+++ /dev/null
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-><TITLE
->Some implementation details</TITLE
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-TITLE="Math library compatibility modes"
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->Prev</A
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->Chapter 13. C and math library overview</TD
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-><DIV
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-><H1
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-><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
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