C99 restrict Keyword

I was pleasently surprised to encounter a C keyword that I had never heard of before. The keyword in question is the restrict keyword which is a type qualifier for pointers and is a formal part of the C99 standard. This keyword allows programmers to declare that pointers which share the same type do not alias each other.  This information can then be used by the compiler to make optimizations when using the pointers. If the data is in fact aliased, the results are undefined.

Consider the following example:

memcpy((void* restrict) dst, (void* restrict) src, size);


This tells the compiler that neither the dst or src pointer paramters overlap and so the compiler is free to apply any optimizations – including optimizations that may result in out of order reads/writes.

Mainstream compilers have varying support for this feature.

  • GCC supports it in C99 mode  – specified via the “-std=c99” option or for non-C99 code by specifying __restrict to enable the keyword as a GCC extension.
  • Microsoft’s Visual Studio .NET 2005/2008 compiler doesn’t support this feature as specified in the C99 standard but does provide similar support using the __restrict specifier. Micorosft also allows this keyword to be specified for both C and C++ code. See the MSDN documentation for more details on Microsofts implementation and differences between it’s support and the C99 specification of restrict.

Finally, it should be noted that this keyword is specific to C and is not specified in the 1998 C++ specification nor is it currently planned for inclusion in the fothcoming C++ specification update.


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