via the
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remaining two arguments, allowing that handler to be reinstated, or
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null pointers can be used if this information is of no interest. An
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exception handling function should take the following form:
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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="PROGRAMLISTING"
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>void
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my_exception_handler(cyg_addrword_t data, cyg_code_t exception, cyg_addrword_t info)
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{
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    &#8230;
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}
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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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>The data argument corresponds to the <TT
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CLASS="PARAMETER"
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><I
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>new_data</I
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></TT
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>
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parameter supplied to <TT
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CLASS="FUNCTION"
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>cyg_exception_set_handler</TT
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>.
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The exception code is provided as well, in case a single handler is
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expected to support multiple exceptions. The <TT
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CLASS="PARAMETER"
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><I
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>info</I
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></TT
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>
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argument will depend on the hardware and on the specific exception.
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      </P
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_exception_clear_handler</TT
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> can be used to
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restore the default handler, if desired. It is also possible for
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software to raise an exception and cause the current handler to be
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invoked, but generally this is useful only for testing.
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      </P
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><P
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>By default the system maintains a single set of global exception
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handlers. However, since exceptions occur synchronously it is
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sometimes useful to handle them on a per-thread basis, and have a
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different set of handlers for each thread. This behaviour can be
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obtained by disabling the configuration option
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<TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_EXCEPTIONS_GLOBAL</TT
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>. If per-thread
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exception handlers are being used then
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<TT
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CLASS="FUNCTION"
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>cyg_exception_set_handler</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_exception_clear_handler</TT
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> apply to the current
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thread. Otherwise they apply to the global set of handlers.
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      </P
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><DIV
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CLASS="CAUTION"
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><P
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></P
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CLASS="CAUTION"
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BORDER="1"
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><TD
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ALIGN="CENTER"
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><B
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>Caution</B
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></TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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><P
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>In the current implementation
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<TT
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CLASS="FUNCTION"
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>cyg_exception_call_handler</TT
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> can only be used on
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the current thread. There is no support for delivering an exception to
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another thread.
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      </P
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></TD
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></TABLE
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></DIV
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>Exceptions at the eCos kernel level refer specifically to
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hardware-related events such as unaligned accesses to memory or
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division by zero. There is no relation with other concepts that are
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also known as exceptions, for example the <TT
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CLASS="LITERAL"
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>throw</TT
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> and
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<TT
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CLASS="LITERAL"
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>catch</TT
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> facilities associated with C++.
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      </P
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></BLOCKQUOTE
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-EXCEPTIONS-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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>If the system is configured with a single set of global exception
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handlers then
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<TT
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CLASS="FUNCTION"
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>cyg_exception_set_handler</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_exception_clear_handler</TT
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> may be called during
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initialization or from thread context. If instead per-thread exception
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handlers are being used then it is not possible to install new
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handlers during initialization because the functions operate
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implicitly on the current thread, so they can only be called from
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thread context. <TT
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CLASS="FUNCTION"
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>cyg_exception_call_handler</TT
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> should
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only be called from thread context.
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      </P
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