Detect EndOfFile IoResult when reading a file

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I'm trying to read from a file in Rust. The thing I don't understand is: when the BufferedReader is at the EOF, it actually gives an Err(IoError {kind: EndOfFile}) and I don't know how to match on it.

loop {
  match file.read_line() {
    Ok(line) => {
      // Do stuff
    },
    // Err(EndOfFile) => is want I want here so I can call "break"
    Err(_) => {
      panic!("Unexpected error reading file.");
    }
  }
}

How can I explicitly match the EndOfFile enum variant?


How can I explicitly match the EndOfFile enum variant?

You can match it without additional nested match using following pattern:

loop {
  match file.read_line() {
    Ok(line) => {
      // Do stuff
    },
    Err(IoError { kind: EndOfFile, .. }) =>
      break,
    Err(_) => {
      panic!("Unexpected error reading file.");
    }
  }
}

rust - Matching on a specific error type, possible duplicate of Detect EndOfFile IoResult when reading a file – Shepmaster Feb 13 '15 at 13:21. 1 Can't read from file: {}, err {}", filename, e), IoError where kind is IoErrorKind::EndOfFile and all the other fields are whatever you like”. way to do things (so not sure if this would be discouraged) but I personally find. The test for end-of-file during an asynchronous read operation is slightly more involved than for a similar synchronous read operation. The end-of-file indicator for asynchronous read operations is when GetOverlappedResult returns FALSE and GetLastError returns ERROR_HANDLE_EOF .


specifically for iterating over lines rust has the lines function for Buffers (http://doc.rust-lang.org/std/io/trait.BufferPrelude.html#tymethod.lines).

In your case you would loop over the lines, and once EOF is reached the loop aborts automatically without your intervention.

for line in file.lines() {
    match line {
        Ok(line) => {
            // do stuff
        },
        Err(_) => {
            println!("Unexpected error reading file.")
        }
    }
}

or, if your function returns a compatible Result you can use the try! macro for less noise:

fn myfun(file: File) -> IoResult<()> {
    for line in file.lines() {
        let line = try!(line);
        // do stuff
    }
}

Delphi in a Nutshell: A Desktop Quick Reference, It is the programmer's responsibility to call IOResult to test the success or failure of each I/O procedure or Error Number Description 100 Read past end of file. Operations that attempt to read at the End-of-File fail, and thus both the eofbit and the failbit end up set. This function can be used to check whether the failure is due to reaching the End-of-File or to some other reason.


Looks like rust post 1.0 has changed IoError to std::io::Error. Also, kind is now hidden in the internals. I had to use a guard to work for a similar type issue (WouldBlock instead of EndOfFile).

e.g.

match some_method_that_might_block() {
    Ok(_) => {
        debug!("worked!");
    },
    Err(ref e @ IoError { kind: std::io::ErrorKind::WouldBlock, .. }) if e.kind() == std::io::ErrorKind::WouldBlock => {
        // ignore WouldBlock
    },
    Err(e) => { debug!("error={:?}", e); }
};

Modula-2 Applied, For example, it is wrong to attempt to open a file which does not exist. This is achieved in FIO by using the boolean IOcheck and the procedure IOresult. The end of the file is detected by the boolean EOF which is set true if after a read  How to detect an end of a file? You can either use the ifstream object ‘fin’ which returns 0 on an end of file or you can use eof() which is a member function of the ios class. It returns a non zero value on reaching the end of file.


For anyone reading this on a modern version of rust post 1.0, the docs now stipulate that reaching the end of a file while attempting to read from it is definitively not an error and should be represented with an Ok(0) (much like other programming languages/frameworks) rather than a hard error.

Since you can directly match against the number of bytes read (unlike std::io::error where the ErrorKind member is hidden and cannot be matched against directly), that makes handling it very easy:

        let mut buf: Vec<u8> = Vec::new();
        loop {
            let bytes_read = match file.read(&mut buf) {
                Ok(0) => break, // end-of-file
                Ok(n) => n,
                Err(e) => // handle error however you see fit,
            };

System.IOResult Function, int IOResult();. File. System. Description. IOResult returns the result of the last by Read, Readln, Eof, Eoln, SeekEof, or SeekEoln on a text file if the file is not To test the failure part of this example, just misspell a file name in the open dialog​. x = 0 with open('a.txt') as f: f.readlines() x = f.tell() f = open('a.txt','a') f.seek(x) f.write('Again Hello World') readlines() reads the entire file & reaches the end. f.tell() returns current location of the file pointer, which is at the end. To cross-validate,


End of File Function --C++, // You must attempt to read info prior to an eof( ) test. while (!fin.eof( )) //if not at end of file, continue reading numbers { cout<<data<<endl;  If no file F is specified, standard input is assumed. Note that calling this function may cause your program to wait: to determine whether you are at EOF, it is necessary to read data. If the file descriptor is not a real file (for instance for standard input or sockets), then this call may seem to hang the program while it is waiting for data to appear or for the file descriptor to be closed.


EOF, Eof returns True if the file-pointer has reached the end of the file, or if the file is empty. In the {$I-} state, use IOResult to check for errors. Open file for reading​. Use fopen to open the file. This function assigns a unique file id to use for reading and writing to the file. Read and display one line at a time until you reach the end of the file. Oranges and lemons, Pineapples and tea. Orangutans and monkeys, Dragonflys or fleas. Close the file.


File handling functions, Functions concerning input and output from and to file. Blockread, Read data from a file into memory. Blockwrite, Write data from memory to a file. Close, Close a file. Eof, Check for end of file IOresult, Return result of last file IO operation. This function returns the end of file location as an offset relative to the beginning of the file. Whenver you read a line in the file (or any portion of the file), check the offset from the read funtion to the value obtained from the EOF function. If the read offset is greater than or equal to the EOF offset, you have reached the end of file.