1.4.13 fseek, ftell: Determine Position and Reposition a File (Sun Studio 12: Fortran Library Reference)

Sun Studio 12: Fortran Library Reference

1.4.13 `fseek`, `ftell`: Determine Position and Reposition a File

fseek and ftell are routines that permit repositioning of a file. ftell returns a file’s current position as an offset of so many bytes from the beginning of the file. At some later point in the program, fseek can use this saved offset value to reposition the file to that same place for reading.

1.4.13.1 `fseek`: Reposition a File on a Logical Unit

The function is called by:

*`INTEGER4 fseek` `n` `= fseek(` `lunit``,` `offset``,` `from` `)`**
`lunit`	*`INTEGER4`**	Input	Open logical unit
`offset`	*`INTEGER4` or `INTEGER8`*	Input	Offset in bytes relative to position specified by from
	An *`INTEGER8` offset value is required when compiled for a 64-bit environment. If a literal constant is supplied, it must be a 64-bit constant, for example: `100_8`**
`from`	*`INTEGER4`**	Input	0=Beginning of file 1=Current position 2=End of file
Return value	*`INTEGER4`**	Output	`n`=0: OK; `n`>0: System error code

Note –

On sequential files, following a call to fseek by an output operation (for example, WRITE) causes all data records following the fseek position to be deleted and replaced by the new data record (and an end-of-file mark). Rewriting a record in place can only be done with direct access files.

Example: fseek()—Reposition MyFile to two bytes from the beginning

        INTEGER*4 fseek, lunit/1/, offset/2/, from/0/, n
        open( UNIT=lunit, FILE=’MyFile’ )
        n = fseek( lunit, offset, from )
        if ( n .gt. 0 ) stop ’fseek error’
        end

Example: Same example in a 64-bit environment and compiled with -m64:

        INTEGER*4 fseek, lunit/1/,  from/0/, n
        INTEGER*8 offset/2/
        open( UNIT=lunit, FILE=’MyFile’ )
        n = fseek( lunit, offset, from )
        if ( n .gt. 0 ) stop ’fseek error’
        end

1.4.13.2 `ftell`: Return Current Position of File

The function is called by:

INTEGER*4 ftell

n = ftell( lunit )

lunit

INTEGER*4

Input

Open logical unit

Return value

INTEGER*4

Output

n>=0: n=Offset in bytes from start of file

n<0: n=System error code

Example: ftell():

       INTEGER*4 ftell, lunit/1/, n
       open( UNIT=lunit, FILE=’MyFile’ )
       ...
       n = ftell( lunit )
       if ( n .lt. 0 ) stop ’ftell error’
       ...

Example: Same example in a 64-bit environment and compiled with -m64:

       INTEGER*4 lunit/1/
       INTEGER*8 ftell, n
       open( UNIT=lunit, FILE=’MyFile’ )
       ...
       n = ftell( lunit )
       if ( n .lt. 0 ) stop ’ftell error’
       ...

See also fseek(3S) and perror(3F); also fseeko64(3F) ftello64(3F).