sysread

/*
 *  call-seq:
 *     ios.sysread(integer )    => string
 *  
 *  Reads <i>integer</i> bytes from <em>ios</em> using a low-level
 *  read and returns them as a string. Do not mix with other methods
 *  that read from <em>ios</em> or you may get unpredictable results.
 *  Raises <code>SystemCallError</code> on error and
 *  <code>EOFError</code> at end of file.
 *     
 *     f = File.new("testfile")
 *     f.sysread(16)   #=> "This is line one"
 */

static VALUE
rb_io_sysread(argc, argv, io)
    int argc;
    VALUE *argv;
    VALUE io;
{
    VALUE len, str;
    rb_io_t *fptr;
    long n, ilen;

    rb_scan_args(argc, argv, "11", &len, &str);
    ilen = NUM2LONG(len);

    if (NIL_P(str)) {
        str = rb_str_new(0, ilen);
    }
    else {
        StringValue(str);
        rb_str_modify(str);
        rb_str_resize(str, ilen);
    }
    if (ilen == 0) return str;

    GetOpenFile(io, fptr);
    rb_io_check_readable(fptr);

    if (READ_DATA_BUFFERED(fptr->f)) {
        rb_raise(rb_eIOError, "sysread for buffered IO");
    }
    rb_str_locktmp(str);

    n = fileno(fptr->f);
    rb_thread_wait_fd(fileno(fptr->f));
    rb_io_check_closed(fptr);
    if (RSTRING(str)->len != ilen) {
        rb_raise(rb_eRuntimeError, "buffer string modified");
    }
    TRAP_BEG;
    n = read(fileno(fptr->f), RSTRING(str)->ptr, ilen);
    TRAP_END;

    rb_str_unlocktmp(str);
    if (n == -1) {
        rb_sys_fail(fptr->path);
    }
    rb_str_resize(str, n);
    if (n == 0 && ilen > 0) {
        rb_eof_error();
    }
    RSTRING(str)->len = n;
    RSTRING(str)->ptr[n] = '\0';
    OBJ_TAINT(str);

    return str;
}