readChar 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 = "", useBytes = FALSE)

Arguments

con	A connection object, or a character string naming a file, or a raw vector.
nchars	integer vector, giving the lengths in characters of (unterminated) character strings to be read or written. Elements must be >= 0 and not NA.
useBytes	logical: For readChar, should nchars be regarded as a number of bytes not characters in a multi-byte locale? For writeChar, see writeLines.
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.

These functions are intended to be used with binary-mode connections. If con is a character string, the functions call file to obtain a binary-mode 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.

Using these functions on a text-mode connection may work but should not be mixed with text-mode access to the connection, especially if the connection was opened with an encoding argument.

See Also

The ‘R Data Import/Export’ manual.

connections, readLines, writeLines, readBin

Examples

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