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Revision: 1.47
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1 root 1.14 =head1 LIBECB - e-C-Builtins
2 root 1.3
3 root 1.14 =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 sf-exg 1.16 It's part of the e-suite of libraries, other members of which include
9 root 1.14 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 sf-exg 1.16 it provides a number of other lowlevel C utilities, such as endianness
18 root 1.14 detection, byte swapping or bit rotations.
19    
20 root 1.24 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 root 1.17
24 root 1.14 More might come.
25 root 1.3
26     =head2 ABOUT THE HEADER
27    
28 root 1.14 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 sf-exg 1.16 There are currently no object files to link to - future versions might
37 root 1.14 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 root 1.1
59 root 1.40 =head2 TYPES / TYPE SUPPORT
60    
61     ecb.h makes sure that the following types are defined (in the expected way):
62    
63 root 1.42 int8_t uint8_t int16_t uint16_t
64     int32_t uint32_t int64_t uint64_t
65     intptr_t uintptr_t ptrdiff_t
66 root 1.40
67     The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
68 root 1.45 platform (currently C<4> or C<8>) and can be used in preprocessor
69     expressions.
70 root 1.40
71 root 1.43 =head2 LANGUAGE/COMPILER VERSIONS
72    
73 sf-exg 1.46 All the following symbols expand to an expression that can be tested in
74 root 1.44 preprocessor instructions as well as treated as a boolean (use C<!!> to
75     ensure it's either C<0> or C<1> if you need that).
76    
77 root 1.43 =over 4
78    
79 root 1.44 =item ECB_C
80    
81     True if the implementation defines the C<__STDC__> macro to a true value,
82     which is typically true for both C and C++ compilers.
83    
84 root 1.43 =item ECB_C99
85    
86 root 1.47 True if the implementation claims to be compliant to C99 (ISO/IEC
87     9899:1999) or any later version.
88    
89     Note that later versions (ECB_C11) remove core features again (for
90     example, variable length arrays).
91 root 1.43
92     =item ECB_C11
93    
94 root 1.47 True if the implementation claims to be compliant to C11 (ISO/IEC
95     9899:2011) or any later version.
96 root 1.44
97     =item ECB_CPP
98    
99     True if the implementation defines the C<__cplusplus__> macro to a true
100     value, which is typically true for C++ compilers.
101    
102     =item ECB_CPP98
103    
104     True if the implementation claims to be compliant to ISO/IEC 14882:1998
105 sf-exg 1.46 (the first C++ ISO standard) or any later version. Typically true for all
106 root 1.44 C++ compilers.
107    
108     =item ECB_CPP11
109    
110     True if the implementation claims to be compliant to ISO/IEC 14882:2011
111 sf-exg 1.46 (C++11) or any later version.
112 root 1.43
113     =item ECB_GCC_VERSION(major,minor)
114    
115     Expands to a true value (suitable for testing in by the preprocessor)
116 sf-exg 1.46 if the compiler used is GNU C and the version is the given version, or
117 root 1.43 higher.
118    
119     This macro tries to return false on compilers that claim to be GCC
120     compatible but aren't.
121    
122     =back
123    
124 root 1.1 =head2 GCC ATTRIBUTES
125    
126 root 1.20 A major part of libecb deals with GCC attributes. These are additional
127 sf-exg 1.26 attributes that you can assign to functions, variables and sometimes even
128 root 1.20 types - much like C<const> or C<volatile> in C.
129    
130     While GCC allows declarations to show up in many surprising places,
131 sf-exg 1.26 but not in many expected places, the safest way is to put attribute
132 root 1.20 declarations before the whole declaration:
133    
134     ecb_const int mysqrt (int a);
135     ecb_unused int i;
136    
137     For variables, it is often nicer to put the attribute after the name, and
138     avoid multiple declarations using commas:
139    
140     int i ecb_unused;
141 root 1.3
142 root 1.1 =over 4
143    
144 root 1.2 =item ecb_attribute ((attrs...))
145 root 1.1
146 root 1.15 A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to
147     nothing on other compilers, so the effect is that only GCC sees these.
148    
149     Example: use the C<deprecated> attribute on a function.
150    
151     ecb_attribute((__deprecated__)) void
152     do_not_use_me_anymore (void);
153 root 1.2
154 root 1.3 =item ecb_unused
155    
156     Marks a function or a variable as "unused", which simply suppresses a
157     warning by GCC when it detects it as unused. This is useful when you e.g.
158     declare a variable but do not always use it:
159    
160 root 1.15 {
161     int var ecb_unused;
162 root 1.3
163 root 1.15 #ifdef SOMECONDITION
164     var = ...;
165     return var;
166     #else
167     return 0;
168     #endif
169     }
170 root 1.3
171 root 1.31 =item ecb_inline
172 root 1.29
173     This is not actually an attribute, but you use it like one. It expands
174     either to C<static inline> or to just C<static>, if inline isn't
175     supported. It should be used to declare functions that should be inlined,
176     for code size or speed reasons.
177    
178     Example: inline this function, it surely will reduce codesize.
179    
180 root 1.31 ecb_inline int
181 root 1.29 negmul (int a, int b)
182     {
183     return - (a * b);
184     }
185    
186 root 1.2 =item ecb_noinline
187    
188 root 1.9 Prevent a function from being inlined - it might be optimised away, but
189 root 1.3 not inlined into other functions. This is useful if you know your function
190     is rarely called and large enough for inlining not to be helpful.
191    
192 root 1.2 =item ecb_noreturn
193    
194 root 1.17 Marks a function as "not returning, ever". Some typical functions that
195     don't return are C<exit> or C<abort> (which really works hard to not
196     return), and now you can make your own:
197    
198     ecb_noreturn void
199     my_abort (const char *errline)
200     {
201     puts (errline);
202     abort ();
203     }
204    
205 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
206     its own, so this is mainly useful for declarations.
207 root 1.17
208 root 1.2 =item ecb_const
209    
210 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
211 root 1.17 much like a mathematical function. It specifically does not read or write
212     any memory any arguments might point to, global variables, or call any
213     non-const functions. It also must not have any side effects.
214    
215     Such a function can be optimised much more aggressively by the compiler -
216     for example, multiple calls with the same arguments can be optimised into
217     a single call, which wouldn't be possible if the compiler would have to
218     expect any side effects.
219    
220     It is best suited for functions in the sense of mathematical functions,
221 sf-exg 1.19 such as a function returning the square root of its input argument.
222 root 1.17
223     Not suited would be a function that calculates the hash of some memory
224     area you pass in, prints some messages or looks at a global variable to
225     decide on rounding.
226    
227     See C<ecb_pure> for a slightly less restrictive class of functions.
228    
229 root 1.2 =item ecb_pure
230    
231 root 1.17 Similar to C<ecb_const>, declares a function that has no side
232     effects. Unlike C<ecb_const>, the function is allowed to examine global
233     variables and any other memory areas (such as the ones passed to it via
234     pointers).
235    
236     While these functions cannot be optimised as aggressively as C<ecb_const>
237     functions, they can still be optimised away in many occasions, and the
238     compiler has more freedom in moving calls to them around.
239    
240     Typical examples for such functions would be C<strlen> or C<memcmp>. A
241     function that calculates the MD5 sum of some input and updates some MD5
242     state passed as argument would I<NOT> be pure, however, as it would modify
243     some memory area that is not the return value.
244    
245 root 1.2 =item ecb_hot
246    
247 root 1.17 This declares a function as "hot" with regards to the cache - the function
248     is used so often, that it is very beneficial to keep it in the cache if
249     possible.
250    
251     The compiler reacts by trying to place hot functions near to each other in
252     memory.
253    
254 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
255 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
256     practise.
257    
258 root 1.2 =item ecb_cold
259    
260 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
261     the cache, or in other words, this function is not called often, or not at
262     speed-critical times, and keeping it in the cache might be a waste of said
263     cache.
264    
265     In addition to placing cold functions together (or at least away from hot
266     functions), this knowledge can be used in other ways, for example, the
267     function will be optimised for size, as opposed to speed, and codepaths
268     leading to calls to those functions can automatically be marked as if
269 root 1.27 C<ecb_expect_false> had been used to reach them.
270 root 1.17
271     Good examples for such functions would be error reporting functions, or
272     functions only called in exceptional or rare cases.
273    
274 root 1.2 =item ecb_artificial
275    
276 root 1.17 Declares the function as "artificial", in this case meaning that this
277     function is not really mean to be a function, but more like an accessor
278     - many methods in C++ classes are mere accessor functions, and having a
279     crash reported in such a method, or single-stepping through them, is not
280     usually so helpful, especially when it's inlined to just a few instructions.
281    
282     Marking them as artificial will instruct the debugger about just this,
283     leading to happier debugging and thus happier lives.
284    
285     Example: in some kind of smart-pointer class, mark the pointer accessor as
286     artificial, so that the whole class acts more like a pointer and less like
287     some C++ abstraction monster.
288    
289     template<typename T>
290     struct my_smart_ptr
291     {
292     T *value;
293    
294     ecb_artificial
295     operator T *()
296     {
297     return value;
298     }
299     };
300    
301 root 1.2 =back
302 root 1.1
303     =head2 OPTIMISATION HINTS
304    
305     =over 4
306    
307 root 1.14 =item bool ecb_is_constant(expr)
308 root 1.1
309 root 1.3 Returns true iff the expression can be deduced to be a compile-time
310     constant, and false otherwise.
311    
312     For example, when you have a C<rndm16> function that returns a 16 bit
313     random number, and you have a function that maps this to a range from
314 root 1.5 0..n-1, then you could use this inline function in a header file:
315 root 1.3
316     ecb_inline uint32_t
317     rndm (uint32_t n)
318     {
319 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
320 root 1.3 }
321    
322     However, for powers of two, you could use a normal mask, but that is only
323     worth it if, at compile time, you can detect this case. This is the case
324     when the passed number is a constant and also a power of two (C<n & (n -
325     1) == 0>):
326    
327     ecb_inline uint32_t
328     rndm (uint32_t n)
329     {
330     return is_constant (n) && !(n & (n - 1))
331     ? rndm16 () & (num - 1)
332 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
333 root 1.3 }
334    
335 root 1.14 =item bool ecb_expect (expr, value)
336 root 1.1
337 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
338     the C<expr> evaluates to C<value> a lot, which can be used for static
339     branch optimisations.
340 root 1.1
341 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
342     C<ecb_expect_false> functions instead.
343 root 1.1
344 root 1.27 =item bool ecb_expect_true (cond)
345 root 1.1
346 root 1.27 =item bool ecb_expect_false (cond)
347 root 1.1
348 root 1.7 These two functions expect a expression that is true or false and return
349     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
350     other conditional statement, it will not change the program:
351    
352     /* these two do the same thing */
353     if (some_condition) ...;
354 root 1.27 if (ecb_expect_true (some_condition)) ...;
355 root 1.7
356 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
357     condition is likely to be true (and for C<ecb_expect_false>, that it is
358     unlikely to be true).
359 root 1.7
360 root 1.9 For example, when you check for a null pointer and expect this to be a
361 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
362 root 1.7
363     void my_free (void *ptr)
364     {
365 root 1.27 if (ecb_expect_false (ptr == 0))
366 root 1.7 return;
367     }
368    
369     Consequent use of these functions to mark away exceptional cases or to
370     tell the compiler what the hot path through a function is can increase
371     performance considerably.
372    
373 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
374     - while these are common aliases, we find that the expect name is easier
375     to understand when quickly skimming code. If you wish, you can use
376     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
377     C<ecb_expect_false> - these are simply aliases.
378    
379 root 1.7 A very good example is in a function that reserves more space for some
380     memory block (for example, inside an implementation of a string stream) -
381 root 1.9 each time something is added, you have to check for a buffer overrun, but
382 root 1.7 you expect that most checks will turn out to be false:
383    
384     /* make sure we have "size" extra room in our buffer */
385     ecb_inline void
386     reserve (int size)
387     {
388 root 1.27 if (ecb_expect_false (current + size > end))
389 root 1.7 real_reserve_method (size); /* presumably noinline */
390     }
391    
392 root 1.14 =item bool ecb_assume (cond)
393 root 1.7
394     Try to tell the compiler that some condition is true, even if it's not
395     obvious.
396    
397     This can be used to teach the compiler about invariants or other
398     conditions that might improve code generation, but which are impossible to
399     deduce form the code itself.
400    
401 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
402 root 1.7 description could be written thus (only C<ecb_assume> was added):
403    
404     ecb_inline void
405     reserve (int size)
406     {
407 root 1.27 if (ecb_expect_false (current + size > end))
408 root 1.7 real_reserve_method (size); /* presumably noinline */
409    
410     ecb_assume (current + size <= end);
411     }
412    
413     If you then call this function twice, like this:
414    
415     reserve (10);
416     reserve (1);
417    
418     Then the compiler I<might> be able to optimise out the second call
419     completely, as it knows that C<< current + 1 > end >> is false and the
420     call will never be executed.
421    
422     =item bool ecb_unreachable ()
423    
424     This function does nothing itself, except tell the compiler that it will
425 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
426 root 1.7 function can be used to implement C<ecb_assume> or similar functions.
427    
428 root 1.14 =item bool ecb_prefetch (addr, rw, locality)
429 root 1.7
430     Tells the compiler to try to prefetch memory at the given C<addr>ess
431 root 1.10 for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of
432 root 1.7 C<0> means that there will only be one access later, C<3> means that
433     the data will likely be accessed very often, and values in between mean
434     something... in between. The memory pointed to by the address does not
435     need to be accessible (it could be a null pointer for example), but C<rw>
436     and C<locality> must be compile-time constants.
437    
438     An obvious way to use this is to prefetch some data far away, in a big
439 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
440 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
441    
442     int sum = 0;
443    
444     for (i = 0; i < N; ++i)
445     {
446     sum += arr [i]
447     ecb_prefetch (arr + i + 128, 0, 0);
448     }
449    
450     It's hard to predict how far to prefetch, and most CPUs that can prefetch
451     are often good enough to predict this kind of behaviour themselves. It
452     gets more interesting with linked lists, especially when you do some fair
453     processing on each list element:
454    
455     for (node *n = start; n; n = n->next)
456     {
457     ecb_prefetch (n->next, 0, 0);
458     ... do medium amount of work with *n
459     }
460    
461     After processing the node, (part of) the next node might already be in
462     cache.
463 root 1.1
464 root 1.2 =back
465 root 1.1
466 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
467 root 1.1
468 root 1.4 =over 4
469    
470 root 1.3 =item bool ecb_big_endian ()
471    
472     =item bool ecb_little_endian ()
473    
474 sf-exg 1.11 These two functions return true if the byte order is big endian
475     (most-significant byte first) or little endian (least-significant byte
476     first) respectively.
477    
478 root 1.24 On systems that are neither, their return values are unspecified.
479    
480 root 1.3 =item int ecb_ctz32 (uint32_t x)
481    
482 root 1.35 =item int ecb_ctz64 (uint64_t x)
483    
484 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
485 root 1.24 equivalently the number of bits set to 0 before the least significant bit
486 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
487    
488 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
489    
490 root 1.35 For example:
491 sf-exg 1.11
492 root 1.15 ecb_ctz32 (3) = 0
493     ecb_ctz32 (6) = 1
494 sf-exg 1.11
495 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
496    
497     =item bool ecb_is_pot64 (uint32_t x)
498    
499     Return true iff C<x> is a power of two or C<x == 0>.
500    
501     For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>.
502    
503 root 1.35 =item int ecb_ld32 (uint32_t x)
504    
505     =item int ecb_ld64 (uint64_t x)
506    
507     Returns the index of the most significant bit set in C<x>, or the number
508     of digits the number requires in binary (so that C<< 2**ld <= x <
509     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
510     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
511     example to see how many bits a certain number requires to be encoded.
512    
513     This function is similar to the "count leading zero bits" function, except
514     that that one returns how many zero bits are "in front" of the number (in
515     the given data type), while C<ecb_ld> returns how many bits the number
516     itself requires.
517    
518 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
519    
520 root 1.3 =item int ecb_popcount32 (uint32_t x)
521    
522 root 1.35 =item int ecb_popcount64 (uint64_t x)
523    
524 root 1.36 Returns the number of bits set to 1 in C<x>.
525    
526     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
527    
528     For example:
529 sf-exg 1.11
530 root 1.15 ecb_popcount32 (7) = 3
531     ecb_popcount32 (255) = 8
532 sf-exg 1.11
533 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
534    
535     =item uint16_t ecb_bitrev16 (uint16_t x)
536    
537     =item uint32_t ecb_bitrev32 (uint32_t x)
538    
539     Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
540     and so on.
541    
542     Example:
543    
544     ecb_bitrev8 (0xa7) = 0xea
545     ecb_bitrev32 (0xffcc4411) = 0x882233ff
546    
547 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
548    
549 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
550    
551 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
552 sf-exg 1.13
553 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
554     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
555     C<ecb_bswap32>).
556    
557     =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
558    
559     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
560 root 1.3
561     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
562    
563 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
564    
565     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
566    
567     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
568    
569     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
570    
571 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
572    
573 root 1.34 These two families of functions return the value of C<x> after rotating
574     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
575     (C<ecb_rotl>).
576 sf-exg 1.11
577 root 1.20 Current GCC versions understand these functions and usually compile them
578 root 1.34 to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on
579     x86).
580 root 1.20
581 root 1.3 =back
582 root 1.1
583     =head2 ARITHMETIC
584    
585 root 1.3 =over 4
586    
587 root 1.14 =item x = ecb_mod (m, n)
588 root 1.3
589 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
590     of the division operation between C<m> and C<n>, using floored
591     division. Unlike the C remainder operator C<%>, this function ensures that
592     the return value is always positive and that the two numbers I<m> and
593     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
594     C<ecb_mod> implements the mathematical modulo operation, which is missing
595     in the language.
596 root 1.14
597 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
598 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
599 root 1.30 type (this typically excludes the minimum signed integer value, the same
600 root 1.25 limitation as for C</> and C<%> in C).
601 sf-exg 1.11
602 root 1.24 Current GCC versions compile this into an efficient branchless sequence on
603 root 1.28 almost all CPUs.
604 root 1.24
605     For example, when you want to rotate forward through the members of an
606     array for increasing C<m> (which might be negative), then you should use
607     C<ecb_mod>, as the C<%> operator might give either negative results, or
608     change direction for negative values:
609    
610     for (m = -100; m <= 100; ++m)
611     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
612    
613 sf-exg 1.37 =item x = ecb_div_rd (val, div)
614    
615     =item x = ecb_div_ru (val, div)
616    
617     Returns C<val> divided by C<div> rounded down or up, respectively.
618     C<val> and C<div> must have integer types and C<div> must be strictly
619 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
620     and with function templates in C++.
621 sf-exg 1.37
622 root 1.3 =back
623 root 1.1
624     =head2 UTILITY
625    
626 root 1.3 =over 4
627    
628 sf-exg 1.23 =item element_count = ecb_array_length (name)
629 root 1.3
630 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
631    
632     int primes[] = { 2, 3, 5, 7, 11 };
633     int sum = 0;
634    
635     for (i = 0; i < ecb_array_length (primes); i++)
636     sum += primes [i];
637    
638 root 1.3 =back
639 root 1.1
640 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
641    
642     These symbols need to be defined before including F<ecb.h> the first time.
643    
644     =over 4
645    
646     =item ECB_NO_THRADS
647    
648     If F<ecb.h> is never used from multiple threads, then this symbol can
649     be defined, in which case memory fences (and similar constructs) are
650     completely removed, leading to more efficient code and fewer dependencies.
651    
652     Setting this symbol to a true value implies C<ECB_NO_SMP>.
653    
654     =item ECB_NO_SMP
655    
656     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
657     multiple threads, but never concurrently (e.g. if the system the program
658     runs on has only a single CPU with a single core, no hyperthreading and so
659     on), then this symbol can be defined, leading to more efficient code and
660     fewer dependencies.
661    
662     =back
663    
664 root 1.1