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### `open_file`, `close_file`, `get_line`, `get_byte`, `put_byte`, `purge_stdin`

`open_file("filename"[,"mode"])`
  :: Opens filename for reading.
`close_file(num)`
  :: Closes the file indicated by a descriptor num.
`get_line([num])`
  :: Reads a line from the file indicated by a descriptor num.
`get_byte(num)`
  :: Reads a byte from the file indicated by a descriptor num.
`put_byte(num,c)`
  :: Writes a byte c to the file indicated by a descriptor num.
`purge_stdin()`
`purge_stdin()`
  :: Clears the buffer for the standard input.

return
  `open_file()` : integer (fild id); `close_file()` : 1; `get_line()` : string; `get_byte()`, `put_byte()` : integer
filename
  file (path) name
mode
  string
num
  non-negative integer (file descriptor)

- `open_file()` opens a file. If mode is not specified, a file is opened for reading. If mode is specified, it is used as the mode specification for C standard I/O function `fopen()`. For example `"w"` requests that the file is truncated to zero length or created for writing. `"a"` requests that the file is opened for writing or created if it does not exist. The stream pointer is set at the end of the file. If successful, it returns a non-negative integer as the file descriptor. Otherwise the system error function is called. Unnecessary files should be closed by `close_file()`.
- `get_line()` reads a line from an opened file and returns the line as a string. If no argument is supplied, it reads a line from the standard input.
- `get_byte()` reads a byte from an opened file and returns the it as an integer.
- `put_byte()` writes a byte from an opened file and returns the the byte as an integer.
- A `get_line()` call after reading the end of file returns an integer 0.
- Strings can be converted into internal forms with string manipulation functions such as `sub_str()`, `eval_str()`.
- `purge_stdin()` clears the buffer for the standard input. When a function receives a character string from `get_line()`, this functions should be called in advance in order to avoid an incorrect behavior which is caused by the characters already exists in the buffer.

    [185] Id = open_file("test");
    0
    [186] get_line(Id);
    12345

    [187] get_line(Id);
    67890

    [188] get_line(Id);
    0
    [189] type(@@);
    0
    [190] close_file(Id);
    1
    [191] open_file("test");
    1
    [192] get_line(1);
    12345

    [193] get_byte(1);
    54                   /* the ASCII code of '6' */
    [194] get_line(1);
    7890                 /* the rest of the last line */
    [195] def test() { return get_line(); }
    [196] def test1() { purge_stdin(); return get_line(); }
    [197] test();
                         /* a remaining newline character has been read */
                         /* returns immediately */
    [198] test1();
    123;                 /* input from a keyboard */
    123;                 /* returned value */

    [199]

`References`
  section `eval_str`, section `str_len`, `str_chr`, `sub_str`.

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