Method: IO#set_encoding_by_bom

Defined in:

io.c

#set_encoding_by_bom ⇒ Encoding^?

If the stream begins with a BOM (byte order marker), consumes the BOM and sets the external encoding accordingly; returns the result encoding if found, or nil otherwise:

File.write('t.tmp', "\u{FEFF}abc")
io = File.open('t.tmp', 'rb')
io.set_encoding_by_bom # => #<Encoding:UTF-8>
io.close

File.write('t.tmp', 'abc')
io = File.open('t.tmp', 'rb')
io.set_encoding_by_bom # => nil
io.close

Raises an exception if the stream is not binmode or its encoding has already been set.

Returns:

• (Encoding, nil)

# File 'io.c', line 9596

static VALUE
rb_io_set_encoding_by_bom(VALUE io)
{
    rb_io_t *fptr;

    GetOpenFile(io, fptr);
    if (!(fptr->mode & FMODE_BINMODE)) {
        rb_raise(rb_eArgError, "ASCII incompatible encoding needs binmode");
    }
    if (fptr->encs.enc2) {
        rb_raise(rb_eArgError, "encoding conversion is set");
    }
    else if (fptr->encs.enc && fptr->encs.enc != rb_ascii8bit_encoding()) {
        rb_raise(rb_eArgError, "encoding is set to %s already",
                 rb_enc_name(fptr->encs.enc));
    }
    if (!io_set_encoding_by_bom(io)) return Qnil;
    return rb_enc_from_encoding(fptr->encs.enc);
}