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(guile.info)Random Access

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Random Access
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 - primitive: seek object offset whence
     Sets the current position of FD/PORT to the integer OFFSET, which
     is interpreted according to the value of WHENCE.

     One of the following variables should be supplied for WHENCE:

      - Variable: SEEK_SET
          Seek from the beginning of the file.

      - Variable: SEEK_CUR
          Seek from the current position.

      - Variable: SEEK_END
          Seek from the end of the file.

     If FD/PORT is a file descriptor, the underlying system call is
     `lseek'.  PORT may be a string port.

     The value returned is the new position in the file.  This means
     that the current position of a port can be obtained using:
          (seek port 0 SEEK_CUR)

 - primitive: fseek object offset whence
     Obsolete.  Almost the same as seek, above, but the return value is
     unspecified.

 - primitive: ftell object
     Returns an integer representing the current position of FD/PORT,
     measured from the beginning.  Equivalent to:
          (seek port 0 SEEK_CUR)

 - primitive: truncate-file object [length]
     Truncates the object referred to by OBJ to at most SIZE bytes.
     OBJ can be a string containing a file name or an integer file
     descriptor or a port.  SIZE may be omitted if OBJ is not a file
     name, in which case the truncation occurs at the current port.
     position.

     The return value is unspecified.