11.6.1.2142
REPOSITION-FILE
 
FILE
 
( ud fileid -- ior )

Reposition the file identified by fileid to ud. ior is the implementation-defined I/O result code. An ambiguous condition exists if the file is positioned outside the file boundaries.

At the conclusion of the operation, FILE-POSITION returns the value ud.

Testing:
: line2 S" Line 2 blah blah blah" ;
: rl1 buf 100 fid1 @ READ-LINE ;
2VARIABLE fp

T{ fn1 R/W OPEN-FILE SWAP fid1 ! -> 0 }T
T{ fid1 @ FILE-SIZE DROP fid1 @ REPOSITION-FILE -> 0 }T
T{ fid1 @ FILE-SIZE -> fid1 @ FILE-POSITION }T

T{ line2 fid1 @ WRITE-FILE -> 0 }T
T{ 10. fid1 @ REPOSITION-FILE -> 0 }T
T{ fid1 @ FILE-POSITION -> 10. 0 }T

T{ 0. fid1 @ REPOSITION-FILE -> 0 }T
T{ rl1 -> line1 SWAP DROP <TRUE> 0 }T
T{ rl1 -> ROT DUP #chars ! }T<TRUE> 0 line2 SWAP DROP
T{ buf #chars @ line2 COMPARE -> 0 }T
T{ rl1 -> 0 <FALSE> 0 }T

T{ fid1 @ FILE-POSITION ROT ROT fp 2! -> 0 }T
T{ fp 2@ fid1 @ FILE-SIZE DROP D= -> <TRUE> }T
T{ S" " fid1 @ WRITE-LINE -> 0 }T
T{ S" " fid1 @ WRITE-LINE -> 0 }T
T{ fp 2@ fid1 @ REPOSITION-FILE -> 0 }T
T{ rl1 -> 0 <TRUE>  0 }T
T{ rl1 -> 0 <TRUE>  0 }T
T{ rl1 -> 0 <FALSE> 0 }T
T{ fid1 @ CLOSE-FILE -> 0 }T