Writing Files in Co-array Fortran

Here’s the translation of the Go code to Co-array Fortran, formatted in Markdown suitable for Hugo:

Our first example demonstrates how to write files in Co-array Fortran. Writing files follows similar patterns to reading files.

program writing_files
  use iso_fortran_env
  implicit none

  character(len=:), allocatable :: content
  integer :: unit, iostat
  character(len=100) :: errmsg

  ! Write a string to a file
  content = "hello" // new_line('a') // "fortran" // new_line('a')
  call write_file("/tmp/dat1", content)

  ! Open a file for writing
  open(newunit=unit, file="/tmp/dat2", status="replace", action="write", iostat=iostat, iomsg=errmsg)
  if (iostat /= 0) then
    print *, "Error opening file: ", trim(errmsg)
    stop
  end if

  ! Write byte values
  write(unit, '(5A1)', advance='no') char(115), char(111), char(109), char(101), new_line('a')
  print *, "Wrote 5 bytes"

  ! Write a string
  write(unit, '(A)') "writes"
  print *, "Wrote 6 bytes"

  ! Flush the buffer
  flush(unit)

  ! Use a buffered write
  write(unit, '(A)') "buffered"
  print *, "Wrote 8 bytes"

  ! Close the file
  close(unit)

contains

  subroutine write_file(filename, content)
    character(len=*), intent(in) :: filename, content
    integer :: unit, iostat
    character(len=100) :: errmsg

    open(newunit=unit, file=filename, status="replace", action="write", iostat=iostat, iomsg=errmsg)
    if (iostat /= 0) then
      print *, "Error opening file: ", trim(errmsg)
      return
    end if

    write(unit, '(A)', iostat=iostat, iomsg=errmsg) content
    if (iostat /= 0) then
      print *, "Error writing to file: ", trim(errmsg)
    end if

    close(unit)
  end subroutine write_file

end program writing_files

This program demonstrates various ways to write files in Co-array Fortran:

We start by writing a string directly to a file using a custom subroutine write_file.
Then, we open a file for more granular writes. In Fortran, it’s common to use the open statement with newunit to get a new unit number automatically.
We write byte values to the file using the char function to convert integers to characters.
We demonstrate writing a string directly to the file.
The flush statement is used to ensure all buffered data is written to the file.
Finally, we show another string write, which could be considered a “buffered” write in the context of Fortran’s I/O system.
We close the file explicitly at the end.

To run this program:

$ gfortran -o writing_files writing_files.f90
$ ./writing_files
Wrote 5 bytes
Wrote 6 bytes
Wrote 8 bytes

Then check the contents of the written files:

$ cat /tmp/dat1
hello
fortran

$ cat /tmp/dat2
some
writes
buffered

Note that Co-array Fortran doesn’t have a direct equivalent to Go’s bufio package for buffered I/O. Fortran’s I/O system handles buffering internally, and the flush statement can be used to ensure data is written to the file.

Next, we’ll look at applying some of these file I/O ideas to standard input and output streams.

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Writing Files in Co-array Fortran