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1 =head1 LIBECB - e-C-Builtins
2
3 =head2 ABOUT LIBECB
4
5 Libecb is currently a simple header file that doesn't require any
6 configuration to use or include in your project.
7
8 It's part of the e-suite of libraries, other members of which include
9 libev and libeio.
10
11 Its homepage can be found here:
12
13 http://software.schmorp.de/pkg/libecb
14
15 It mainly provides a number of wrappers around many compiler built-ins,
16 together with replacement functions for other compilers. In addition
17 to this, it provides a number of other lowlevel C utilities, such as
18 endianness detection, byte swapping or bit rotations.
19
20 Or in other words, things that should be built into any standard C
21 system, but aren't, implemented as efficient as possible with GCC (clang,
22 msvc...), and still correct with other compilers.
23
24 More might come.
25
26 =head2 ABOUT THE HEADER
27
28 At the moment, all you have to do is copy F<ecb.h> somewhere where your
29 compiler can find it and include it:
30
31 #include <ecb.h>
32
33 The header should work fine for both C and C++ compilation, and gives you
34 all of F<inttypes.h> in addition to the ECB symbols.
35
36 There are currently no object files to link to - future versions might
37 come with an (optional) object code library to link against, to reduce
38 code size or gain access to additional features.
39
40 It also currently includes everything from F<inttypes.h>.
41
42 =head2 ABOUT THIS MANUAL / CONVENTIONS
43
44 This manual mainly describes each (public) function available after
45 including the F<ecb.h> header. The header might define other symbols than
46 these, but these are not part of the public API, and not supported in any
47 way.
48
49 When the manual mentions a "function" then this could be defined either as
50 as inline function, a macro, or an external symbol.
51
52 When functions use a concrete standard type, such as C<int> or
53 C<uint32_t>, then the corresponding function works only with that type. If
54 only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
55 the corresponding function relies on C to implement the correct types, and
56 is usually implemented as a macro. Specifically, a "bool" in this manual
57 refers to any kind of boolean value, not a specific type.
58
59 =head2 TYPES / TYPE SUPPORT
60
61 ecb.h makes sure that the following types are defined (in the expected way):
62
63 int8_t uint8_
64 int16_t uint16_t
65 int32_t uint32_
66 int64_t uint64_t
67 int_fast8_t uint_fast8_t
68 int_fast16_t uint_fast16_t
69 int_fast32_t uint_fast32_t
70 int_fast64_t uint_fast64_t
71 intptr_t uintptr_t
72
73 The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
74 platform (currently C<4> or C<8>) and can be used in preprocessor
75 expressions.
76
77 For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
78
79 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
80
81 All the following symbols expand to an expression that can be tested in
82 preprocessor instructions as well as treated as a boolean (use C<!!> to
83 ensure it's either C<0> or C<1> if you need that).
84
85 =over
86
87 =item ECB_C
88
89 True if the implementation defines the C<__STDC__> macro to a true value,
90 while not claiming to be C++, i..e C, but not C++.
91
92 =item ECB_C99
93
94 True if the implementation claims to be compliant to C99 (ISO/IEC
95 9899:1999) or any later version, while not claiming to be C++.
96
97 Note that later versions (ECB_C11) remove core features again (for
98 example, variable length arrays).
99
100 =item ECB_C11, ECB_C17
101
102 True if the implementation claims to be compliant to C11/C17 (ISO/IEC
103 9899:2011, :20187) or any later version, while not claiming to be C++.
104
105 =item ECB_CPP
106
107 True if the implementation defines the C<__cplusplus__> macro to a true
108 value, which is typically true for C++ compilers.
109
110 =item ECB_CPP11, ECB_CPP14, ECB_CPP17
111
112 True if the implementation claims to be compliant to C++11/C++14/C++17
113 (ISO/IEC 14882:2011, :2014, :2017) or any later version.
114
115 Note that many C++20 features will likely have their own feature test
116 macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>).
117
118 =item ECB_OPTIMIZE_SIZE
119
120 Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
121 can also be defined before including F<ecb.h>, in which case it will be
122 unchanged.
123
124 =item ECB_GCC_VERSION (major, minor)
125
126 Expands to a true value (suitable for testing by the preprocessor) if the
127 compiler used is GNU C and the version is the given version, or higher.
128
129 This macro tries to return false on compilers that claim to be GCC
130 compatible but aren't.
131
132 =item ECB_EXTERN_C
133
134 Expands to C<extern "C"> in C++, and a simple C<extern> in C.
135
136 This can be used to declare a single external C function:
137
138 ECB_EXTERN_C int printf (const char *format, ...);
139
140 =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
141
142 These two macros can be used to wrap multiple C<extern "C"> definitions -
143 they expand to nothing in C.
144
145 They are most useful in header files:
146
147 ECB_EXTERN_C_BEG
148
149 int mycfun1 (int x);
150 int mycfun2 (int x);
151
152 ECB_EXTERN_C_END
153
154 =item ECB_STDFP
155
156 If this evaluates to a true value (suitable for testing by the
157 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
158 and double/binary64 representations internally I<and> the endianness of
159 both types match the endianness of C<uint32_t> and C<uint64_t>.
160
161 This means you can just copy the bits of a C<float> (or C<double>) to an
162 C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
163 without having to think about format or endianness.
164
165 This is true for basically all modern platforms, although F<ecb.h> might
166 not be able to deduce this correctly everywhere and might err on the safe
167 side.
168
169 =item ECB_64BIT_NATIVE
170
171 Evaluates to a true value (suitable for both preprocessor and C code
172 testing) if 64 bit integer types on this architecture are evaluated
173 "natively", that is, with similar speeds as 32 bit integers. While 64 bit
174 integer support is very common (and in fact required by libecb), 32 bit
175 cpus have to emulate operations on them, so you might want to avoid them.
176
177 =item ECB_AMD64, ECB_AMD64_X32
178
179 These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
180 ABI, respectively, and undefined elsewhere.
181
182 The designers of the new X32 ABI for some inexplicable reason decided to
183 make it look exactly like amd64, even though it's completely incompatible
184 to that ABI, breaking about every piece of software that assumed that
185 C<__x86_64> stands for, well, the x86-64 ABI, making these macros
186 necessary.
187
188 =back
189
190 =head2 MACRO TRICKERY
191
192 =over
193
194 =item ECB_CONCAT (a, b)
195
196 Expands any macros in C<a> and C<b>, then concatenates the result to form
197 a single token. This is mainly useful to form identifiers from components,
198 e.g.:
199
200 #define S1 str
201 #define S2 cpy
202
203 ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
204
205 =item ECB_STRINGIFY (arg)
206
207 Expands any macros in C<arg> and returns the stringified version of
208 it. This is mainly useful to get the contents of a macro in string form,
209 e.g.:
210
211 #define SQL_LIMIT 100
212 sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
213
214 =item ECB_STRINGIFY_EXPR (expr)
215
216 Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
217 is a valid expression. This is useful to catch typos or cases where the
218 macro isn't available:
219
220 #include <errno.h>
221
222 ECB_STRINGIFY (EDOM); // "33" (on my system at least)
223 ECB_STRINGIFY_EXPR (EDOM); // "33"
224
225 // now imagine we had a typo:
226
227 ECB_STRINGIFY (EDAM); // "EDAM"
228 ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
229
230 =back
231
232 =head2 ATTRIBUTES
233
234 A major part of libecb deals with additional attributes that can be
235 assigned to functions, variables and sometimes even types - much like
236 C<const> or C<volatile> in C. They are implemented using either GCC
237 attributes or other compiler/language specific features. Attributes
238 declarations must be put before the whole declaration:
239
240 ecb_const int mysqrt (int a);
241 ecb_unused int i;
242
243 =over
244
245 =item ecb_unused
246
247 Marks a function or a variable as "unused", which simply suppresses a
248 warning by the compiler when it detects it as unused. This is useful when
249 you e.g. declare a variable but do not always use it:
250
251 {
252 ecb_unused int var;
253
254 #ifdef SOMECONDITION
255 var = ...;
256 return var;
257 #else
258 return 0;
259 #endif
260 }
261
262 =item ecb_deprecated
263
264 Similar to C<ecb_unused>, but marks a function, variable or type as
265 deprecated. This makes some compilers warn when the type is used.
266
267 =item ecb_deprecated_message (message)
268
269 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
270 used instead of a generic depreciation message when the object is being
271 used.
272
273 =item ecb_inline
274
275 Expands either to (a compiler-specific equivalent of) C<static inline> or
276 to just C<static>, if inline isn't supported. It should be used to declare
277 functions that should be inlined, for code size or speed reasons.
278
279 Example: inline this function, it surely will reduce codesize.
280
281 ecb_inline int
282 negmul (int a, int b)
283 {
284 return - (a * b);
285 }
286
287 =item ecb_noinline
288
289 Prevents a function from being inlined - it might be optimised away, but
290 not inlined into other functions. This is useful if you know your function
291 is rarely called and large enough for inlining not to be helpful.
292
293 =item ecb_noreturn
294
295 Marks a function as "not returning, ever". Some typical functions that
296 don't return are C<exit> or C<abort> (which really works hard to not
297 return), and now you can make your own:
298
299 ecb_noreturn void
300 my_abort (const char *errline)
301 {
302 puts (errline);
303 abort ();
304 }
305
306 In this case, the compiler would probably be smart enough to deduce it on
307 its own, so this is mainly useful for declarations.
308
309 =item ecb_restrict
310
311 Expands to the C<restrict> keyword or equivalent on compilers that support
312 them, and to nothing on others. Must be specified on a pointer type or
313 an array index to indicate that the memory doesn't alias with any other
314 restricted pointer in the same scope.
315
316 Example: multiply a vector, and allow the compiler to parallelise the
317 loop, because it knows it doesn't overwrite input values.
318
319 void
320 multiply (ecb_restrict float *src,
321 ecb_restrict float *dst,
322 int len, float factor)
323 {
324 int i;
325
326 for (i = 0; i < len; ++i)
327 dst [i] = src [i] * factor;
328 }
329
330 =item ecb_const
331
332 Declares that the function only depends on the values of its arguments,
333 much like a mathematical function. It specifically does not read or write
334 any memory any arguments might point to, global variables, or call any
335 non-const functions. It also must not have any side effects.
336
337 Such a function can be optimised much more aggressively by the compiler -
338 for example, multiple calls with the same arguments can be optimised into
339 a single call, which wouldn't be possible if the compiler would have to
340 expect any side effects.
341
342 It is best suited for functions in the sense of mathematical functions,
343 such as a function returning the square root of its input argument.
344
345 Not suited would be a function that calculates the hash of some memory
346 area you pass in, prints some messages or looks at a global variable to
347 decide on rounding.
348
349 See C<ecb_pure> for a slightly less restrictive class of functions.
350
351 =item ecb_pure
352
353 Similar to C<ecb_const>, declares a function that has no side
354 effects. Unlike C<ecb_const>, the function is allowed to examine global
355 variables and any other memory areas (such as the ones passed to it via
356 pointers).
357
358 While these functions cannot be optimised as aggressively as C<ecb_const>
359 functions, they can still be optimised away in many occasions, and the
360 compiler has more freedom in moving calls to them around.
361
362 Typical examples for such functions would be C<strlen> or C<memcmp>. A
363 function that calculates the MD5 sum of some input and updates some MD5
364 state passed as argument would I<NOT> be pure, however, as it would modify
365 some memory area that is not the return value.
366
367 =item ecb_hot
368
369 This declares a function as "hot" with regards to the cache - the function
370 is used so often, that it is very beneficial to keep it in the cache if
371 possible.
372
373 The compiler reacts by trying to place hot functions near to each other in
374 memory.
375
376 Whether a function is hot or not often depends on the whole program,
377 and less on the function itself. C<ecb_cold> is likely more useful in
378 practise.
379
380 =item ecb_cold
381
382 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
383 the cache, or in other words, this function is not called often, or not at
384 speed-critical times, and keeping it in the cache might be a waste of said
385 cache.
386
387 In addition to placing cold functions together (or at least away from hot
388 functions), this knowledge can be used in other ways, for example, the
389 function will be optimised for size, as opposed to speed, and codepaths
390 leading to calls to those functions can automatically be marked as if
391 C<ecb_expect_false> had been used to reach them.
392
393 Good examples for such functions would be error reporting functions, or
394 functions only called in exceptional or rare cases.
395
396 =item ecb_artificial
397
398 Declares the function as "artificial", in this case meaning that this
399 function is not really meant to be a function, but more like an accessor
400 - many methods in C++ classes are mere accessor functions, and having a
401 crash reported in such a method, or single-stepping through them, is not
402 usually so helpful, especially when it's inlined to just a few instructions.
403
404 Marking them as artificial will instruct the debugger about just this,
405 leading to happier debugging and thus happier lives.
406
407 Example: in some kind of smart-pointer class, mark the pointer accessor as
408 artificial, so that the whole class acts more like a pointer and less like
409 some C++ abstraction monster.
410
411 template<typename T>
412 struct my_smart_ptr
413 {
414 T *value;
415
416 ecb_artificial
417 operator T *()
418 {
419 return value;
420 }
421 };
422
423 =back
424
425 =head2 OPTIMISATION HINTS
426
427 =over
428
429 =item bool ecb_is_constant (expr)
430
431 Returns true iff the expression can be deduced to be a compile-time
432 constant, and false otherwise.
433
434 For example, when you have a C<rndm16> function that returns a 16 bit
435 random number, and you have a function that maps this to a range from
436 0..n-1, then you could use this inline function in a header file:
437
438 ecb_inline uint32_t
439 rndm (uint32_t n)
440 {
441 return (n * (uint32_t)rndm16 ()) >> 16;
442 }
443
444 However, for powers of two, you could use a normal mask, but that is only
445 worth it if, at compile time, you can detect this case. This is the case
446 when the passed number is a constant and also a power of two (C<n & (n -
447 1) == 0>):
448
449 ecb_inline uint32_t
450 rndm (uint32_t n)
451 {
452 return is_constant (n) && !(n & (n - 1))
453 ? rndm16 () & (num - 1)
454 : (n * (uint32_t)rndm16 ()) >> 16;
455 }
456
457 =item ecb_expect (expr, value)
458
459 Evaluates C<expr> and returns it. In addition, it tells the compiler that
460 the C<expr> evaluates to C<value> a lot, which can be used for static
461 branch optimisations.
462
463 Usually, you want to use the more intuitive C<ecb_expect_true> and
464 C<ecb_expect_false> functions instead.
465
466 =item bool ecb_expect_true (cond)
467
468 =item bool ecb_expect_false (cond)
469
470 These two functions expect a expression that is true or false and return
471 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
472 other conditional statement, it will not change the program:
473
474 /* these two do the same thing */
475 if (some_condition) ...;
476 if (ecb_expect_true (some_condition)) ...;
477
478 However, by using C<ecb_expect_true>, you tell the compiler that the
479 condition is likely to be true (and for C<ecb_expect_false>, that it is
480 unlikely to be true).
481
482 For example, when you check for a null pointer and expect this to be a
483 rare, exceptional, case, then use C<ecb_expect_false>:
484
485 void my_free (void *ptr)
486 {
487 if (ecb_expect_false (ptr == 0))
488 return;
489 }
490
491 Consequent use of these functions to mark away exceptional cases or to
492 tell the compiler what the hot path through a function is can increase
493 performance considerably.
494
495 You might know these functions under the name C<likely> and C<unlikely>
496 - while these are common aliases, we find that the expect name is easier
497 to understand when quickly skimming code. If you wish, you can use
498 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
499 C<ecb_expect_false> - these are simply aliases.
500
501 A very good example is in a function that reserves more space for some
502 memory block (for example, inside an implementation of a string stream) -
503 each time something is added, you have to check for a buffer overrun, but
504 you expect that most checks will turn out to be false:
505
506 /* make sure we have "size" extra room in our buffer */
507 ecb_inline void
508 reserve (int size)
509 {
510 if (ecb_expect_false (current + size > end))
511 real_reserve_method (size); /* presumably noinline */
512 }
513
514 =item ecb_assume (cond)
515
516 Tries to tell the compiler that some condition is true, even if it's not
517 obvious. This is not a function, but a statement: it cannot be used in
518 another expression.
519
520 This can be used to teach the compiler about invariants or other
521 conditions that might improve code generation, but which are impossible to
522 deduce form the code itself.
523
524 For example, the example reservation function from the C<ecb_expect_false>
525 description could be written thus (only C<ecb_assume> was added):
526
527 ecb_inline void
528 reserve (int size)
529 {
530 if (ecb_expect_false (current + size > end))
531 real_reserve_method (size); /* presumably noinline */
532
533 ecb_assume (current + size <= end);
534 }
535
536 If you then call this function twice, like this:
537
538 reserve (10);
539 reserve (1);
540
541 Then the compiler I<might> be able to optimise out the second call
542 completely, as it knows that C<< current + 1 > end >> is false and the
543 call will never be executed.
544
545 =item ecb_unreachable ()
546
547 This function does nothing itself, except tell the compiler that it will
548 never be executed. Apart from suppressing a warning in some cases, this
549 function can be used to implement C<ecb_assume> or similar functionality.
550
551 =item ecb_prefetch (addr, rw, locality)
552
553 Tells the compiler to try to prefetch memory at the given C<addr>ess
554 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
555 C<0> means that there will only be one access later, C<3> means that
556 the data will likely be accessed very often, and values in between mean
557 something... in between. The memory pointed to by the address does not
558 need to be accessible (it could be a null pointer for example), but C<rw>
559 and C<locality> must be compile-time constants.
560
561 This is a statement, not a function: you cannot use it as part of an
562 expression.
563
564 An obvious way to use this is to prefetch some data far away, in a big
565 array you loop over. This prefetches memory some 128 array elements later,
566 in the hope that it will be ready when the CPU arrives at that location.
567
568 int sum = 0;
569
570 for (i = 0; i < N; ++i)
571 {
572 sum += arr [i]
573 ecb_prefetch (arr + i + 128, 0, 0);
574 }
575
576 It's hard to predict how far to prefetch, and most CPUs that can prefetch
577 are often good enough to predict this kind of behaviour themselves. It
578 gets more interesting with linked lists, especially when you do some fair
579 processing on each list element:
580
581 for (node *n = start; n; n = n->next)
582 {
583 ecb_prefetch (n->next, 0, 0);
584 ... do medium amount of work with *n
585 }
586
587 After processing the node, (part of) the next node might already be in
588 cache.
589
590 =back
591
592 =head2 BIT FIDDLING / BIT WIZARDRY
593
594 =over
595
596 =item bool ecb_big_endian ()
597
598 =item bool ecb_little_endian ()
599
600 These two functions return true if the byte order is big endian
601 (most-significant byte first) or little endian (least-significant byte
602 first) respectively.
603
604 On systems that are neither, their return values are unspecified.
605
606 =item int ecb_ctz32 (uint32_t x)
607
608 =item int ecb_ctz64 (uint64_t x)
609
610 =item int ecb_ctz (T x) [C++]
611
612 Returns the index of the least significant bit set in C<x> (or
613 equivalently the number of bits set to 0 before the least significant bit
614 set), starting from 0. If C<x> is 0 the result is undefined.
615
616 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
617
618 The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
619 C<uint32_t> and C<uint64_t> types.
620
621 For example:
622
623 ecb_ctz32 (3) = 0
624 ecb_ctz32 (6) = 1
625
626 =item bool ecb_is_pot32 (uint32_t x)
627
628 =item bool ecb_is_pot64 (uint32_t x)
629
630 =item bool ecb_is_pot (T x) [C++]
631
632 Returns true iff C<x> is a power of two or C<x == 0>.
633
634 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
635
636 The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
637 C<uint32_t> and C<uint64_t> types.
638
639 =item int ecb_ld32 (uint32_t x)
640
641 =item int ecb_ld64 (uint64_t x)
642
643 =item int ecb_ld64 (T x) [C++]
644
645 Returns the index of the most significant bit set in C<x>, or the number
646 of digits the number requires in binary (so that C<< 2**ld <= x <
647 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
648 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
649 example to see how many bits a certain number requires to be encoded.
650
651 This function is similar to the "count leading zero bits" function, except
652 that that one returns how many zero bits are "in front" of the number (in
653 the given data type), while C<ecb_ld> returns how many bits the number
654 itself requires.
655
656 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
657
658 The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
659 C<uint32_t> and C<uint64_t> types.
660
661 =item int ecb_popcount32 (uint32_t x)
662
663 =item int ecb_popcount64 (uint64_t x)
664
665 =item int ecb_popcount (T x) [C++]
666
667 Returns the number of bits set to 1 in C<x>.
668
669 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
670
671 The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
672 C<uint32_t> and C<uint64_t> types.
673
674 For example:
675
676 ecb_popcount32 (7) = 3
677 ecb_popcount32 (255) = 8
678
679 =item uint8_t ecb_bitrev8 (uint8_t x)
680
681 =item uint16_t ecb_bitrev16 (uint16_t x)
682
683 =item uint32_t ecb_bitrev32 (uint32_t x)
684
685 =item T ecb_bitrev (T x) [C++]
686
687 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
688 and so on.
689
690 The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
691
692 Example:
693
694 ecb_bitrev8 (0xa7) = 0xea
695 ecb_bitrev32 (0xffcc4411) = 0x882233ff
696
697 =item T ecb_bitrev (T x) [C++]
698
699 Overloaded C++ bitrev function.
700
701 C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
702
703 =item uint32_t ecb_bswap16 (uint32_t x)
704
705 =item uint32_t ecb_bswap32 (uint32_t x)
706
707 =item uint64_t ecb_bswap64 (uint64_t x)
708
709 =item T ecb_bswap (T x)
710
711 These functions return the value of the 16-bit (32-bit, 64-bit) value
712 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
713 C<ecb_bswap32>).
714
715 The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
716 C<uint32_t> and C<uint64_t> types.
717
718 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
719
720 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
721
722 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
723
724 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
725
726 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
727
728 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
729
730 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
731
732 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
733
734 These two families of functions return the value of C<x> after rotating
735 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
736 (C<ecb_rotl>). There are no restrictions on the value C<count>, i.e. both
737 zero and values equal or larger than the word width work correctly. Also,
738 notwithstanding C<count> being unsigned, negative numbers work and shift
739 to the opposite direction.
740
741 Current GCC/clang versions understand these functions and usually compile
742 them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
743 on x86).
744
745 =item T ecb_rotl (T x, unsigned int count) [C++]
746
747 =item T ecb_rotr (T x, unsigned int count) [C++]
748
749 Overloaded C++ rotl/rotr functions.
750
751 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
752
753 =back
754
755 =head2 BIT MIXING, HASHING
756
757 Sometimes you have an integer and want to distribute its bits well, for
758 example, to use it as a hash in a hashtable. A common example is pointer
759 values, which often only have a limited range (e.g. low and high bits are
760 often zero).
761
762 The following functions try to mix the bits to get a good bias-free
763 distribution. They were mainly made for pointers, but the underlying
764 integer functions are exposed as well.
765
766 As an added benefit, the functions are reversible, so if you find it
767 convenient to store only the hash value, you can recover the original
768 pointer from the hash ("unmix"), as long as your pinters are 32 or 64 bit
769 (if this isn't the case on your platform, drop us a note and we will add
770 functions for other bit widths).
771
772 The unmix functions are very slightly slower than the mix functions, so
773 it is equally very slightly preferable to store the original values wehen
774 convenient.
775
776 The underlying algorithm if subject to change, so currently these
777 functions are not suitable for persistent hash tables, as their result
778 value can change between diferent versions of libecb.
779
780 =over
781
782 =item uintptr_t ecb_ptrmix (void *ptr)
783
784 Mixes the bits of a pointer so the result is suitable for hash table
785 lookups. In other words, this hashes the pointer value.
786
787 =item uintptr_t ecb_ptrmix (T *ptr) [C++]
788
789 Overload the C<ecb_ptrmix> function to work for any pointer in C++.
790
791 =item void *ecb_ptrunmix (uintptr_t v)
792
793 Unmix the hash value into the original pointer. This only works as long
794 as the hash value is not truncated, i.e. you used C<uintptr_t> (or
795 equivalent) throughout to store it.
796
797 =item T *ecb_ptrunmix<T> (uintptr_t v) [C++]
798
799 The somewhat less useful template version of C<ecb_ptrunmix> for
800 C++. Example:
801
802 sometype *myptr;
803 uintptr_t hash = ecb_ptrmix (myptr);
804 sometype *orig = ecb_ptrunmix<sometype> (hash);
805
806 =item uint32_t ecb_mix32 (uint32_t v)
807
808 =item uint64_t ecb_mix64 (uint64_t v)
809
810 Sometimes you don't have a pointer but an integer whose values are very
811 badly distributed. In this case you cna sue these integer versions of the
812 mixing function. No C++ template is provided currently.
813
814 =item uint32_t ecb_unmix32 (uint32_t v)
815
816 =item uint64_t ecb_unmix64 (uint64_t v)
817
818 The reverse of the C<ecb_mix> functions - they take a mixed/hashed value
819 and recover the original value.
820
821 =back
822
823 =head2 HOST ENDIANNESS CONVERSION
824
825 =over
826
827 =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
828
829 =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
830
831 =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
832
833 =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
834
835 =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
836
837 =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
838
839 Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
840
841 The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
842 where C<be> and C<le> stand for big endian and little endian, respectively.
843
844 =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
845
846 =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
847
848 =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
849
850 =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
851
852 =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
853
854 =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
855
856 Like above, but converts I<from> host byte order to the specified
857 endianness.
858
859 =back
860
861 In C++ the following additional template functions are supported:
862
863 =over
864
865 =item T ecb_be_to_host (T v)
866
867 =item T ecb_le_to_host (T v)
868
869 =item T ecb_host_to_be (T v)
870
871 =item T ecb_host_to_le (T v)
872
873 =back
874
875 These functions work like their C counterparts, above, but use templates,
876 which make them useful in generic code.
877
878 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
879 (so unlike their C counterparts, there is a version for C<uint8_t>, which
880 again can be useful in generic code).
881
882 =head2 UNALIGNED LOAD/STORE
883
884 These function load or store unaligned multi-byte values.
885
886 =over
887
888 =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
889
890 =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
891
892 =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
893
894 These functions load an unaligned, unsigned 16, 32 or 64 bit value from
895 memory.
896
897 =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
898
899 =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
900
901 =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
902
903 =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
904
905 =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
906
907 =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
908
909 Like above, but additionally convert from big endian (C<be>) or little
910 endian (C<le>) byte order to host byte order while doing so.
911
912 =item ecb_poke_u16_u (void *ptr, uint16_t v)
913
914 =item ecb_poke_u32_u (void *ptr, uint32_t v)
915
916 =item ecb_poke_u64_u (void *ptr, uint64_t v)
917
918 These functions store an unaligned, unsigned 16, 32 or 64 bit value to
919 memory.
920
921 =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
922
923 =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
924
925 =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
926
927 =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
928
929 =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
930
931 =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
932
933 Like above, but additionally convert from host byte order to big endian
934 (C<be>) or little endian (C<le>) byte order while doing so.
935
936 =back
937
938 In C++ the following additional template functions are supported:
939
940 =over
941
942 =item T ecb_peek<T> (const void *ptr)
943
944 =item T ecb_peek_be<T> (const void *ptr)
945
946 =item T ecb_peek_le<T> (const void *ptr)
947
948 =item T ecb_peek_u<T> (const void *ptr)
949
950 =item T ecb_peek_be_u<T> (const void *ptr)
951
952 =item T ecb_peek_le_u<T> (const void *ptr)
953
954 Similarly to their C counterparts, these functions load an unsigned 8, 16,
955 32 or 64 bit value from memory, with optional conversion from big/little
956 endian.
957
958 Since the type cannot be deduced, it has to be specified explicitly, e.g.
959
960 uint_fast16_t v = ecb_peek<uint16_t> (ptr);
961
962 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
963
964 Unlike their C counterparts, these functions support 8 bit quantities
965 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
966 all of which hopefully makes them more useful in generic code.
967
968 =item ecb_poke (void *ptr, T v)
969
970 =item ecb_poke_be (void *ptr, T v)
971
972 =item ecb_poke_le (void *ptr, T v)
973
974 =item ecb_poke_u (void *ptr, T v)
975
976 =item ecb_poke_be_u (void *ptr, T v)
977
978 =item ecb_poke_le_u (void *ptr, T v)
979
980 Again, similarly to their C counterparts, these functions store an
981 unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to
982 big/little endian.
983
984 C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
985
986 Unlike their C counterparts, these functions support 8 bit quantities
987 (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
988 all of which hopefully makes them more useful in generic code.
989
990 =back
991
992 =head2 FAST INTEGER TO STRING
993
994 Libecb defines a set of very fast integer to decimal string (or integer
995 to ascii, short C<i2a>) functions. These work by converting the integer
996 to a fixed point representation and then successively multiplying out
997 the topmost digits. Unlike some other, also very fast, libraries, ecb's
998 algorithm should be completely branchless per digit, and does not rely on
999 the presence of special cpu functions (such as clz).
1000
1001 There is a high level API that takes an C<int32_t>, C<uint32_t>,
1002 C<int64_t> or C<uint64_t> as argument, and a low-level API, which is
1003 harder to use but supports slightly more formatting options.
1004
1005 =head3 HIGH LEVEL API
1006
1007 The high level API consists of four functions, one each for C<int32_t>,
1008 C<uint32_t>, C<int64_t> and C<uint64_t>:
1009
1010 Example:
1011
1012 char buf[ECB_I2A_MAX_DIGITS + 1];
1013 char *end = ecb_i2a_i32 (buf, 17262);
1014 *end = 0;
1015 // buf now contains "17262"
1016
1017 =over
1018
1019 =item ECB_I2A_I32_DIGITS (=11)
1020
1021 =item char *ecb_i2a_u32 (char *ptr, uint32_t value)
1022
1023 Takes an C<uint32_t> I<value> and formats it as a decimal number starting
1024 at I<ptr>, using at most C<ECB_I2A_I32_DIGITS> characters. Returns a
1025 pointer to just after the generated string, where you would normally put
1026 the terminating C<0> character. This function outputs the minimum number
1027 of digits.
1028
1029 =item ECB_I2A_U32_DIGITS (=10)
1030
1031 =item char *ecb_i2a_i32 (char *ptr, int32_t value)
1032
1033 Same as C<ecb_i2a_u32>, but formats a C<int32_t> value, including a minus
1034 sign if needed.
1035
1036 =item ECB_I2A_I64_DIGITS (=20)
1037
1038 =item char *ecb_i2a_u64 (char *ptr, uint64_t value)
1039
1040 =item ECB_I2A_U64_DIGITS (=21)
1041
1042 =item char *ecb_i2a_i64 (char *ptr, int64_t value)
1043
1044 Similar to their 32 bit counterparts, these take a 64 bit argument.
1045
1046 =item ECB_I2A_MAX_DIGITS (=21)
1047
1048 Instead of using a type specific length macro, you can just use
1049 C<ECB_I2A_MAX_DIGITS>, which is good enough for any C<ecb_i2a> function.
1050
1051 =back
1052
1053 =head3 LOW-LEVEL API
1054
1055 The functions above use a number of low-level APIs which have some strict
1056 limitations, but can be used as building blocks (studying C<ecb_i2a_i32>
1057 and related functions is recommended).
1058
1059 There are three families of functions: functions that convert a number
1060 to a fixed number of digits with leading zeroes (C<ecb_i2a_0N>, C<0>
1061 for "leading zeroes"), functions that generate up to N digits, skipping
1062 leading zeroes (C<_N>), and functions that can generate more digits, but
1063 the leading digit has limited range (C<_xN>).
1064
1065 None of the functions deal with negative numbers.
1066
1067 Example: convert an IP address in an u32 into dotted-quad:
1068
1069 uint32_t ip = 0x0a000164; // 10.0.1.100
1070 char ips[3 * 4 + 3 + 1];
1071 char *ptr = ips;
1072 ptr = ecb_i2a_3 (ptr, ip >> 24 ); *ptr++ = '.';
1073 ptr = ecb_i2a_3 (ptr, (ip >> 16) & 0xff); *ptr++ = '.';
1074 ptr = ecb_i2a_3 (ptr, (ip >> 8) & 0xff); *ptr++ = '.';
1075 ptr = ecb_i2a_3 (ptr, ip & 0xff); *ptr++ = 0;
1076 printf ("ip: %s\n", ips); // prints "ip: 10.0.1.100"
1077
1078 =over
1079
1080 =item char *ecb_i2a_02 (char *ptr, uint32_t value) // 32 bit
1081
1082 =item char *ecb_i2a_03 (char *ptr, uint32_t value) // 32 bit
1083
1084 =item char *ecb_i2a_04 (char *ptr, uint32_t value) // 32 bit
1085
1086 =item char *ecb_i2a_05 (char *ptr, uint32_t value) // 64 bit
1087
1088 =item char *ecb_i2a_06 (char *ptr, uint32_t value) // 64 bit
1089
1090 =item char *ecb_i2a_07 (char *ptr, uint32_t value) // 64 bit
1091
1092 =item char *ecb_i2a_08 (char *ptr, uint32_t value) // 64 bit
1093
1094 =item char *ecb_i2a_09 (char *ptr, uint32_t value) // 64 bit
1095
1096 The C<< ecb_i2a_0I<N> > functions take an unsigned I<value> and convert
1097 them to exactly I<N> digits, returning a pointer to the first character
1098 after the digits. The I<value> must be in range. The functions marked with
1099 I<32 bit> do their calculations internally in 32 bit, the ones marked with
1100 I<64 bit> internally use 64 bit integers, which might be slow on 32 bit
1101 architectures (the high level API decides on 32 vs. 64 bit versions using
1102 C<ECB_64BIT_NATIVE>).
1103
1104 =item char *ecb_i2a_2 (char *ptr, uint32_t value) // 32 bit
1105
1106 =item char *ecb_i2a_3 (char *ptr, uint32_t value) // 32 bit
1107
1108 =item char *ecb_i2a_4 (char *ptr, uint32_t value) // 32 bit
1109
1110 =item char *ecb_i2a_5 (char *ptr, uint32_t value) // 64 bit
1111
1112 =item char *ecb_i2a_6 (char *ptr, uint32_t value) // 64 bit
1113
1114 =item char *ecb_i2a_7 (char *ptr, uint32_t value) // 64 bit
1115
1116 =item char *ecb_i2a_8 (char *ptr, uint32_t value) // 64 bit
1117
1118 =item char *ecb_i2a_9 (char *ptr, uint32_t value) // 64 bit
1119
1120 Similarly, the C<< ecb_i2a_I<N> > functions take an unsigned I<value>
1121 and convert them to at most I<N> digits, suppressing leading zeroes, and
1122 returning a pointer to the first character after the digits.
1123
1124 =item ECB_I2A_MAX_X5 (=59074)
1125
1126 =item char *ecb_i2a_x5 (char *ptr, uint32_t value) // 32 bit
1127
1128 =item ECB_I2A_MAX_X10 (=2932500665)
1129
1130 =item char *ecb_i2a_x10 (char *ptr, uint32_t value) // 64 bit
1131
1132 The C<< ecb_i2a_xI<N> >> functions are similar to the C<< ecb_i2a_I<N> >
1133 functions, but they can generate one digit more, as long as the number
1134 is within range, which is given by the symbols C<ECB_I2A_MAX_X5> (almost
1135 16 bit range) and C<ECB_I2A_MAX_X10> (a bit more than 31 bit range),
1136 respectively.
1137
1138 For example, the digit part of a 32 bit signed integer just fits into the
1139 C<ECB_I2A_MAX_X10> range, so while C<ecb_i2a_x10> cannot convert a 10
1140 digit number, it can convert all 32 bit signed numbers. Sadly, it's not
1141 good enough for 32 bit unsigned numbers.
1142
1143 =back
1144
1145 =head2 FLOATING POINT FIDDLING
1146
1147 =over
1148
1149 =item ECB_INFINITY [-UECB_NO_LIBM]
1150
1151 Evaluates to positive infinity if supported by the platform, otherwise to
1152 a truly huge number.
1153
1154 =item ECB_NAN [-UECB_NO_LIBM]
1155
1156 Evaluates to a quiet NAN if supported by the platform, otherwise to
1157 C<ECB_INFINITY>.
1158
1159 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
1160
1161 Same as C<ldexpf>, but always available.
1162
1163 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
1164
1165 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
1166
1167 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
1168
1169 These functions each take an argument in the native C<float> or C<double>
1170 type and return the IEEE 754 bit representation of it (binary16/half,
1171 binary32/single or binary64/double precision).
1172
1173 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
1174 will be the most significant bit, followed by exponent and mantissa.
1175
1176 This function should work even when the native floating point format isn't
1177 IEEE compliant, of course at a speed and code size penalty, and of course
1178 also within reasonable limits (it tries to convert NaNs, infinities and
1179 denormals, but will likely convert negative zero to positive zero).
1180
1181 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1182 be able to completely optimise away the 32 and 64 bit functions.
1183
1184 These functions can be helpful when serialising floats to the network - you
1185 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
1186
1187 Another use for these functions is to manipulate floating point values
1188 directly.
1189
1190 Silly example: toggle the sign bit of a float.
1191
1192 /* On gcc-4.7 on amd64, */
1193 /* this results in a single add instruction to toggle the bit, and 4 extra */
1194 /* instructions to move the float value to an integer register and back. */
1195
1196 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
1197
1198 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
1199
1200 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
1201
1202 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
1203
1204 The reverse operation of the previous function - takes the bit
1205 representation of an IEEE binary16, binary32 or binary64 number (half,
1206 single or double precision) and converts it to the native C<float> or
1207 C<double> format.
1208
1209 This function should work even when the native floating point format isn't
1210 IEEE compliant, of course at a speed and code size penalty, and of course
1211 also within reasonable limits (it tries to convert normals and denormals,
1212 and might be lucky for infinities, and with extraordinary luck, also for
1213 negative zero).
1214
1215 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1216 be able to optimise away this function completely.
1217
1218 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
1219
1220 =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
1221
1222 Convert a IEEE binary32/single precision to binary16/half format, and vice
1223 versa, handling all details (round-to-nearest-even, subnormals, infinity
1224 and NaNs) correctly.
1225
1226 These are functions are available under C<-DECB_NO_LIBM>, since
1227 they do not rely on the platform floating point format. The
1228 C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
1229 usually what you want.
1230
1231 =back
1232
1233 =head2 ARITHMETIC
1234
1235 =over
1236
1237 =item x = ecb_mod (m, n)
1238
1239 Returns C<m> modulo C<n>, which is the same as the positive remainder
1240 of the division operation between C<m> and C<n>, using floored
1241 division. Unlike the C remainder operator C<%>, this function ensures that
1242 the return value is always positive and that the two numbers I<m> and
1243 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1244 C<ecb_mod> implements the mathematical modulo operation, which is missing
1245 in the language.
1246
1247 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1248 negatable, that is, both C<m> and C<-m> must be representable in its
1249 type (this typically excludes the minimum signed integer value, the same
1250 limitation as for C</> and C<%> in C).
1251
1252 Current GCC/clang versions compile this into an efficient branchless
1253 sequence on almost all CPUs.
1254
1255 For example, when you want to rotate forward through the members of an
1256 array for increasing C<m> (which might be negative), then you should use
1257 C<ecb_mod>, as the C<%> operator might give either negative results, or
1258 change direction for negative values:
1259
1260 for (m = -100; m <= 100; ++m)
1261 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1262
1263 =item x = ecb_div_rd (val, div)
1264
1265 =item x = ecb_div_ru (val, div)
1266
1267 Returns C<val> divided by C<div> rounded down or up, respectively.
1268 C<val> and C<div> must have integer types and C<div> must be strictly
1269 positive. Note that these functions are implemented with macros in C
1270 and with function templates in C++.
1271
1272 =back
1273
1274 =head2 UTILITY
1275
1276 =over
1277
1278 =item element_count = ecb_array_length (name)
1279
1280 Returns the number of elements in the array C<name>. For example:
1281
1282 int primes[] = { 2, 3, 5, 7, 11 };
1283 int sum = 0;
1284
1285 for (i = 0; i < ecb_array_length (primes); i++)
1286 sum += primes [i];
1287
1288 =back
1289
1290 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1291
1292 These symbols need to be defined before including F<ecb.h> the first time.
1293
1294 =over
1295
1296 =item ECB_NO_THREADS
1297
1298 If F<ecb.h> is never used from multiple threads, then this symbol can
1299 be defined, in which case memory fences (and similar constructs) are
1300 completely removed, leading to more efficient code and fewer dependencies.
1301
1302 Setting this symbol to a true value implies C<ECB_NO_SMP>.
1303
1304 =item ECB_NO_SMP
1305
1306 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1307 multiple threads, but never concurrently (e.g. if the system the program
1308 runs on has only a single CPU with a single core, no hyperthreading and so
1309 on), then this symbol can be defined, leading to more efficient code and
1310 fewer dependencies.
1311
1312 =item ECB_NO_LIBM
1313
1314 When defined to C<1>, do not export any functions that might introduce
1315 dependencies on the math library (usually called F<-lm>) - these are
1316 marked with [-UECB_NO_LIBM].
1317
1318 =back
1319
1320 =head1 UNDOCUMENTED FUNCTIONALITY
1321
1322 F<ecb.h> is full of undocumented functionality as well, some of which is
1323 intended to be internal-use only, some of which we forgot to document, and
1324 some of which we hide because we are not sure we will keep the interface
1325 stable.
1326
1327 While you are welcome to rummage around and use whatever you find useful
1328 (we can't stop you), keep in mind that we will change undocumented
1329 functionality in incompatible ways without thinking twice, while we are
1330 considerably more conservative with documented things.
1331
1332 =head1 AUTHORS
1333
1334 C<libecb> is designed and maintained by:
1335
1336 Emanuele Giaquinta <e.giaquinta@glauco.it>
1337 Marc Alexander Lehmann <schmorp@schmorp.de>
1338
1339