R: Functions to Manipulate Connections

connections {base} | R Documentation

## Functions to Manipulate Connections

### Description

Functions to create, open and close connections.

### Usage

    file(description = "", open = "", blocking = TRUE,
         encoding = getOption("encoding"))
    pipe(description, open = "", encoding = getOption("encoding"))
    fifo(description = "", open = "", blocking = FALSE,
         encoding = getOption("encoding"))
    gzfile(description, open = "", encoding = getOption("encoding"),
           compression = 6)
    url(description, open = "", blocking = TRUE,
        encoding = getOption("encoding"))
    socketConnection(host = "localhost", port, server = FALSE,
                     blocking = FALSE, open = "a+",
                     encoding = getOption("encoding"))

    open(con, open = "r", blocking = TRUE, ...)
    close(con, type = "rw", ...)

    isOpen(con, rw = "")
    isIncomplete(con)

### Arguments

`description` | character. A description of the connection. For `file` and `pipe` this is a path to the file to be opened. For `url` it is a complete URL, including schemes (`http://`, `ftp://` or `file://`). `file` also accepts complete URLs.
`con` | a connection.
`host` | character. Host name for port.
`port` | integer. The TCP port number.
`server` | logical. Should the socket be a client or a server?
`open` | character. A description of how to open the connection (if at all). See Details for possible values.
`blocking` | logical. See `Blocking' section below.
`encoding` | An integer vector of length 256.
`compression` | integer in 0–9. The amount of compression to be applied when writing, from none to maximal. The default is a good space/time compromise.
`type` | character. Currently ignored.
`where` | integer. A file position (relative to the origin specified by `origin`), or `NA`.
`rw` | character. Empty or `"read"` or `"write"`, partial matches allowed.
`...` | arguments passed to or from other methods.

### Details

The first six functions create connections. By default the connection is not opened (except for `socketConnection`, but may be opened by setting a non-empty value of argument `open`. `gzfile` applies to files compressed by `gzip`: such connections can only be binary.

All platforms support file connections and `url("file://")` connections. The other types may be partially implemented or not implemented at all. (They do work on most Unix platforms, and all but `fifo` on Windows.)

Proxies can be specified for `url` connections: see `download.file`.

`open`, `close` and `seek` are generic functions: the following applies to the methods relevant to connections.

`open` opens a connection. In general functions using connections will open them if they are not open, but then close them again, so to leave a connection open call `open` explicitly.

`close` closes and destroys a connection.

Possible values for the mode `open` to open a connection are

`"r"` or `"rt"`
  Open for reading in text mode.
`"w"` or `"wt"`
  Open for writing in text mode.
`"a"` or `"at"`
  Open for appending in text mode.
`"rb"`
  Open for reading in binary mode.
`"wb"`
  Open for writing in binary mode.
`"ab"`
  Open for appending in binary mode.
`"r+"`, `"r+b"`
  Open for reading and writing.
`"w+"`, `"w+b"`
  Open for reading and writing.
`"r+"`, `"r+b"`
  Open for reading and writing, truncating file initially.
`"a+"`, `"a+b"`
  Open for reading and appending.

Not all modes are applicable to all connections: for example URLs can only be opened for reading. Only file and socket connections can be opened for reading and writing/appending. For many connections there is little or no difference between text and binary modes, but there is for file-like connections on Windows, and `pushBack` is text-oriented and is only allowed on connections open for reading in text mode.

If for a `file` connection the description is `""`, the file is immediately opened in `"w+"` mode and unlinked from the file system. This provides a temporary file to write to and then read from.

The encoding vector is used to map the input from a file or pipe to the platform's native character set. Supplied examples are `native.enc` as well as `MacRoman`, `WinAnsi` and `ISOLatin1`, whose actual encoding is platform-dependent. Missing characters are mapped to a space in these encodings.

### Value

`file`, `pipe`, `fifo`, `url`, `gzfile` and `socketConnection` return a connection object which inherits from class `"connection"` and has a first more specific class.
`isOpen` returns a logical value, whether the connection is currently open.
`isIncomplete` returns a logical value, whether last read attempt was blocked, or for an output text connection whether there is unflushed output.

### Blocking

The default condition for all but fifo and socket connections is to be in blocking mode. In that mode, functions do not return to the R evaluator until they are complete. In non-blocking mode, operations return as soon as possible, so on input they will return with whatever input is available (possibly none) and for output they will return whether or not the write succeeded.

The function `readLines` behaves differently in respect of incomplete last lines in the two modes: see its help page.

Even when a connection is in blocking mode, attempts are made to ensure that it does not block the event loop and hence the operation of GUI parts of R. These do not always succeed, and the whole process will be blocked during a DNS lookup on Unix, for example.

Most blocking operations on URLs and sockets are subject to the timeout set by `options("timeout")`. Note that this is a timeout for no response at all, not for the whole operation.

### Fifos

Fifos default to non-blocking. That follows Svr4 and it probably most natural, but it does have some implications. In particular, opening a non-blocking fifo connection for writing (only) will fail unless some other process is reading on the fifo.

Opening a fifo for both reading and writing (in any mode: one can only append to fifos) connects both sides of the fifo to the R process, and provides an similar facility to `file()`.

### Note

R's connections are modelled on those in S version 4 (see Chambers, 1998). However R also goes well beyond the Svr4 model, for example in output text connections and URL, gzfile and socket connections.

The default mode in R is `"r"` except for socket connections. This differs from Svr4, where it is the equivalent of `"r+"`, known as `"*"`.

### References

Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.

### See Also

`textConnection`, `seek`, `readLines`, `showConnections`, `pushBack`.

`capabilities` to see if `gzfile`, `url`, `fifo` and `socketConnection` are supported by this build of R.

### Examples

    zz <- file("ex.data", "w")  # open an output file connection
    cat("TITLE extra line", "2 3 5 7", "", "11 13 17", file = zz, sep = "\n")
    cat("One more line\n", file = zz)
    close(zz)
    readLines("ex.data")
    unlink("ex.data")

    if(capabilities("libz")) {
      zz <- gzfile("ex.gz", "w")  # compressed file
      cat("TITLE extra line", "2 3 5 7", "", "11 13 17", file = zz, sep = "\n")
      close(zz)
      print(readLines(gzfile("ex.gz")))
      unlink("ex.gz")
    }

    ## An example of a file open for reading and writing
    Tfile <- file("test1", "w+")
    c(isOpen(Tfile, "r"), isOpen(Tfile, "w")) # both TRUE
    cat("abc\ndef\n", file=Tfile)
    readLines(Tfile)
    seek(Tfile, 0, rw="r") # reset to beginning
    readLines(Tfile)
    cat("ghi\n", file=Tfile)
    readLines(Tfile)
    close(Tfile)
    unlink("test1")

    ## We can do the same thing with an anonymous file.
    Tfile <- file()
    cat("abc\ndef\n", file=Tfile)
    readLines(Tfile)
    close(Tfile)

    if(capabilities("fifo")) {
      zz <- fifo("foo", "w+")
      writeLines("abc", zz)
      print(readLines(zz))
      close(zz)
      unlink("foo")
    }

    ## Unix examples of use of pipes

    # read listing of current directory
    readLines(pipe("ls -1"))

    # remove trailing commas. Suppose
    % cat data2
    450, 390, 467, 654,  30, 542, 334, 432, 421,
    357, 497, 493, 550, 549, 467, 575, 578, 342,
    446, 547, 534, 495, 979, 479
    # Then read this by
    scan(pipe("sed -e s/,$// data2"), sep=",")

    # convert decimal point to comma in output
    zz <- pipe(paste("sed s/\\./,/ >", "outfile"), "w")
    cat(format(round(rnorm(100), 4)), sep = "\n", file = zz)
    close(zz)
    file.show("outfile", delete.file=TRUE)

    ## example for Unix machine running a finger daemon

    con <- socketConnection(port = 79, blocking = TRUE)
    writeLines(paste(system("whoami", intern=TRUE), "\r", sep=""), con)
    gsub(" *$", "", readLines(con))
    close(con)

    ## two R processes communicating via non-blocking sockets
    # R process 1
    con1 <- socketConnection(port = 6011, server=TRUE)
    writeLines(LETTERS, con1)
    close(con1)

    # R process 2
    con2 <- socketConnection(Sys.info()["nodename"], port = 6011)
    # as non-blocking, may need to loop for input
    readLines(con2)
    while(isIncomplete(con2)) {Sys.sleep(1); readLines(con2)}
    close(con2)

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