IO#readpartial

Reads up to maxlen bytes from the stream; returns a string (either a new string or the given out_string). Its encoding is:

• The unchanged encoding of out_string, if out_string is given.

• ASCII-8BIT, otherwise.

• Contains maxlen bytes from the stream, if available.

• Otherwise contains all available bytes, if any available.

• Otherwise is an empty string.

With the single non-negative integer argument maxlen given, returns a new string:

f = File.new('t.txt')
f.readpartial(20) # => "First line\nSecond l"
f.readpartial(20) # => "ine\n\nFourth line\n"
f.readpartial(20) # => "Fifth line\n"
f.readpartial(20) # Raises EOFError.
f.close

With both argument maxlen and string argument out_string given, returns modified out_string:

f = File.new('t.txt')
s = 'foo'
f.readpartial(20, s) # => "First line\nSecond l"
s = 'bar'
f.readpartial(0, s)  # => ""
f.close

This method is useful for a stream such as a pipe, a socket, or a tty. It blocks only when no data is immediately available. This means that it blocks only when all of the following are true:

• The byte buffer in the stream is empty.

• The content of the stream is empty.

• The stream is not at EOF.

When blocked, the method waits for either more data or EOF on the stream:

• If more data is read, the method returns the data.

• If EOF is reached, the method raises EOFError.

When not blocked, the method responds immediately:

• Returns data from the buffer if there is any.

• Otherwise returns data from the stream if there is any.

• Otherwise raises EOFError if the stream has reached EOF.

Note that this method is similar to sysread. The differences are:

• If the byte buffer is not empty, read from the byte buffer instead of “sysread for buffered IO (IOError)”.

• It doesn’t cause Errno::EWOULDBLOCK and Errno::EINTR. When readpartial meets EWOULDBLOCK and EINTR by read system call, readpartial retries the system call.

The latter means that readpartial is non-blocking-flag insensitive. It blocks on the situation IO#sysread causes Errno::EWOULDBLOCK as if the fd is blocking mode.

Examples:

#                        # Returned      Buffer Content    Pipe Content
r, w = IO.pipe           #
w << 'abc'               #               ""                "abc".
r.readpartial(4096)      # => "abc"      ""                ""
r.readpartial(4096)      # (Blocks because buffer and pipe are empty.)

#                        # Returned      Buffer Content    Pipe Content
r, w = IO.pipe           #
w << 'abc'               #               ""                "abc"
w.close                  #               ""                "abc" EOF
r.readpartial(4096)      # => "abc"      ""                 EOF
r.readpartial(4096)      # raises EOFError

#                        # Returned      Buffer Content    Pipe Content
r, w = IO.pipe           #
w << "abc\ndef\n"        #               ""                "abc\ndef\n"
r.gets                   # => "abc\n"    "def\n"           ""
w << "ghi\n"             #               "def\n"           "ghi\n"
r.readpartial(4096)      # => "def\n"    ""                "ghi\n"
r.readpartial(4096)      # => "ghi\n"    ""                ""