Retrieve data from file written in FORTRAN during program run - fortran

Question

I am trying to write a series of values for time (real values) into a dat file in FORTRAN. This is a part of an MPI code and the code runs for a long time. So I would like to extract data at every time step and print it into a file and read the file any time during the execution of the program. Currently, the problem I am facing is, the values of time are not written into the file until the program ends. I have put the open statement before the do loop and the close statement after the end of do loop.
The parts of my code look like:
open(unit=57,file='inst.dat')
do loop starts
.
.
.
write(57,*) time
.
.
.
end do
close(57)

Answer 1

try call flush(unit). Check your compiler docs as this is i think an extension.
You mention MPI: For parallel codes I think you need to give each thread its own file/unit,
or take other measures to avoid conflicts.

Answer 2

From Gfortran manual:
Beginning with the Fortran 2003 standard, there is a FLUSH statement that should be preferred over the FLUSH intrinsic.
The FLUSH intrinsic and the Fortran 2003 FLUSH statement have identical effect: they flush the runtime library's I/O buffer so that the data becomes visible to other processes. This does not guarantee that the data is committed to disk.
On POSIX systems, you can request that all data is transferred to the storage device by calling the fsync function, with the POSIX file descriptor of the I/O unit as argument (retrieved with GNU intrinsic FNUM). The following example shows how:
! Declare the interface for POSIX fsync function
interface
function fsync (fd) bind(c,name="fsync")
use iso_c_binding, only: c_int
integer(c_int), value :: fd
integer(c_int) :: fsync
end function fsync
end interface
! Variable declaration
integer :: ret
! Opening unit 10
open (10,file="foo")
! ...
! Perform I/O on unit 10
! ...
! Flush and sync
flush(10)
ret = fsync(fnum(10))
! Handle possible error
if (ret /= 0) stop "Error calling FSYNC"

Answer 3

How about closing the file after every time step (assuming a reasonable amount of time elapses between time steps)?
do loop starts
.
.
!Note: an if statement should wrap the following so that it is
!only called by one processor.
open(unit=57,file='inst.dat')
write(57,*) time
close(57)
.
.
end do
Alternatively if the time between time steps is short, writing the data after blocks of 10, 100, ... iterations may be more efficient.