ViewVC Help
View File | Revision Log | Show Annotations | Download File
/cvs/libecb/ecb.pod
Revision: 1.61
Committed: Thu Feb 12 12:37:33 2015 UTC (9 years, 4 months ago) by sf-exg
Branch: MAIN
Changes since 1.60: +3 -3 lines
Log Message:
Doc fix.

File Contents

# Content
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 GCC built-ins, together
16 with replacement functions for other compilers. In addition to this,
17 it provides a number of other lowlevel C utilities, such as endianness
18 detection, byte swapping or bit rotations.
19
20 Or in other words, things that should be built into any standard C system,
21 but aren't, implemented as efficient as possible with GCC, and still
22 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_t int16_t uint16_t
64 int32_t uint32_t int64_t uint64_t
65 intptr_t uintptr_t
66
67 The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68 platform (currently C<4> or C<8>) and can be used in preprocessor
69 expressions.
70
71 For C<ptrdiff_t> and C<size_t> use C<stddef.h>.
72
73 =head2 LANGUAGE/COMPILER VERSIONS
74
75 All the following symbols expand to an expression that can be tested in
76 preprocessor instructions as well as treated as a boolean (use C<!!> to
77 ensure it's either C<0> or C<1> if you need that).
78
79 =over 4
80
81 =item ECB_C
82
83 True if the implementation defines the C<__STDC__> macro to a true value,
84 while not claiming to be C++.
85
86 =item ECB_C99
87
88 True if the implementation claims to be compliant to C99 (ISO/IEC
89 9899:1999) or any later version, while not claiming to be C++.
90
91 Note that later versions (ECB_C11) remove core features again (for
92 example, variable length arrays).
93
94 =item ECB_C11
95
96 True if the implementation claims to be compliant to C11 (ISO/IEC
97 9899:2011) or any later version, while not claiming to be C++.
98
99 =item ECB_CPP
100
101 True if the implementation defines the C<__cplusplus__> macro to a true
102 value, which is typically true for C++ compilers.
103
104 =item ECB_CPP11
105
106 True if the implementation claims to be compliant to ISO/IEC 14882:2011
107 (C++11) or any later version.
108
109 =item ECB_GCC_VERSION (major, minor)
110
111 Expands to a true value (suitable for testing in by the preprocessor)
112 if the compiler used is GNU C and the version is the given version, or
113 higher.
114
115 This macro tries to return false on compilers that claim to be GCC
116 compatible but aren't.
117
118 =item ECB_EXTERN_C
119
120 Expands to C<extern "C"> in C++, and a simple C<extern> in C.
121
122 This can be used to declare a single external C function:
123
124 ECB_EXTERN_C int printf (const char *format, ...);
125
126 =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
127
128 These two macros can be used to wrap multiple C<extern "C"> definitions -
129 they expand to nothing in C.
130
131 They are most useful in header files:
132
133 ECB_EXTERN_C_BEG
134
135 int mycfun1 (int x);
136 int mycfun2 (int x);
137
138 ECB_EXTERN_C_END
139
140 =item ECB_STDFP
141
142 If this evaluates to a true value (suitable for testing in by the
143 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
144 and double/binary64 representations internally I<and> the endianness of
145 both types match the endianness of C<uint32_t> and C<uint64_t>.
146
147 This means you can just copy the bits of a C<float> (or C<double>) to an
148 C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
149 without having to think about format or endianness.
150
151 This is true for basically all modern platforms, although F<ecb.h> might
152 not be able to deduce this correctly everywhere and might err on the safe
153 side.
154
155 =item ECB_AMD64, ECB_AMD64_X32
156
157 These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
158 ABI, respectively, and undefined elsewhere.
159
160 The designers of the new X32 ABI for some inexplicable reason decided to
161 make it look exactly like amd64, even though it's completely incompatible
162 to that ABI, breaking about every piece of software that assumed that
163 C<__x86_64> stands for, well, the x86-64 ABI, making these macros
164 necessary.
165
166 =back
167
168 =head2 ATTRIBUTES
169
170 A major part of libecb deals with additional attributes that can be
171 assigned to functions, variables and sometimes even types - much like
172 C<const> or C<volatile> in C. They are implemented using either GCC
173 attributes or other compiler/language specific features. Attributes
174 declarations must be put before the whole declaration:
175
176 ecb_const int mysqrt (int a);
177 ecb_unused int i;
178
179 =over 4
180
181 =item ecb_unused
182
183 Marks a function or a variable as "unused", which simply suppresses a
184 warning by GCC when it detects it as unused. This is useful when you e.g.
185 declare a variable but do not always use it:
186
187 {
188 ecb_unused int var;
189
190 #ifdef SOMECONDITION
191 var = ...;
192 return var;
193 #else
194 return 0;
195 #endif
196 }
197
198 =item ecb_deprecated
199
200 Similar to C<ecb_unused>, but marks a function, variable or type as
201 deprecated. This makes some compilers warn when the type is used.
202
203 =item ecb_inline
204
205 Expands either to C<static inline> or to just C<static>, if inline
206 isn't supported. It should be used to declare functions that should be
207 inlined, for code size or speed reasons.
208
209 Example: inline this function, it surely will reduce codesize.
210
211 ecb_inline int
212 negmul (int a, int b)
213 {
214 return - (a * b);
215 }
216
217 =item ecb_noinline
218
219 Prevent a function from being inlined - it might be optimised away, but
220 not inlined into other functions. This is useful if you know your function
221 is rarely called and large enough for inlining not to be helpful.
222
223 =item ecb_noreturn
224
225 Marks a function as "not returning, ever". Some typical functions that
226 don't return are C<exit> or C<abort> (which really works hard to not
227 return), and now you can make your own:
228
229 ecb_noreturn void
230 my_abort (const char *errline)
231 {
232 puts (errline);
233 abort ();
234 }
235
236 In this case, the compiler would probably be smart enough to deduce it on
237 its own, so this is mainly useful for declarations.
238
239 =item ecb_restrict
240
241 Expands to the C<restrict> keyword or equivalent on compilers that support
242 them, and to nothing on others. Must be specified on a pointer type or
243 an array index to indicate that the memory doesn't alias with any other
244 restricted pointer in the same scope.
245
246 Example: multiply a vector, and allow the compiler to parallelise the
247 loop, because it knows it doesn't overwrite input values.
248
249 void
250 multiply (ecb_restrict float *src,
251 ecb_restrict float *dst,
252 int len, float factor)
253 {
254 int i;
255
256 for (i = 0; i < len; ++i)
257 dst [i] = src [i] * factor;
258 }
259
260 =item ecb_const
261
262 Declares that the function only depends on the values of its arguments,
263 much like a mathematical function. It specifically does not read or write
264 any memory any arguments might point to, global variables, or call any
265 non-const functions. It also must not have any side effects.
266
267 Such a function can be optimised much more aggressively by the compiler -
268 for example, multiple calls with the same arguments can be optimised into
269 a single call, which wouldn't be possible if the compiler would have to
270 expect any side effects.
271
272 It is best suited for functions in the sense of mathematical functions,
273 such as a function returning the square root of its input argument.
274
275 Not suited would be a function that calculates the hash of some memory
276 area you pass in, prints some messages or looks at a global variable to
277 decide on rounding.
278
279 See C<ecb_pure> for a slightly less restrictive class of functions.
280
281 =item ecb_pure
282
283 Similar to C<ecb_const>, declares a function that has no side
284 effects. Unlike C<ecb_const>, the function is allowed to examine global
285 variables and any other memory areas (such as the ones passed to it via
286 pointers).
287
288 While these functions cannot be optimised as aggressively as C<ecb_const>
289 functions, they can still be optimised away in many occasions, and the
290 compiler has more freedom in moving calls to them around.
291
292 Typical examples for such functions would be C<strlen> or C<memcmp>. A
293 function that calculates the MD5 sum of some input and updates some MD5
294 state passed as argument would I<NOT> be pure, however, as it would modify
295 some memory area that is not the return value.
296
297 =item ecb_hot
298
299 This declares a function as "hot" with regards to the cache - the function
300 is used so often, that it is very beneficial to keep it in the cache if
301 possible.
302
303 The compiler reacts by trying to place hot functions near to each other in
304 memory.
305
306 Whether a function is hot or not often depends on the whole program,
307 and less on the function itself. C<ecb_cold> is likely more useful in
308 practise.
309
310 =item ecb_cold
311
312 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
313 the cache, or in other words, this function is not called often, or not at
314 speed-critical times, and keeping it in the cache might be a waste of said
315 cache.
316
317 In addition to placing cold functions together (or at least away from hot
318 functions), this knowledge can be used in other ways, for example, the
319 function will be optimised for size, as opposed to speed, and codepaths
320 leading to calls to those functions can automatically be marked as if
321 C<ecb_expect_false> had been used to reach them.
322
323 Good examples for such functions would be error reporting functions, or
324 functions only called in exceptional or rare cases.
325
326 =item ecb_artificial
327
328 Declares the function as "artificial", in this case meaning that this
329 function is not really meant to be a function, but more like an accessor
330 - many methods in C++ classes are mere accessor functions, and having a
331 crash reported in such a method, or single-stepping through them, is not
332 usually so helpful, especially when it's inlined to just a few instructions.
333
334 Marking them as artificial will instruct the debugger about just this,
335 leading to happier debugging and thus happier lives.
336
337 Example: in some kind of smart-pointer class, mark the pointer accessor as
338 artificial, so that the whole class acts more like a pointer and less like
339 some C++ abstraction monster.
340
341 template<typename T>
342 struct my_smart_ptr
343 {
344 T *value;
345
346 ecb_artificial
347 operator T *()
348 {
349 return value;
350 }
351 };
352
353 =back
354
355 =head2 OPTIMISATION HINTS
356
357 =over 4
358
359 =item bool ecb_is_constant (expr)
360
361 Returns true iff the expression can be deduced to be a compile-time
362 constant, and false otherwise.
363
364 For example, when you have a C<rndm16> function that returns a 16 bit
365 random number, and you have a function that maps this to a range from
366 0..n-1, then you could use this inline function in a header file:
367
368 ecb_inline uint32_t
369 rndm (uint32_t n)
370 {
371 return (n * (uint32_t)rndm16 ()) >> 16;
372 }
373
374 However, for powers of two, you could use a normal mask, but that is only
375 worth it if, at compile time, you can detect this case. This is the case
376 when the passed number is a constant and also a power of two (C<n & (n -
377 1) == 0>):
378
379 ecb_inline uint32_t
380 rndm (uint32_t n)
381 {
382 return is_constant (n) && !(n & (n - 1))
383 ? rndm16 () & (num - 1)
384 : (n * (uint32_t)rndm16 ()) >> 16;
385 }
386
387 =item bool ecb_expect (expr, value)
388
389 Evaluates C<expr> and returns it. In addition, it tells the compiler that
390 the C<expr> evaluates to C<value> a lot, which can be used for static
391 branch optimisations.
392
393 Usually, you want to use the more intuitive C<ecb_expect_true> and
394 C<ecb_expect_false> functions instead.
395
396 =item bool ecb_expect_true (cond)
397
398 =item bool ecb_expect_false (cond)
399
400 These two functions expect a expression that is true or false and return
401 C<1> or C<0>, respectively, so when used in the condition of an C<if> or
402 other conditional statement, it will not change the program:
403
404 /* these two do the same thing */
405 if (some_condition) ...;
406 if (ecb_expect_true (some_condition)) ...;
407
408 However, by using C<ecb_expect_true>, you tell the compiler that the
409 condition is likely to be true (and for C<ecb_expect_false>, that it is
410 unlikely to be true).
411
412 For example, when you check for a null pointer and expect this to be a
413 rare, exceptional, case, then use C<ecb_expect_false>:
414
415 void my_free (void *ptr)
416 {
417 if (ecb_expect_false (ptr == 0))
418 return;
419 }
420
421 Consequent use of these functions to mark away exceptional cases or to
422 tell the compiler what the hot path through a function is can increase
423 performance considerably.
424
425 You might know these functions under the name C<likely> and C<unlikely>
426 - while these are common aliases, we find that the expect name is easier
427 to understand when quickly skimming code. If you wish, you can use
428 C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
429 C<ecb_expect_false> - these are simply aliases.
430
431 A very good example is in a function that reserves more space for some
432 memory block (for example, inside an implementation of a string stream) -
433 each time something is added, you have to check for a buffer overrun, but
434 you expect that most checks will turn out to be false:
435
436 /* make sure we have "size" extra room in our buffer */
437 ecb_inline void
438 reserve (int size)
439 {
440 if (ecb_expect_false (current + size > end))
441 real_reserve_method (size); /* presumably noinline */
442 }
443
444 =item bool ecb_assume (cond)
445
446 Try to tell the compiler that some condition is true, even if it's not
447 obvious.
448
449 This can be used to teach the compiler about invariants or other
450 conditions that might improve code generation, but which are impossible to
451 deduce form the code itself.
452
453 For example, the example reservation function from the C<ecb_expect_false>
454 description could be written thus (only C<ecb_assume> was added):
455
456 ecb_inline void
457 reserve (int size)
458 {
459 if (ecb_expect_false (current + size > end))
460 real_reserve_method (size); /* presumably noinline */
461
462 ecb_assume (current + size <= end);
463 }
464
465 If you then call this function twice, like this:
466
467 reserve (10);
468 reserve (1);
469
470 Then the compiler I<might> be able to optimise out the second call
471 completely, as it knows that C<< current + 1 > end >> is false and the
472 call will never be executed.
473
474 =item bool ecb_unreachable ()
475
476 This function does nothing itself, except tell the compiler that it will
477 never be executed. Apart from suppressing a warning in some cases, this
478 function can be used to implement C<ecb_assume> or similar functions.
479
480 =item bool ecb_prefetch (addr, rw, locality)
481
482 Tells the compiler to try to prefetch memory at the given C<addr>ess
483 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
484 C<0> means that there will only be one access later, C<3> means that
485 the data will likely be accessed very often, and values in between mean
486 something... in between. The memory pointed to by the address does not
487 need to be accessible (it could be a null pointer for example), but C<rw>
488 and C<locality> must be compile-time constants.
489
490 An obvious way to use this is to prefetch some data far away, in a big
491 array you loop over. This prefetches memory some 128 array elements later,
492 in the hope that it will be ready when the CPU arrives at that location.
493
494 int sum = 0;
495
496 for (i = 0; i < N; ++i)
497 {
498 sum += arr [i]
499 ecb_prefetch (arr + i + 128, 0, 0);
500 }
501
502 It's hard to predict how far to prefetch, and most CPUs that can prefetch
503 are often good enough to predict this kind of behaviour themselves. It
504 gets more interesting with linked lists, especially when you do some fair
505 processing on each list element:
506
507 for (node *n = start; n; n = n->next)
508 {
509 ecb_prefetch (n->next, 0, 0);
510 ... do medium amount of work with *n
511 }
512
513 After processing the node, (part of) the next node might already be in
514 cache.
515
516 =back
517
518 =head2 BIT FIDDLING / BIT WIZARDRY
519
520 =over 4
521
522 =item bool ecb_big_endian ()
523
524 =item bool ecb_little_endian ()
525
526 These two functions return true if the byte order is big endian
527 (most-significant byte first) or little endian (least-significant byte
528 first) respectively.
529
530 On systems that are neither, their return values are unspecified.
531
532 =item int ecb_ctz32 (uint32_t x)
533
534 =item int ecb_ctz64 (uint64_t x)
535
536 Returns the index of the least significant bit set in C<x> (or
537 equivalently the number of bits set to 0 before the least significant bit
538 set), starting from 0. If C<x> is 0 the result is undefined.
539
540 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
541
542 For example:
543
544 ecb_ctz32 (3) = 0
545 ecb_ctz32 (6) = 1
546
547 =item bool ecb_is_pot32 (uint32_t x)
548
549 =item bool ecb_is_pot64 (uint32_t x)
550
551 Return true iff C<x> is a power of two or C<x == 0>.
552
553 For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
554
555 =item int ecb_ld32 (uint32_t x)
556
557 =item int ecb_ld64 (uint64_t x)
558
559 Returns the index of the most significant bit set in C<x>, or the number
560 of digits the number requires in binary (so that C<< 2**ld <= x <
561 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
562 to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
563 example to see how many bits a certain number requires to be encoded.
564
565 This function is similar to the "count leading zero bits" function, except
566 that that one returns how many zero bits are "in front" of the number (in
567 the given data type), while C<ecb_ld> returns how many bits the number
568 itself requires.
569
570 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
571
572 =item int ecb_popcount32 (uint32_t x)
573
574 =item int ecb_popcount64 (uint64_t x)
575
576 Returns the number of bits set to 1 in C<x>.
577
578 For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
579
580 For example:
581
582 ecb_popcount32 (7) = 3
583 ecb_popcount32 (255) = 8
584
585 =item uint8_t ecb_bitrev8 (uint8_t x)
586
587 =item uint16_t ecb_bitrev16 (uint16_t x)
588
589 =item uint32_t ecb_bitrev32 (uint32_t x)
590
591 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
592 and so on.
593
594 Example:
595
596 ecb_bitrev8 (0xa7) = 0xea
597 ecb_bitrev32 (0xffcc4411) = 0x882233ff
598
599 =item uint32_t ecb_bswap16 (uint32_t x)
600
601 =item uint32_t ecb_bswap32 (uint32_t x)
602
603 =item uint64_t ecb_bswap64 (uint64_t x)
604
605 These functions return the value of the 16-bit (32-bit, 64-bit) value
606 C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
607 C<ecb_bswap32>).
608
609 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
610
611 =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
612
613 =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
614
615 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
616
617 =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
618
619 =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
620
621 =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
622
623 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
624
625 These two families of functions return the value of C<x> after rotating
626 all the bits by C<count> positions to the right (C<ecb_rotr>) or left
627 (C<ecb_rotl>).
628
629 Current GCC versions understand these functions and usually compile them
630 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
631 x86).
632
633 =back
634
635 =head2 FLOATING POINT FIDDLING
636
637 =over 4
638
639 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
640
641 =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
642
643 These functions each take an argument in the native C<float> or C<double>
644 type and return the IEEE 754 bit representation of it.
645
646 The bit representation is just as IEEE 754 defines it, i.e. the sign bit
647 will be the most significant bit, followed by exponent and mantissa.
648
649 This function should work even when the native floating point format isn't
650 IEEE compliant, of course at a speed and code size penalty, and of course
651 also within reasonable limits (it tries to convert NaNs, infinities and
652 denormals, but will likely convert negative zero to positive zero).
653
654 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
655 be able to optimise away this function completely.
656
657 These functions can be helpful when serialising floats to the network - you
658 can serialise the return value like a normal uint32_t/uint64_t.
659
660 Another use for these functions is to manipulate floating point values
661 directly.
662
663 Silly example: toggle the sign bit of a float.
664
665 /* On gcc-4.7 on amd64, */
666 /* this results in a single add instruction to toggle the bit, and 4 extra */
667 /* instructions to move the float value to an integer register and back. */
668
669 x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
670
671 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
672
673 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
674
675 =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM]
676
677 The reverse operation of the previous function - takes the bit
678 representation of an IEEE binary16, binary32 or binary64 number and
679 converts it to the native C<float> or C<double> format.
680
681 This function should work even when the native floating point format isn't
682 IEEE compliant, of course at a speed and code size penalty, and of course
683 also within reasonable limits (it tries to convert normals and denormals,
684 and might be lucky for infinities, and with extraordinary luck, also for
685 negative zero).
686
687 On all modern platforms (where C<ECB_STDFP> is true), the compiler should
688 be able to optimise away this function completely.
689
690 =back
691
692 =head2 ARITHMETIC
693
694 =over 4
695
696 =item x = ecb_mod (m, n)
697
698 Returns C<m> modulo C<n>, which is the same as the positive remainder
699 of the division operation between C<m> and C<n>, using floored
700 division. Unlike the C remainder operator C<%>, this function ensures that
701 the return value is always positive and that the two numbers I<m> and
702 I<m' = m + i * n> result in the same value modulo I<n> - in other words,
703 C<ecb_mod> implements the mathematical modulo operation, which is missing
704 in the language.
705
706 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
707 negatable, that is, both C<m> and C<-m> must be representable in its
708 type (this typically excludes the minimum signed integer value, the same
709 limitation as for C</> and C<%> in C).
710
711 Current GCC versions compile this into an efficient branchless sequence on
712 almost all CPUs.
713
714 For example, when you want to rotate forward through the members of an
715 array for increasing C<m> (which might be negative), then you should use
716 C<ecb_mod>, as the C<%> operator might give either negative results, or
717 change direction for negative values:
718
719 for (m = -100; m <= 100; ++m)
720 int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
721
722 =item x = ecb_div_rd (val, div)
723
724 =item x = ecb_div_ru (val, div)
725
726 Returns C<val> divided by C<div> rounded down or up, respectively.
727 C<val> and C<div> must have integer types and C<div> must be strictly
728 positive. Note that these functions are implemented with macros in C
729 and with function templates in C++.
730
731 =back
732
733 =head2 UTILITY
734
735 =over 4
736
737 =item element_count = ecb_array_length (name)
738
739 Returns the number of elements in the array C<name>. For example:
740
741 int primes[] = { 2, 3, 5, 7, 11 };
742 int sum = 0;
743
744 for (i = 0; i < ecb_array_length (primes); i++)
745 sum += primes [i];
746
747 =back
748
749 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
750
751 These symbols need to be defined before including F<ecb.h> the first time.
752
753 =over 4
754
755 =item ECB_NO_THREADS
756
757 If F<ecb.h> is never used from multiple threads, then this symbol can
758 be defined, in which case memory fences (and similar constructs) are
759 completely removed, leading to more efficient code and fewer dependencies.
760
761 Setting this symbol to a true value implies C<ECB_NO_SMP>.
762
763 =item ECB_NO_SMP
764
765 The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
766 multiple threads, but never concurrently (e.g. if the system the program
767 runs on has only a single CPU with a single core, no hyperthreading and so
768 on), then this symbol can be defined, leading to more efficient code and
769 fewer dependencies.
770
771 =item ECB_NO_LIBM
772
773 When defined to C<1>, do not export any functions that might introduce
774 dependencies on the math library (usually called F<-lm>) - these are
775 marked with [-UECB_NO_LIBM].
776
777 =back
778
779