Opening multiple files in Fortran 90

Question

I would like to open 10,000 files with file names starting from abc25000 until abc35000 and copy some information into each file. The code I have written is as below:

PROGRAM puppy
IMPLICIT NONE

integer :: i
CHARACTER(len=3) :: n1
CHARACTER(len=5) :: cnum
CHARACTER(len=8) :: n2

loop1: do i = 25000 ,35000  !in one frame

  n1='abc'
  write(cnum,'(i5)') i
  n2=n1//cnum
  print*, n2
  open(unit=i ,file=n2)

enddo loop1

end

This code is supposed to generate files starting from abc24000 until abc35000 but it stops about half way saying that

At line 17 of file test-openFile.f90 (unit = 26021, file = '')

Fortran runtime error: Too many open files

What do I need to do to fix the above code?

Answer 1

This limit is set by your OS. If you're using a Unix/Linux variant, you can check the limit using from the command line using ulimit -n, and raise it using ulimit -n 16384. You'll need to set a limit greater than 10000 to allow for all the other files that the shell will have open. You may also need admin privileges to do this.

I regularly bump the limit up to 2048 to run Fortran programs, but never as high as 10000. However, I echo the other answers that, if possible, it's better to restructure your program to close each file before opening the next.

Answer 2

You need to work on the files one at a time (or in small groups that do not exceed the limitation imposed by the operating system).

for each file:
  open file
  write
  close file