readChar                package:base                R Documentation

Transfer Character Strings To and From Connections

Description:

     Transfer character strings to and from connections, without
     assuming they are null-terminated on the connection.

Usage:

     readChar(con, nchars, useBytes = FALSE)

     writeChar(object, con,
               nchars = nchar(object, type="chars"), eos = "")

Arguments:

     con: A connection object, or a character string naming a file, or
          a raw vector.

  nchars: integer, giving the lengths in characters of (unterminated)
          character strings to be read or written.  Must be >= 0 and
          not missing.

useBytes: logical: should 'nchars' be regarded as a number of bytes not
          characters in a multi-byte locale?

  object: A character vector to be written to the connection, at least
          as long as 'nchars'.

     eos: 'end of string': character string .  The terminator to be
          written after each string, followed by an ASCII 'nul'; use
          'NULL' for no terminator at all.

Details:

     These functions complement 'readBin' and 'writeBin' which read and
     write C-style zero-terminated character strings.  They are for
     strings of known length, and can optionally write an end-of-string
     mark.  They are intended only for character strings valid in the
     current locale.

     If 'con' is a character string, the functions call 'file' to
     obtain an file connection which is opened for the duration of the
     function call.

     If the connection is open it is read/written from its current
     position.  If it is not open, it is opened for the duration of the
     call in an appropriate mode (binary read or write) and then closed
     again.  An open connection must be in binary mode.

     If 'readChar' is called with 'con' a raw vector, the data in the
     vector is used as input.  If 'writeChar' is called with 'con' a
     raw vector, it is just an indication that a raw vector should be
     returned.  

     Character strings containing ASCII 'nul'(s) will be read correctly
     by 'readChar' but truncated at the first 'nul' with a warning.

     If the character length requested for 'readChar' is longer than
     the data available on the connection, what is available is
     returned.  For 'writeChar' if too many characters are requested
     the output is zero-padded, with a warning.

     Missing strings are written as 'NA'.

Value:

     For 'readChar', a character vector of length the number of items
     read (which might be less than 'length(nchars)').

     For 'writeChar', a raw vector (if 'con' is a raw vector) or
     invisibly 'NULL'.

Note:

     Earlier versions of R allowed embedded nul bytes within character
     strings, but not R >= 2.8.0.  'readChar' was commonly used to read
     fixed-size zero-padded byte fields for which 'readBin' was
     unsuitable.  'readChar' can still be used for such fields if there
     are no embedded nuls: otherwise 'readBin(what="raw")' provides an
     alternative.

     'nchars' will be interpreted in bytes not characters in a
     non-UTF-8 multi-byte locale, with a warning.

     There is little validity checking of UTF-8 reads.

See Also:

     The _R Data Import/Export_ manual.

     'connections', 'readLines', 'writeLines', 'readBin'

Examples:

     ## test fixed-length strings
     zz <- file("testchar", "wb")
     x <- c("a", "this will be truncated", "abc")
     nc <- c(3, 10, 3)
     writeChar(x, zz, nc, eos=NULL)
     writeChar(x, zz, eos="\r\n")
     close(zz)

     zz <- file("testchar", "rb")
     readChar(zz, nc)
     readChar(zz, nchar(x)+3) # need to read the terminator explicitly
     close(zz)
     unlink("testchar")

