Operations on external files

Last updated on 2026-02-21 | Edit this page

Overview

Questions

  • How do I open and close files for input and output?
  • How do I handle any errors that might occur during I/O?

Objectives

  • Learn how to open and close files for input and output using a unit number.
  • Be able to read and write to an open file and handle errors that might arise.
  • Move around an open file using read to move forwards and backspace() and rewind() to move back.

Operations on external files

Writing data to, or reading data from, an external file is an essential part of a useful application.

Writing a file

File handles, or unit numbers are obtained using open(), and are used to direct the output of write() to the relevant file.

FORTRAN 

  integer :: myunit

  open(newunit = myunit, file = 'filename.dat', form = 'formatted', &
       action = 'write', status = 'new')

  write(unit = myunit, *) data1    ! write data first record
  write(unit = myunit, *) data2    ! second record ... and so on

  close(unit = myunit, status = 'keep')

To read back the same data, we might use:

FORTRAN 

   open(newunit = myunit, file = 'filename.dat', form = 'formatted', &
        action = "read", status = "old")

   read(unit = myunit, *) data1
   read(unit = myunit, *) data2

   close (unit = myunit, status = 'keep')

Unit numbers

A valid unit number can be assigned by using the newunit option to open() – this has been available since Fortran 2008. There is no need to choose your own (and it can be error-prone to do so).

Note that a given file can only be connected to one unit number at any given time.

open()

The arguments seen above include:

  • file: literal string or character variable or expression with the name of the file in the file system;
  • form: formatted or unformatted;
  • action: read, write or readwrite. An error may occur if inappropriate actions are performed;
  • status: one of old, new, replace, scratch, or unknown.

A file with status old is expected to exist, while an attempt to create a new file when one already exists with the same name will result in an error. If replace is specified, any existing file will be overwritten by a new file. If scratch is specified, a temporary file is created which will be deleted when close() is executed (or at the end of the program). The system will automatically choose a name for a scratch file if no file argument is present. The default status is unknown, which means the status is system dependent.

Formally, only the unit number is mandatory (and must appear first), but there is no reason not to provide as much information as possible.

close()

The unit number is again mandatory. The status is either keep or delete. If the status is omitted, the default is keep, except for scratch files.

inquire()

The inquire statement offers a way to obtain information on the current state of either unit numbers or files. There are a large number of optional arguments. One common usage is to check whether a file exists:

FORTRAN 

  logical :: exists
  inquire( file = 'filename.dat', exist = exists)

Internal files

In some situations, it may be convenient to use a formatted read to generate a new string in memory (in the same way as sprintf in C). Fortran uses a so-called internal file, which is usually a character string. E.g.,

FORTRAN 

   character (len = 10) :: buffer
   integer              :: ival
   ...
   read(buffer, fmt = "i10") ival

Here buffer takes the place of the input unit. This can be used, e.g., to create format strings at run time.

Recovery from errors

Operations on external files can be error-prone. While there is no formal exception mechanism in Fortran, some ability to recover is available.

Consider the following schematic example:

FORTRAN 

subroutine read_my_file_format(myunit, ..., ierr)

  integer, intent(in)   :: myunit
  integer, intent(out)  :: ierr          ! error code

  character (len = 256) :: msg

  read (myunit, ..., err = 999, iomsg = msg) ...

  ! Everything was ok
  ierr = 0
  return

999 continue
  ierr = -1
  print *, "Error reading file: ", trim(msg)

  return
end subroutine real_my_file_format

We assume the relevant file has been opened successfully, and is connected to unit number myunit. If an error occurs at the point of the read statement, the err argument directs control to be transferred to the statement will label 999. In this case the system should provide a meaningful message describing the error.

If the read is successful, the routine completes at the first return statement with intent out ierr = 0.

Error handling for open() and close()

Both open and close statements provide optional arguments for error handling, illustrated schematically here with open():

FORTRAN 

  integer :: ierr
  character (len = 128) :: msg

  open( newunit = myunit, ..., err = 900, iostat = ierr, iomsg = msg)
  • err: is a label in the same scope to which control is transferred on error;
  • iostat: an integer variable which is zero on success but positive if an error has occurred;
  • iomsg: if an error occurs, a system-dependent message will be assigned to msg.

If neither err nor iostat arguments are present, then an error may result in immediate termination of the program.

The variable msg should be a scalar character variable; the message will be truncated or padded appropriately.

Error handling for read() and write()

Error handling facilities for read() and write() are similar, and are illustrated here:

FORTRAN 

  write (myunit, myformat, end = 999, err = 998, iostat = ierr, iomsg = msg) ..
  • end: label in same scope to which control is transferred on end-of-file;
  • err: label in same scope to which control is transferred on any other error; the label may be the same as end;
  • iostat: the integer error code is negative if end-of-record or end-of-file, or positive if another error (e.g., bad format conversion) or zero on success;
  • iomsg: should return a useful message on error.

The two negative iostat cases can be distinguished via calls to the intrinsic functions

FORTRAN 

  is_iostat_end(ierr)
  is_iostat_eor(ierr)

stop statement

If one really can’t continue, then execution can be terminated immediately via the stop statement. This has an optional message string argument.

FORTRAN 

  stop "Cannot continue"

In general one should try to recover by returning control to the caller, so stop is a last resort.

Moving around an open file

It is sometimes useful to be able to reposition oneself in an open file so that a given record can be read (or written) more than once. This can be done using backspace() with the relevant unit number. Formally

FORTRAN 

  backspace([unit = ] unit-number [, iostat = iostat] [, err = label])

This steps back one record.

It is also possible to reposition to the beginning of the file, again with the relevant connected unit number using rewind().

FORTRAN 

  rewind([unit = ] unit-number [, iostat = iostat] [, err = label])

Exercise (20 minutes)

Challenge

Writing a simple image

Portable bit map (PBM, PGM, PPM) is a very simple image format which can be expressed as an ASCII file. See the description at Wikipedia.

In the template program1.f90, we establish a two-dimensional logical array of a fixed size, and we want to write out a file that can be viewed in “P1” format (file extension .pbm ) by an image viewer.

The data is initialised with a suitable pattern to check that the image is correct (it is the right way up and is not a mirror). A little care is required in the rows and columns (look closely at the example).

Provide a module with the subroutine write_pbm() with three arguments as in the template program. Note the template program expects a module solution_module.

Additional exercises: repeat for integer data (to “P2” format .pgm) and floating point data (to “P3” format .ppm).

You can check any image you produce by either downloading it to your machine with scp or rsync, or you can view it directly on ARCHER2 by logging in with X11 forwarding enabled, running module load imagemagick and then display test49.pbm.

A solution to this problem appears in the later episode on interfaces.

You should be able to produce an output PBM that looks like the example shown below.

Sample PBM output.
Key Points
  • Writing data to, or reading data from, an external file is an essential part of a useful application.
  • Unit numbers are integers which are used as handles to open files.
  • Errors from I/O can be handled with labels and by examining the returned message.