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<!-- Copyright (C) 2003 Red Hat, Inc. --> <!-- This material may be distributed only subject to the terms --> <!-- and conditions set forth in the Open Publication License, v1.0 --> <!-- or later (the latest version is presently available at --> <!-- http://www.opencontent.org/openpub/). --> <!-- Distribution of the work or derivative of the work in any --> <!-- standard (paper) book form is prohibited unless prior --> <!-- permission is obtained from the copyright holder. --> <HTML ><HEAD ><TITLE >Some implementation details</TITLE ><meta name="MSSmartTagsPreventParsing" content="TRUE"> <META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="eCos Reference Manual" HREF="ecos-ref.html"><LINK REL="UP" TITLE="C and math library overview" HREF="c-and-math-library-overview.html"><LINK REL="PREVIOUS" TITLE="Math library compatibility modes" HREF="math-library-compatibility-modes.html"><LINK REL="NEXT" TITLE="Thread safety" HREF="libc-thread-safety.html"></HEAD ><BODY CLASS="SECT1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >eCos Reference Manual</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="math-library-compatibility-modes.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 13. C and math library overview</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="libc-thread-safety.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="LIBC-IMPLEMENTATION-DETAILS">Some implementation details</H1 ><P >Here are some details about the implementation which might be interesting, although they do not affect the ISO-defined semantics of the library. </P ><P ></P ><UL ><LI ><P >It is possible to configure <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >eCos</I ></SPAN > to have the standard C library without the kernel. You might want to do this to use less memory. But if you disable the kernel, you will be unable to use memory allocation, thread-safety and certain stdio functions such as input. Other C library functionality is unaffected.</P ></LI ><LI ><P >The opaque type returned by <TT CLASS="FUNCTION" >clock()</TT > is called clock_t, and is implemented as a 64 bit integer. The value returned by <TT CLASS="FUNCTION" >clock()</TT > is only correct if the kernel is configured with real-time clock support, as determined by the CYGVAR_KERNEL_COUNTERS_CLOCK configuration option in <TT CLASS="FILENAME" >kernel.h</TT > .</P ></LI ><LI ><P >The FILE type is not implemented as a structure, but rather as a CYG_ADDRESS. </P ></LI ><LI ><P >The GNU C compiler will place its own <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >built-in</I ></SPAN > implementations instead of some C library functions. This can be turned off with the <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >-fno-builtin</I ></SPAN > option. The functions affected by this are <TT CLASS="FUNCTION" >abs()</TT > , <TT CLASS="FUNCTION" >cos()</TT > , <TT CLASS="FUNCTION" >fabs()</TT > , <TT CLASS="FUNCTION" >labs()</TT > , <TT CLASS="FUNCTION" >memcmp()</TT > , <TT CLASS="FUNCTION" >memcpy()</TT > , <TT CLASS="FUNCTION" >sin()</TT > , <TT CLASS="FUNCTION" >sqrt()</TT > , <TT CLASS="FUNCTION" >strcmp()</TT > , <TT CLASS="FUNCTION" >strcpy()</TT > , and <TT CLASS="FUNCTION" >strlen()</TT > .</P ></LI ><LI ><P >For faster execution speed you should avoid this option and let the compiler use its built-ins. This can be turned off by invoking <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >GCC</I ></SPAN > with the <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >-fno-builtin</I ></SPAN > option. </P ></LI ><LI ><P ><TT CLASS="FUNCTION" >memcpy()</TT > and <TT CLASS="FUNCTION" >memset()</TT > are located in the infrastructure package, not in the C library package. This is because the compiler calls these functions, and the kernel needs to resolve them even if the C library is not configured. </P ></LI ><LI ><P >Error codes such as EDOM and ERANGE, as well as <TT CLASS="FUNCTION" >strerror()</TT > , are implemented in the <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >error</I ></SPAN > package. The error package is separate from the rest of the C and math libraries so that the rest of <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >eCos</I ></SPAN > can use these error handling facilities even if the C library is not configured. </P ></LI ><LI ><P >When <TT CLASS="FUNCTION" >free()</TT > is invoked, heap memory will normally be coalesced. If the CYGSEM_KERNEL_MEMORY_COALESCE configuration parameter is not set, memory will not be coalesced, which might cause programs to fail. </P ></LI ><LI ><P >Signals, as implemented by <TT CLASS="FILENAME" ><signal.h></TT >, are guaranteed to work correctly if raised using the <TT CLASS="FUNCTION" >raise()</TT > function from a normal working program context. Using signals from within an ISR or DSR context is not expected to work. Also, it is not guaranteed that if CYGSEM_LIBC_SIGNALS_HWEXCEPTIONS is set, that handling a signal using <TT CLASS="FUNCTION" >signal()</TT > will necessarily catch that form of exception. For example, it may be expected that a divide-by-zero error would be caught by handling <TT CLASS="VARNAME" >SIGFPE</TT >. However it depends on the underlying HAL implementation to implement the required hardware exception. And indeed the hardware itself may not be capable of detecting these exceptions so it may not be possible for the HAL implementer to do this in any case. Despite this lack of guarantees in this respect, the signals implementation is still ISO C compliant since ISO C does not offer any such guarantees either. </P ></LI ><LI ><P >The <TT CLASS="FUNCTION" >getenv()</TT > function is implemented (unless the CYGPKG_LIBC_ENVIRONMENT configuration option is turned off), but there is no shell or <TT CLASS="FUNCTION" >putenv()</TT > function to set the environment dynamically. The environment is set in a global variable environ, declared as:</P ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="PROGRAMLISTING" >extern char **environ; // Standard environment definition</PRE ></TD ></TR ></TABLE ><P >The environment can be statically initialized at startup time using the CYGDAT_LIBC_DEFAULT_ENVIRONMENT option. If so, remember that the final entry of the array initializer must be NULL. </P ></LI ></UL ><P >Here is a minimal <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >eCos</I ></SPAN > program which demonstrates the use of environments (see also the test case in <TT CLASS="FILENAME" >language/c/libc/current/tests/stdlib/getenv.c</TT >): </P ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="PROGRAMLISTING" >#include <stdio.h> #include <stdlib.h> // Main header for stdlib functions extern char **environ; // Standard environment definition int main( int argc, char *argv[] ) { char *str; char *env[] = { "PATH=/usr/local/bin:/usr/bin", "HOME=/home/fred", "TEST=1234=5678", "home=hatstand", NULL }; printf("Display the current PATH environment variable\n"); environ = (char **)&env; str = getenv("PATH"); if (str==NULL) { printf("The current PATH is unset\n"); } else { printf("The current PATH is \"%s\"\n", str); } return 0; } </PRE ></TD ></TR ></TABLE ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="math-library-compatibility-modes.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="ecos-ref.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="libc-thread-safety.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Math library compatibility modes</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="c-and-math-library-overview.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Thread safety</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
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