Method: IO#read

Defined in:

io.c

#read(maxlen = nil, out_string = nil) ⇒ nil

Reads bytes from the stream; the stream must be opened for reading (see Access Modes):

• If maxlen is nil, reads all bytes using the stream’s data mode.

• Otherwise reads up to maxlen bytes in binary mode.

Returns a string (either a new string or the given out_string) containing the bytes read. The encoding of the string depends on both maxLen and out_string:

• maxlen is nil: uses internal encoding of self (regardless of whether out_string was given).

• maxlen not nil:

  • out_string given: encoding of out_string not modified.

  • out_string not given: ASCII-8BIT is used.

Without Argument out_string

When argument out_string is omitted, the returned value is a new string:

f = File.new('t.txt')
f.read
# => "First line\nSecond line\n\nFourth line\nFifth line\n"
f.rewind
f.read(30) # => "First line\r\nSecond line\r\n\r\nFou"
f.read(30) # => "rth line\r\nFifth line\r\n"
f.read(30) # => nil
f.close

If maxlen is zero, returns an empty string.

With Argument out_string

When argument out_string is given, the returned value is out_string, whose content is replaced:

f = File.new('t.txt')
s = 'foo'      # => "foo"
f.read(nil, s) # => "First line\nSecond line\n\nFourth line\nFifth line\n"
s              # => "First line\nSecond line\n\nFourth line\nFifth line\n"
f.rewind
s = 'bar'
f.read(30, s)  # => "First line\r\nSecond line\r\n\r\nFou"
s              # => "First line\r\nSecond line\r\n\r\nFou"
s = 'baz'
f.read(30, s)  # => "rth line\r\nFifth line\r\n"
s              # => "rth line\r\nFifth line\r\n"
s = 'bat'
f.read(30, s)  # => nil
s              # => ""
f.close

Note that this method behaves like the fread() function in C. This means it retries to invoke read(2) system calls to read data with the specified maxlen (or until EOF).

This behavior is preserved even if the stream is in non-blocking mode. (This method is non-blocking-flag insensitive as other methods.)

If you need the behavior like a single read(2) system call, consider #readpartial, #read_nonblock, and #sysread.

Related: IO#write.

# File 'io.c', line 3802

static VALUE
io_read(int argc, VALUE *argv, VALUE io)
{
    rb_io_t *fptr;
    long n, len;
    VALUE length, str;
    int shrinkable;
#if RUBY_CRLF_ENVIRONMENT
    int previous_mode;
#endif

    rb_scan_args(argc, argv, "02", &length, &str);

    if (NIL_P(length)) {
        GetOpenFile(io, fptr);
        rb_io_check_char_readable(fptr);
        return read_all(fptr, remain_size(fptr), str);
    }
    len = NUM2LONG(length);
    if (len < 0) {
        rb_raise(rb_eArgError, "negative length %ld given", len);
    }

    shrinkable = io_setstrbuf(&str,len);

    GetOpenFile(io, fptr);
    rb_io_check_byte_readable(fptr);
    if (len == 0) {
        io_set_read_length(str, 0, shrinkable);
        return str;
    }

    READ_CHECK(fptr);
#if RUBY_CRLF_ENVIRONMENT
    previous_mode = set_binary_mode_with_seek_cur(fptr);
#endif
    n = io_fread(str, 0, len, fptr);
    io_set_read_length(str, n, shrinkable);
#if RUBY_CRLF_ENVIRONMENT
    if (previous_mode == O_TEXT) {
        setmode(fptr->fd, O_TEXT);
    }
#endif
    if (n == 0) return Qnil;

    return str;
}