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A general description is here.
There is a subtle change in behaviour for numeric index values 2^31 and larger. These never used to be legitimate and so were treated as ‘NA’, sometimes with a warning. They are now legal for long vectors so there is no longer a warning, and ‘x[2^31] <- y’ will now extend the vector on a 64-bit platform and give an error on a 32-bit one.
It is now possible for 64-bit builds to allocate amounts of memory limited only by the OS. It may be wise to use OS facilities (e.g. ‘ulimit’ in a ‘bash’ shell, ‘limit’ in ‘csh’), to set limits on overall memory consumption of an R process, particularly in a multi-user environment. A number of packages need a limit of at least 4GB of virtual memory to load.
64-bit Windows builds of R are by default limited in memory usage to the amount of RAM installed: this limit can be changed by command-line option ‘--max-mem-size’ or setting environment variable ‘R_MAX_MEM_SIZE’.
The functions ‘.C()’, ‘.Call()’, ‘.External()’ and ‘.Fortran()’ now
check that they are called with an unnamed first argument: the formal
arguments were changed from ‘name=’ to ‘.NAME=’ in R 2.13.0, but some
packages were still using the old name.
The functions ‘.C()’, ‘.Call()’, ‘.External()’ and ‘.Fortran()’ now give an error (rather than a warning) if called with a named first argument.
New functions ‘cite()’ and ‘citeNatbib()’ have been added, to allow generation of in-text citations from ‘"bibentry"’ objects. A ‘cite()’ function may be added to ‘bibstyle()’ environments.
‘dist()’ can produce dissimilarity objects for more than 65536 rows (but for example ‘hclust()’ cannot process such objects).
‘serialize()’ to a raw vector is no longer limited in size (except by resources) on 64-bit platforms.
Many calls to ‘.C()’ have been replaced by ‘.Call()’ to allow long vectors to be supported (now or in the future). Regrettably several packages had copied the non-API ‘.C()’ calls and so failed.
A foreign function call (‘.C()’ etc) in a package without a ‘PACKAGE’ argument will only look in the first DLL specified in the ‘NAMESPACE’ file of the package rather than searching all loaded DLLs. A few packages needed ‘PACKAGE’ arguments added.
When ‘R CMD build’ is run in an encoding other than the one specified in the package's ‘DESCRIPTION’ file it tries harder to expand the ‘authors@R’ field in the specified encoding. (PR#14958.)
The functions ‘.C()’, ‘.Call()’, ‘.External()’ and ‘.Fortran()’ now check that they are called with an unnamed first argument: the formal arguments were changed from ‘name=’ to ‘.NAME=’ in R 2.13.0, but some packages were still using the old name. This is currently a warning, but will be an error in future.
Long custom line types would sometimes segfault on the cairographics-based devices. (PR#15055.)