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A general description is here.
Optional use of ‘libcurl’ (version 7.28.0 from Oct 2012 or later) for Internet access (including on Windows):
• ‘capabilities("libcurl")’ reports if this is available.
• ‘libcurlVersion()’ reports the version in use, and other details of the ‘"libcurl"’ build including which URL schemes it supports.
• ‘curlGetHeaders()’ retrieves the headers for ‘http://’, ‘https://’, ‘ftp://’ and ‘ftps://’ URLs: analysis of these headers can provide insights into the `existence' of a URL (it might for example be permanently redirected) and is so used in ‘R CMD check --as-cran’.
• ‘download.file()’ has a new optional method ‘"libcurl"’ which will handle more URL schemes, follow redirections, and allows simultaneous downloads of multiple URLs.
• ‘url()’ has a new method ‘"libcurl"’ which handles more URL schemes and follows redirections. The default method is controlled by a new option ‘url.method’, which applies also to the opening of URLs _via_ ‘file()’ (which happens implicitly in functions such as ‘read.table’.)
• When ‘file()’ or ‘url()’ is invoked with a ‘https://’ or ‘ftps://’ URL which the current method cannot handle, it switches to a suitable method if one is available.
(Windows.) The DLLs ‘internet.dll’ and ‘internet2.dll’ have been merged. In this version it is safe to switch (repeatedly) between the internal and Windows internet functions within an R session.
The Windows internet functions are still selected by flag ‘--internet2’ or ‘setInternet2()’. This can be overridden for an ‘url()’ connection _via_ its new ‘method’ argument.
‘parent.env<-’ can no longer modify the parent of a locked namespace or namespace imports environment. Contributed by Karl Millar.
‘R CMD INSTALL’ gains an option ‘--built-timestamp=STAMP’ allowing 100% reproducible package building, thanks to Dirk Eddelbuettel.
‘R CMD check --as-cran’ checks existence and accessibility of URLs in the ‘DESCRIPTION’ file and in the help files.
‘R CMD check --as-cran’ reports non-ASCII characters in R source files in case there is no package encoding.
‘R CMD check --as-cran’ reports (apparent) S3 methods exported but not registered. (This can also be selected by an environment variable: see the ‘R Internals’ manual.)