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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Some implementation details</TITLE
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TITLE="C and math library overview"
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TITLE="Math library compatibility modes"
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TITLE="Thread safety"
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><TR
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><TH
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COLSPAN="3"
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>eCos Reference Manual</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="math-library-compatibility-modes.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 13. C and math library overview</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="libc-thread-safety.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="LIBC-IMPLEMENTATION-DETAILS">Some implementation details</H1
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><P
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>Here are some details about the implementation
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which might be interesting, although they do not affect the ISO-defined
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semantics of the library. </P
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><P
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></P
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><UL
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><LI
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><P
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>It is possible to configure
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<SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>eCos</I
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></SPAN
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>
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 to have the standard C library without the kernel. You might want
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to do this to use less memory. But if you disable the kernel, you
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will be unable to use memory allocation, thread-safety and certain
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stdio functions such as input. Other C library functionality is
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unaffected.</P
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></LI
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><LI
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><P
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>The opaque type returned by
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<TT
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CLASS="FUNCTION"
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>clock()</TT
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>
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 is called clock_t, and is implemented as a 64 bit integer.
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The value returned by
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<TT
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CLASS="FUNCTION"
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>clock()</TT
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>
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 is only correct if the kernel is configured with real-time clock
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support, as determined by the CYGVAR_KERNEL_COUNTERS_CLOCK
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configuration option in
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<TT
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CLASS="FILENAME"
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>kernel.h</TT
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>
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.</P
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></LI
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><LI
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><P
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>The FILE type is not implemented as a structure, but rather
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as a CYG_ADDRESS. </P
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></LI
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><LI
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><P
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>The GNU C compiler will place its own <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>built-in</I
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></SPAN
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> implementations
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instead of some C library functions. This can be turned off with
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the <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>-fno-builtin</I
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></SPAN
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> option. The functions affected
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by this are
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<TT
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CLASS="FUNCTION"
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>abs()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>cos()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>fabs()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>labs()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>memcmp()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>memcpy()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>sin()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>sqrt()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>strcmp()</TT
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>
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,
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<TT
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CLASS="FUNCTION"
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>strcpy()</TT
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>
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, and
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<TT
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CLASS="FUNCTION"
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>strlen()</TT
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>
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.</P
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></LI
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><LI
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><P
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>For faster execution speed you should avoid this option
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and let the compiler use its built-ins. This can be turned off by
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invoking
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<SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>GCC</I
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></SPAN
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>
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 with the <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>-fno-builtin</I
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></SPAN
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> option. </P
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></LI
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><LI
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><P
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><TT
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CLASS="FUNCTION"
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>memcpy()</TT
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>
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 and
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<TT
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CLASS="FUNCTION"
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>memset()</TT
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>
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 are located in the infrastructure package, not in the C library
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package. This is because the compiler calls these functions, and
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the kernel needs to resolve them even if the C library is not configured. </P
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></LI
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><LI
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><P
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>Error codes such as EDOM and ERANGE, as well as
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<TT
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CLASS="FUNCTION"
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>strerror()</TT
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>
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, are implemented in the <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>error</I
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></SPAN
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> package. The
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error package is separate from the rest of the C and math libraries
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so that the rest of
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<SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>eCos</I
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></SPAN
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>
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 can use these error handling facilities even if the C library is
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not configured. </P
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></LI
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><LI
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><P
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>When
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<TT
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CLASS="FUNCTION"
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>free()</TT
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>
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 is invoked, heap memory will normally be coalesced. If the CYGSEM_KERNEL_MEMORY_COALESCE
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configuration parameter is not set, memory will not be coalesced,
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which might cause programs to fail. </P
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></LI
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><LI
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><P
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>Signals, as implemented by
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<TT
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CLASS="FILENAME"
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>&lt;signal.h&gt;</TT
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>, are guaranteed to work
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correctly if raised using the
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<TT
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CLASS="FUNCTION"
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>raise()</TT
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>
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 function from a normal working program context. Using signals from
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within an ISR or DSR context is not expected to work. Also, it is
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not guaranteed that if CYGSEM_LIBC_SIGNALS_HWEXCEPTIONS
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is set, that handling a signal using
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<TT
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CLASS="FUNCTION"
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>signal()</TT
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>
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 will necessarily catch that form of exception. For example, it
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may be expected that a divide-by-zero error would be caught by handling
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<TT
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CLASS="VARNAME"
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>SIGFPE</TT
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>. However it depends on the underlying HAL implementation to implement
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the required hardware exception. And indeed the hardware itself
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may not be capable of detecting these exceptions so it may not be
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possible for the HAL implementer to do this in any case. Despite
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this lack of guarantees in this respect, the signals implementation
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is still ISO C compliant since ISO C does not offer any such guarantees
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either. </P
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></LI
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><LI
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><P
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>The
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<TT
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CLASS="FUNCTION"
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>getenv()</TT
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>
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 function is implemented (unless the CYGPKG_LIBC_ENVIRONMENT configuration
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option is turned off), but there is no shell or
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<TT
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CLASS="FUNCTION"
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>putenv()</TT
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>
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 function to set the environment dynamically. The environment is
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set in a global variable environ, declared as:</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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>extern char **environ; // Standard environment definition</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 environment can be statically initialized at startup time
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using the CYGDAT_LIBC_DEFAULT_ENVIRONMENT
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option. If so, remember that the final entry of the array initializer
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must be NULL. </P
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></LI
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></UL
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><P
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>Here is a minimal <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>eCos</I
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></SPAN
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> program which
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demonstrates the use of environments (see also the test case in <TT
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CLASS="FILENAME"
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>language/c/libc/current/tests/stdlib/getenv.c</TT
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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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>#include &lt;stdio.h&gt;
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#include &lt;stdlib.h&gt; // Main header for stdlib functions
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extern char **environ; // Standard environment definition
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int
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main( int argc, char *argv[] )
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{
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 char *str;
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 char *env[] = { "PATH=/usr/local/bin:/usr/bin",
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 "HOME=/home/fred",
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 "TEST=1234=5678",
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 "home=hatstand",
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 NULL };
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 printf("Display the current PATH environment variable\n");
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 environ = (char **)&amp;env;
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 str = getenv("PATH");
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 if (str==NULL) {
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  printf("The current PATH is unset\n");
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 } else {
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  printf("The current PATH is \"%s\"\n", str);
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 }
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 return 0;
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} </PRE
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></TD
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></TR
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></TABLE
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="math-library-compatibility-modes.html"
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ACCESSKEY="P"
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>Prev</A
420 
></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="ecos-ref.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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WIDTH="33%"
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ALIGN="right"
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><A
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HREF="libc-thread-safety.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Math library compatibility modes</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="c-and-math-library-overview.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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WIDTH="33%"
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ALIGN="right"
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>Thread safety</TD
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