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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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<HTML
><HEAD
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>Some implementation details</TITLE
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><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"
>&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
></TD
></TR
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[/] [openrisc/] [trunk/] [rtos/] [ecos-2.0/] [doc/] [html/] [ref/] [libc-implementation-details.html] - Blame information for rev 510

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1	28	unneback	`<!-- Copyright (C) 2003 Red Hat, Inc. -->`
2			`<!-- This material may be distributed only subject to the terms -->`
3			`<!-- and conditions set forth in the Open Publication License, v1.0 -->`
4			`<!-- or later (the latest version is presently available at -->`
5			`<!-- http://www.opencontent.org/openpub/). -->`
6			`<!-- Distribution of the work or derivative of the work in any -->`
7			`<!-- standard (paper) book form is prohibited unless prior -->`
8			`<!-- permission is obtained from the copyright holder. -->`
9			`<HTML`
10			`><HEAD`
11			`><TITLE`
12			`>Some implementation details</TITLE`
13			`><meta name="MSSmartTagsPreventParsing" content="TRUE">`
14			`<META`
15			`NAME="GENERATOR"`
16			`CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+`
17			`"><LINK`
18			`REL="HOME"`
19			`TITLE="eCos Reference Manual"`
20			`HREF="ecos-ref.html"><LINK`
21			`REL="UP"`
22			`TITLE="C and math library overview"`
23			`HREF="c-and-math-library-overview.html"><LINK`
24			`REL="PREVIOUS"`
25			`TITLE="Math library compatibility modes"`
26			`HREF="math-library-compatibility-modes.html"><LINK`
27			`REL="NEXT"`
28			`TITLE="Thread safety"`
29			`HREF="libc-thread-safety.html"></HEAD`
30			`><BODY`
31			`CLASS="SECT1"`
32			`BGCOLOR="#FFFFFF"`
33			`TEXT="#000000"`
34			`LINK="#0000FF"`
35			`VLINK="#840084"`
36			`ALINK="#0000FF"`
37			`><DIV`
38			`CLASS="NAVHEADER"`
39			`><TABLE`
40			`SUMMARY="Header navigation table"`
41			`WIDTH="100%"`
42			`BORDER="0"`
43			`CELLPADDING="0"`
44			`CELLSPACING="0"`
45			`><TR`
46			`><TH`
47			`COLSPAN="3"`
48			`ALIGN="center"`
49			`>eCos Reference Manual</TH`
50			`></TR`
51			`><TR`
52			`><TD`
53			`WIDTH="10%"`
54			`ALIGN="left"`
55			`VALIGN="bottom"`
56			`><A`
57			`HREF="math-library-compatibility-modes.html"`
58			`ACCESSKEY="P"`
59			`>Prev</A`
60			`></TD`
61			`><TD`
62			`WIDTH="80%"`
63			`ALIGN="center"`
64			`VALIGN="bottom"`
65			`>Chapter 13. C and math library overview</TD`
66			`><TD`
67			`WIDTH="10%"`
68			`ALIGN="right"`
69			`VALIGN="bottom"`
70			`><A`
71			`HREF="libc-thread-safety.html"`
72			`ACCESSKEY="N"`
73			`>Next</A`
74			`></TD`
75			`></TR`
76			`></TABLE`
77			`><HR`
78			`ALIGN="LEFT"`
79			`WIDTH="100%"></DIV`
80			`><DIV`
81			`CLASS="SECT1"`
82			`><H1`
83			`CLASS="SECT1"`
84			`><A`
85			`NAME="LIBC-IMPLEMENTATION-DETAILS">Some implementation details</H1`
86			`><P`
87			`>Here are some details about the implementation`
88			`which might be interesting, although they do not affect the ISO-defined`
89			`semantics of the library. </P`
90			`><P`
91			`></P`
92			`><UL`
93			`><LI`
94			`><P`
95			`>It is possible to configure`
96			`<SPAN`
97			`CLASS="emphasis"`
98			`><I`
99			`CLASS="EMPHASIS"`
100			`>eCos</I`
101			`></SPAN`
102			`>`
103			`to have the standard C library without the kernel. You might want`
104			`to do this to use less memory. But if you disable the kernel, you`
105			`will be unable to use memory allocation, thread-safety and certain`
106			`stdio functions such as input. Other C library functionality is`
107			`unaffected.</P`
108			`></LI`
109			`><LI`
110			`><P`
111			`>The opaque type returned by`
112			`<TT`
113			`CLASS="FUNCTION"`
114			`>clock()</TT`
115			`>`
116			`is called clock_t, and is implemented as a 64 bit integer.`
117			`The value returned by`
118			`<TT`
119			`CLASS="FUNCTION"`
120			`>clock()</TT`
121			`>`
122			`is only correct if the kernel is configured with real-time clock`
123			`support, as determined by the CYGVAR_KERNEL_COUNTERS_CLOCK`
124			`configuration option in`
125			`<TT`
126			`CLASS="FILENAME"`
127			`>kernel.h</TT`
128			`>`
129			`.</P`
130			`></LI`
131			`><LI`
132			`><P`
133			`>The FILE type is not implemented as a structure, but rather`
134			`as a CYG_ADDRESS. </P`
135			`></LI`
136			`><LI`
137			`><P`
138			`>The GNU C compiler will place its own <SPAN`
139			`CLASS="emphasis"`
140			`><I`
141			`CLASS="EMPHASIS"`
142			`>built-in</I`
143			`></SPAN`
144			`> implementations`
145			`instead of some C library functions. This can be turned off with`
146			`the <SPAN`
147			`CLASS="emphasis"`
148			`><I`
149			`CLASS="EMPHASIS"`
150			`>-fno-builtin</I`
151			`></SPAN`
152			`> option. The functions affected`
153			`by this are`
154			`<TT`
155			`CLASS="FUNCTION"`
156			`>abs()</TT`
157			`>`
158			`,`
159			`<TT`
160			`CLASS="FUNCTION"`
161			`>cos()</TT`
162			`>`
163			`,`
164			`<TT`
165			`CLASS="FUNCTION"`
166			`>fabs()</TT`
167			`>`
168			`,`
169			`<TT`
170			`CLASS="FUNCTION"`
171			`>labs()</TT`
172			`>`
173			`,`
174			`<TT`
175			`CLASS="FUNCTION"`
176			`>memcmp()</TT`
177			`>`
178			`,`
179			`<TT`
180			`CLASS="FUNCTION"`
181			`>memcpy()</TT`
182			`>`
183			`,`
184			`<TT`
185			`CLASS="FUNCTION"`
186			`>sin()</TT`
187			`>`
188			`,`
189			`<TT`
190			`CLASS="FUNCTION"`
191			`>sqrt()</TT`
192			`>`
193			`,`
194			`<TT`
195			`CLASS="FUNCTION"`
196			`>strcmp()</TT`
197			`>`
198			`,`
199			`<TT`
200			`CLASS="FUNCTION"`
201			`>strcpy()</TT`
202			`>`
203			`, and`
204			`<TT`
205			`CLASS="FUNCTION"`
206			`>strlen()</TT`
207			`>`
208			`.</P`
209			`></LI`
210			`><LI`
211			`><P`
212			`>For faster execution speed you should avoid this option`
213			`and let the compiler use its built-ins. This can be turned off by`
214			`invoking`
215			`<SPAN`
216			`CLASS="emphasis"`
217			`><I`
218			`CLASS="EMPHASIS"`
219			`>GCC</I`
220			`></SPAN`
221			`>`
222			`with the <SPAN`
223			`CLASS="emphasis"`
224			`><I`
225			`CLASS="EMPHASIS"`
226			`>-fno-builtin</I`
227			`></SPAN`
228			`> option. </P`
229			`></LI`
230			`><LI`
231			`><P`
232			`><TT`
233			`CLASS="FUNCTION"`
234			`>memcpy()</TT`
235			`>`
236			`and`
237			`<TT`
238			`CLASS="FUNCTION"`
239			`>memset()</TT`
240			`>`
241			`are located in the infrastructure package, not in the C library`
242			`package. This is because the compiler calls these functions, and`
243			`the kernel needs to resolve them even if the C library is not configured. </P`
244			`></LI`
245			`><LI`
246			`><P`
247			`>Error codes such as EDOM and ERANGE, as well as`
248			`<TT`
249			`CLASS="FUNCTION"`
250			`>strerror()</TT`
251			`>`
252			`, are implemented in the <SPAN`
253			`CLASS="emphasis"`
254			`><I`
255			`CLASS="EMPHASIS"`
256			`>error</I`
257			`></SPAN`
258			`> package. The`
259			`error package is separate from the rest of the C and math libraries`
260			`so that the rest of`
261			`<SPAN`
262			`CLASS="emphasis"`
263			`><I`
264			`CLASS="EMPHASIS"`
265			`>eCos</I`
266			`></SPAN`
267			`>`
268			`can use these error handling facilities even if the C library is`
269			`not configured. </P`
270			`></LI`
271			`><LI`
272			`><P`
273			`>When`
274			`<TT`
275			`CLASS="FUNCTION"`
276			`>free()</TT`
277			`>`
278			`is invoked, heap memory will normally be coalesced. If the CYGSEM_KERNEL_MEMORY_COALESCE`
279			`configuration parameter is not set, memory will not be coalesced,`
280			`which might cause programs to fail. </P`
281			`></LI`
282			`><LI`
283			`><P`
284			`>Signals, as implemented by`
285			`<TT`
286			`CLASS="FILENAME"`
287			`><signal.h></TT`
288			`>, are guaranteed to work`
289			`correctly if raised using the`
290			`<TT`
291			`CLASS="FUNCTION"`
292			`>raise()</TT`
293			`>`
294			`function from a normal working program context. Using signals from`
295			`within an ISR or DSR context is not expected to work. Also, it is`
296			`not guaranteed that if CYGSEM_LIBC_SIGNALS_HWEXCEPTIONS`
297			`is set, that handling a signal using`
298			`<TT`
299			`CLASS="FUNCTION"`
300			`>signal()</TT`
301			`>`
302			`will necessarily catch that form of exception. For example, it`
303			`may be expected that a divide-by-zero error would be caught by handling`
304			`<TT`
305			`CLASS="VARNAME"`
306			`>SIGFPE</TT`
307			`>. However it depends on the underlying HAL implementation to implement`
308			`the required hardware exception. And indeed the hardware itself`
309			`may not be capable of detecting these exceptions so it may not be`
310			`possible for the HAL implementer to do this in any case. Despite`
311			`this lack of guarantees in this respect, the signals implementation`
312			`is still ISO C compliant since ISO C does not offer any such guarantees`
313			`either. </P`
314			`></LI`
315			`><LI`
316			`><P`
317			`>The`
318			`<TT`
319			`CLASS="FUNCTION"`
320			`>getenv()</TT`
321			`>`
322			`function is implemented (unless the CYGPKG_LIBC_ENVIRONMENT configuration`
323			`option is turned off), but there is no shell or`
324			`<TT`
325			`CLASS="FUNCTION"`
326			`>putenv()</TT`
327			`>`
328			`function to set the environment dynamically. The environment is`
329			`set in a global variable environ, declared as:</P`
330			`><TABLE`
331			`BORDER="5"`
332			`BGCOLOR="#E0E0F0"`
333			`WIDTH="70%"`
334			`><TR`
335			`><TD`
336			`><PRE`
337			`CLASS="PROGRAMLISTING"`
338			`>extern char **environ; // Standard environment definition</PRE`
339			`></TD`
340			`></TR`
341			`></TABLE`
342			`><P`
343			`>The environment can be statically initialized at startup time`
344			`using the CYGDAT_LIBC_DEFAULT_ENVIRONMENT`
345			`option. If so, remember that the final entry of the array initializer`
346			`must be NULL. </P`
347			`></LI`
348			`></UL`
349			`><P`
350			`>Here is a minimal <SPAN`
351			`CLASS="emphasis"`
352			`><I`
353			`CLASS="EMPHASIS"`
354			`>eCos</I`
355			`></SPAN`
356			`> program which`
357			`demonstrates the use of environments (see also the test case in <TT`
358			`CLASS="FILENAME"`
359			`>language/c/libc/current/tests/stdlib/getenv.c</TT`
360			`>): </P`
361			`><TABLE`
362			`BORDER="5"`
363			`BGCOLOR="#E0E0F0"`
364			`WIDTH="70%"`
365			`><TR`
366			`><TD`
367			`><PRE`
368			`CLASS="PROGRAMLISTING"`
369			`>#include <stdio.h>`
370			`#include <stdlib.h> // Main header for stdlib functions`
371
372			`extern char **environ; // Standard environment definition`
373
374			`int`
375			`main( int argc, char *argv[] )`
376			`{`
377			`char *str;`
378			`char *env[] = { "PATH=/usr/local/bin:/usr/bin",`
379			`"HOME=/home/fred",`
380			`"TEST=1234=5678",`
381			`"home=hatstand",`
382			`NULL };`
383
384			`printf("Display the current PATH environment variable\n");`
385
386			`environ = (char **)&env;`
387
388			`str = getenv("PATH");`
389
390			`if (str==NULL) {`
391			`printf("The current PATH is unset\n");`
392			`} else {`
393			`printf("The current PATH is \"%s\"\n", str);`
394			`}`
395			`return 0;`
396			`} </PRE`
397			`></TD`
398			`></TR`
399			`></TABLE`
400			`></DIV`
401			`><DIV`
402			`CLASS="NAVFOOTER"`
403			`><HR`
404			`ALIGN="LEFT"`
405			`WIDTH="100%"><TABLE`
406			`SUMMARY="Footer navigation table"`
407			`WIDTH="100%"`
408			`BORDER="0"`
409			`CELLPADDING="0"`
410			`CELLSPACING="0"`
411			`><TR`
412			`><TD`
413			`WIDTH="33%"`
414			`ALIGN="left"`
415			`VALIGN="top"`
416			`><A`
417			`HREF="math-library-compatibility-modes.html"`
418			`ACCESSKEY="P"`
419			`>Prev</A`
420			`></TD`
421			`><TD`
422			`WIDTH="34%"`
423			`ALIGN="center"`
424			`VALIGN="top"`
425			`><A`
426			`HREF="ecos-ref.html"`
427			`ACCESSKEY="H"`
428			`>Home</A`
429			`></TD`
430			`><TD`
431			`WIDTH="33%"`
432			`ALIGN="right"`
433			`VALIGN="top"`
434			`><A`
435			`HREF="libc-thread-safety.html"`
436			`ACCESSKEY="N"`
437			`>Next</A`
438			`></TD`
439			`></TR`
440			`><TR`
441			`><TD`
442			`WIDTH="33%"`
443			`ALIGN="left"`
444			`VALIGN="top"`
445			`>Math library compatibility modes</TD`
446			`><TD`
447			`WIDTH="34%"`
448			`ALIGN="center"`
449			`VALIGN="top"`
450			`><A`
451			`HREF="c-and-math-library-overview.html"`
452			`ACCESSKEY="U"`
453			`>Up</A`
454			`></TD`
455			`><TD`
456			`WIDTH="33%"`
457			`ALIGN="right"`
458			`VALIGN="top"`
459			`>Thread safety</TD`
460			`></TR`
461			`></TABLE`
462			`></DIV`
463			`></BODY`
464			`></HTML`
465			`>`