问题
I have an unformatted Fortran file, containing strings of different lengths, and I'm having trouble reading these strings with Fortran itself.
Sample program:
program test
implicit none
character(len=200) :: line
open(32,file="testfile",form="unformatted",action="write")
write(32) "A test string"
write(32) "Another longer test string"
close(32)
open(33,file="testfile",form="unformatted",action="read")
read(33) line
write(6,*) trim(line)
read(33) line
write(6,*) trim(line)
close(33)
end program test
This fails (compiled with gfortran) with:
At line 11 of file test.f90 (unit = 33, file = 'testfile')
Fortran runtime error: I/O past end of record on unformatted file
I could get it to work by trying the read with decreasing lengths and backspacing (read_string subroutine), but this looks highly inefficient:
program test
implicit none
character(len=200) :: line
open(32,file="testfile",form="unformatted",action="write")
write(32) "A test string"
write(32) "Another longer test string"
close(32)
open(33,file="testfile",form="unformatted",action="read")
call read_string(33,line)
write(6,*) trim(line)
call read_string(33,line)
write(6,*) trim(line)
close(33)
contains
subroutine read_string(u,string)
integer, intent(in) :: u
character(len=*), intent(out) :: string
integer :: i, error
do i=len(string),0,-1
read(u,iostat=error) string(:i)
if (error == 0) then
string(i+1:) = ''
exit
end if
backspace(u)
end do
end subroutine read_string
end program test
Is there a better way to read variable-length strings from unformatted files?
回答1:
I slightly reworked your sample program, reading the file in binary. This works for Intel's compiler; gfortran does not know the binary format, so ymmv. See where I got my ideas at Intel's reference on record types
program test
implicit none
character(len=200) :: line
integer(4) recl_at_start, recl_at_end
open(32,file="testfile",form="unformatted",action="write")
write(32) "A test string"
write(32) "Another longer test string"
close(32)
! initialization is required to fill the line with blanks
! because trim() does not work on line filled with zero characters
line = ""
open(33,file="testfile",form="binary",action="read")
read(33) recl_at_start
read(33) line(1:recl_at_start)
read(33) recl_at_end
write(6,*) trim(line)
read(33) recl_at_start
read(33) line(1:recl_at_start)
read(33) recl_at_end
write(6,*) trim(line)
close(33)
end program test
its output is
A test string
Another longer test string
Now that you know the line length, trim() is not really necessary anymore. Just use
write(6,*) line(1:recl_at_start)
This also prevents troubles when you add "A shorter test string" to the data.
来源：https://stackoverflow.com/questions/19464417/reading-strings-from-unformatted-files-variable-record-length