14.2.18 End of File and Errors

Once a file has been opened its status can be acquired. As an example the feof functions determines if the end of the file has been reached. This can be very useful when reading small parts of a file at a time. The following example shows how to read one line at a time from a file until the end has been reached.

filename = "myfile.txt";
fid = fopen (filename, "r");
while (! feof (fid) )
  text_line = fgetl (fid);
endwhile
fclose (fid);

Note that in some situations it is more efficient to read the entire contents of a file and then process it, than it is to read it line by line. This has the potential advantage of removing the loop in the above code.

: status = feof (fid)

Return 1 if an end-of-file condition has been encountered for the file specified by file descriptor fid and 0 otherwise.

Note that feof will only return 1 if the end of the file has already been encountered, not if the next read operation will result in an end-of-file condition.

See also: fread, frewind, fseek, fclear, fopen.

: msg = ferror (fid)

: [msg, err] = ferror (fid)

: […] = ferror (fid, "clear")

Query the error status of the stream specified by file descriptor fid.

If an error condition exists then return a string msg describing the error. Otherwise, return an empty string "".

The second input "clear" is optional. If supplied, the error state on the stream will be cleared.

The optional second output is a numeric indication of the error status. err is 1 if an error condition has been encountered and 0 otherwise.

Note that ferror indicates if an error has already occurred, not whether the next operation will result in an error condition.

See also: fclear, fopen.

: fclear (fid)

Clear the stream state for the file specified by the file descriptor fid.

See also: ferror, fopen.

: freport ()

Print a list of which files have been opened, and whether they are open for reading, writing, or both.

For example:

freport ()

     -|  number  mode  arch       name
     -|  ------  ----  ----       ----
     -|     0     r    ieee-le    stdin
     -|     1     w    ieee-le    stdout
     -|     2     w    ieee-le    stderr
     -|     3     r    ieee-le    myfile

See also: fopen, fclose, is_valid_file_id.