Manual Reference Pages - backspace (7fortran)
NAME
backspace(7f) - [IO:FILE POSITIONING] - backspace one record on specified I/O unit
SYNOPSIS
BACKSPACE file-unit-number
BACKSPACE([UNIT=]unit-number][,IOMSG=iomsg][,IOSTAT=iostat][,ERR=label])
DESCRIPTION
backspace(7f) positions the specified file back to the beginning of the current record or if already at the beginning of a record, back to the beginning of the previous record.
If the file is at its initial point, the position of the file is not changed.
It is most often used when a program has partially read a line and then wants to go back and reread the line using the information from the previous read(7f),
backspace(7f) is rarely used in new code as the subsequent addition of Fortran features such as non-advancing I/O and internal reads into a CHARACTER variable (which can be read from multiple times) are typically far more efficient and provide much of the same functionality when re-reading the current line.
Backspacing is very inefficient on many current platforms. Reading a file with stream-I/O and indexing relevant line positions to return to; or using direct-access files is far more efficient than backspacing through a file when moving back large numbers of lines on Linux and Unix platforms.
A unit open for direct access or unformatted access cannot be referenced by backspace(7f). backspace(7f) only works with formatted sequential files that may be repositioned. So it does not generally work with standard input from a terminal, pipes, and other formatted sequential file types that cannot be rewound or positioned.
Backspacing over records written using list-directed or namelist formatting is prohibited. It will usually work, but since the compiler is free to write list-directed or namelist output on a varying number of lines it is not supported, as it is not certain what data is on which line unless the program itself searches for particular strings.
Backspacing a file that is connected but does not exist is prohibited.
If a BACKSPACE statement causes the implicit writing of an endfile record, the file is positioned before the record that precedes the endfile record.
If the preceding record is an endfile record, the file is positioned before the endfile record.
OPTIONS
UNIT : unit number of file to backspace one line on. A unit open for direct access or unformatted access cannot be referenced by a BACKSPACE. IOSTAT : a compiler-specific number that indicates an error occurred if non-zero. IOMSG : a message describing error IOSTAT if IOSTAT is not zero. ERR : a label number to jump to if an error occurs
EXAMPLE
An example of a BACKSPACE statement is:
program demo_backspace implicit none character(len=256) :: line character(len=256) :: mssge integer :: i integer :: j integer :: ios integer,allocatable :: iarr(:)Results:! create a basic sequential file open(10,file=’dem_backspace.txt’) ! open a file do i=1,30 ! write lines to it write(10,’(a,i3,*(i3))’) ’line ’,i, (j,j=1,i) enddo
! back up several lines do i=1,14 backspace(10, iostat=ios,iomsg=mssge) if(ios.ne.0)then write(*,’(*(a))’) ’*dem_backspace* ERROR:’,mssge endif enddo read(10,’(a)’)line write(*,*)’back at a previous record !’
! read line as a string write(*,’("string=",a)’)trim(line)
! backspace so can read again as numbers backspace(10) ! read part of a line numerically to get size of array to read read(10,’(5x,i3)’)i allocate(iarr(i))
! reread line just reading array backspace(10) read(10,’(8x,*(i3))’)iarr write(*,’(*(g0,1x))’)’size=’,i,’array=’,iarr
!! Note: writing a new line will truncate file !! to current record position
close(10,status=’delete’)
end program demo_backspace
> back at a previous record ! > string=line 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 > size= 17 array= 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17Fortran intrinsic descriptions (license: MIT) @urbanjost
| Nemo Release 3.1 | backspace (7fortran) | August 19, 2024 |
