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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Some implementation details</TITLE
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><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"
>&lt;signal.h&gt;</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 &lt;stdio.h&gt;
#include &lt;stdlib.h&gt; // 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 **)&amp;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
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