File

import

f = Import('file');

File is implemented as a Set named File.

File {
  fd;
  errno;
  open(path, op, prio);
  lseek(offset, whence);
  read(nbytes);
  write(data);
  close();
  errmsg();
  size();
};

fd

fd is the file descriptor that is like an integer in C. But in Melang, this property is read-only. It will be set by method open. Usually, we don’t have to access it.

errno

errno is an integer as in C. This number is an error number set by every file operation method in this Set.

open

Open a file in the specified mode.

Input:

• path - this is a file path string.

• op - a string that indicates the operation including read, write and append. Its value is composed of three parts:

  • r - read
  • w - write
  • a - append
  • + - not truncate

  e.g.

  • rw - read and write
  • ra - read from the end of file
  • aw - append to write

• prio - an integer indicates the file access authority, e.g. 0777. This argument only used on opening a nonexistent file. It is optional. If not given, 0644 will be set by default.

Return value:

• true will be returned on success, otherwise false returned and errno will be set.

lseek

Repositions the offset of the file descriptor to the argument offset, according to the directive whence.

Input:

• offset - an integer offset.
• whence - a string value including three values:
  • begin - from the beginning of the file.
  • current - from current position.
  • end - from the end of the file.

Return value:

• true will be returned on success, otherwise false returned and errno will be set.

read

Read nbytes from current position of the openned file.

Input:

• nbytes - an integer indicates how many bytes read in this time.

Return value:

• nbytes data in file will be returned on success, otherwise false returned and errno will be set.

write

Write data in the file.

Input:

• data - must be a string.

Return value:

• the number of bytes written in file will be returned on success, otherwise false returned and errno will be set.

close

Close file.

Input: None

Return value:

• Always be nil.

errmsg

Get error message.

Input: None

Return value:

• a string error message will be returned.

size

Get file size.

Input: None

Return value:

• an integer file bytes will be returned on success, otherwise false returned.

Example

Create a test file before Melang execution.

$ echo "hello" > tempfile

Execute Melang program:

sys = Import('sys');
F = Import('file');
f = $F; // or f = $File; both are the same.
if (f.open('tempfile', 'rw+') != false) { //read and write file and file content won't be ereased
  f.lseek(1, 'begin');
  f.write('hi all');
  f.lseek(0, 'begin');
  sys.print(f.read(f.size()));
} fi

The output is:

hi all

And the content in tempfile is:

hi all