[Bug libfortran/59513] [4.8/4.9/4.10 Regression] Fortran runtime error: Sequential READ or WRITE not allowed after EOF marker, possibly use REWIND or BACKSPACE

[manfred99 at gmx dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59513

--- Comment #25 from Manfred Schwarb <manfred99 at gmx dot ch> ---
OK, thanks Jerry and Dominique for the explanations.

It seems the correct syntax then is:
      READ(lun,END=100) buffer
      GOTO 101
100   BACKSPACE(lun)
101   WRITE(lun,*) "whatever"

Not that the above code would make sense to me as a mere user,
but it works both with gfortran 4.5 and 4.6 upwards.

I just wrote a small test program, the output is identical for
gfortran 4.5 and 4.6+:
      CHARACTER*20 buffer,buffer2
      INTEGER i

      buffer=""
      buffer2=""
      OPEN(10,FILE="test.txt")
      WRITE(10,'(a)') "0123456789"
      CLOSE(10)

      OPEN(10,FILE="test.txt",POSITION="APPEND")
      CALL ftell(10,i)
      print*,"file position in append mode:",i
      CLOSE(10)

      OPEN(10,FILE="test.txt")
      READ(10,'(a20)') buffer
      print*,"X",buffer,"X"
      CALL ftell(10,i)
      print*,"file position after reading buffer:",i
CC      BACKSPACE(10)  ! then below write will overwrite first line
      WRITE(10,'(a)') "ABC"
      CLOSE(10)
      CALL system("cat test.txt")

      OPEN(10,FILE="test.txt")
      WRITE(10,'(a)') "0123456789"
      CLOSE(10)

      OPEN(10,FILE="test.txt")
      READ(10,'(a20)',END=100) buffer,buffer2
      GOTO 101
  100 CALL ftell(10,i)
      print*,"END clause encountered: file position:",i
      BACKSPACE(10)
  101 CALL ftell(10,i)
      print*,"file position:",i
      print*,"X",buffer,"X",buffer2,"X"
      WRITE(10,'(a)') "ABC"
      CLOSE(10)
      CALL system("cat test.txt")

      END