C/CPP__restrict__(ImplementationSpecific)Keyword

from : What does the restrict keyword mean in C&＃43;&＃43;? - Stack Overflow

I was always unsure, what does the restrict keyword mean in C&＃43;&＃43;?

Does it mean the two or more pointer given to the function does not overlap? What else does it mean?

&＃61;&＃61;&＃61;> a pointer with "restrict" keyword or the implementation/compiler specific version of it, means that the content refered to by the pointer can and only can be accessed through the said pointer.

c&＃43;&＃43;restrict-qualifier

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• 34

  restrict is a c99 keyword. Yes, Rpbert S. Barnes, I know that most compilers support __restrict__. You will note that anything with double underscores is, by definition, implementation specific and thus NOT C&＃43;&＃43;, but a compiler specific version of it. 

  – KitsuneYMG

   Jan 6 &＃39;10 at 9:31

• 8

  What? Just because it&＃39;s implementation specific does not make it not C&＃43;&＃43;; the C&＃43;&＃43; allows for implementation specific stuff explicitly, and does not disallow it or render it not C&＃43;&＃43;. 

  – Alice

   May 29 &＃39;14 at 5:02

• 7

  &＃64;Alice KitsuneYMG means that it&＃39;s not part of ISO C&＃43;&＃43;, and is instead considered a C&＃43;&＃43; extension. Compiler creators are allowed to make and distribute their own extensions, which coexist with ISO C&＃43;&＃43; and act as part of a usually-less-or-non-portable unofficial addition to C&＃43;&＃43;. Examples would be MS&＃39;s old Managed C&＃43;&＃43;, and their more recent C&＃43;&＃43;/CLI. Other examples would be preprocessor directives and macros supplied by some compilers, such as the common #warning directive, or the function signature macros (__PRETTY_FUNCTION__ on GCC, __FUNCSIG__ on MSVC, etc.). 

  – Justin Time - Reinstate Monica

   Mar 17 &＃39;16 at 21:57 

• 6

  &＃64;Alice To my knowledge, C&＃43;&＃43;11 doesn&＃39;t mandate full support for all of C99, nor do C&＃43;&＃43;14 or what I know of C&＃43;&＃43;17. restrict isn&＃39;t considered a C&＃43;&＃43; keyword (see en.cppreference.com/w/cpp/keyword ), and in fact, the only mention of restrict in the C&＃43;&＃43;11 standard (see open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf , a copy of the FDIS with minor editorial changes, §17.2 [library.c], PDF page 413) states that: 

  – Justin Time - Reinstate Monica

   Mar 20 &＃39;16 at 17:10

• 6

  &＃64;Alice How so? I stated the part that says that restrict is to be omitted from (excluded from, left out of) C standard library function signatures and semantics when those functions are included in the C&＃43;&＃43; standard library. Or in other words, I stated the fact that says that if a C standard library function&＃39;s signature contains restrict in C, the restrict keyword must be removed from the C&＃43;&＃43; equivalent&＃39;s signature. 

  – Justin Time - Reinstate Monica

(Most Voted Answer)

In his paper, Memory Optimization, Christer Ericson says that while restrict is not part of the C&＃43;&＃43; standard yet, that it is supported by many compilers and he recommends it&＃39;s usage when available:

restrict keyword

! New to 1999 ANSI/ISO C standard

! Not in C&＃43;&＃43; standard yet, but supported by many C&＃43;&＃43; compilers

! A hint only, so may do nothing and still be conforming

A restrict-qualified pointer (or reference)...

! ...is basically a promise to the compiler that for the scope of the pointer, the target of the pointer will only be accessed through that pointer (and pointers copied from it).

In C&＃43;&＃43; compilers that support it it should probably behave the same as in C.

See this SO post for details: Realistic usage of the C99 ‘restrict’ keyword?

Take half an hour to skim through Ericson&＃39;s paper, it&＃39;s interesting and worth the time.

Edit

I also found that IBM&＃39;s AIX C/C&＃43;&＃43; compiler supports the __restrict__ keyword.

g&＃43;&＃43; also seems to support this as the following program compiles cleanly on g&＃43;&＃43;:

#include int foo(int * __restrict__ a, int * __restrict__ b) {return *a &＃43; *b;
}int main(void) {int a &＃61; 1, b &＃61; 1, c;c &＃61; foo(&a, &b);printf("c &＃61;&＃61; %d\n", c);return 0;
}


I also found a nice article on the use of restrict:

Demystifying The Restrict Keyword

Edit2

I ran across an article which specifically discusses the use of restrict in C&＃43;&＃43; programs:

Load-hit-stores and the __restrict keyword

Also, Microsoft Visual C&＃43;&＃43; also supports the __restrict keyword.

The Memory Optimisation paper link is dead, here&＃39;s a link to the audio from his GDC presentation. gdcvault.com/play/1022689/Memory 

– Grimeh

 Dec 8 &＃39;16 at 20:21

• 1

  &＃64;EnnMichael: Obviously if you&＃39;re going to use it in a portable C&＃43;&＃43; project, you should #ifndef __GNUC__ #define __restrict__ /* no-op */ or similar. And define it to __restrict if _MSC_VER is defined. 

  – Peter Cordes

   Apr 27 &＃39;17 at 22:43