fortran, File access question (Fortran 77)

File access question (Fortran 77) - please e-mail answer

Author	Message
N8T #1 / 3	File access question (Fortran 77) - please e-mail answer In general, sequential files have variable length records, while direct access files have fixed length records which must be known to the program. On some systems, it may be possible to make a file which is formatted and happens to have fixed length records, which might be accessible both directly and sequentially, but this certainly isn't guaranteed by the standard. Tim
Mon, 07 Feb 2000 03:00:00 GMT

ROBERT GREE #2 / 3	File access question (Fortran 77) - please e-mail answer This is a multi-part message in MIME format. ------=_NextPart_000_01BCADA4.5DEBAF60 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit According to "fortran for Scientists and Engineers" by David Weinman: "In standard Fortran 77, it is not possible to create a sequential file that can later be accessed directly." Why would this be so? I'm more interested in the mechanism that prevents this access, than in the rationale behind it. So far as I know, the sequential and direct access files do not include, for example, header records that define their types, record size (in the case of Direct access) etc.. The reason isn't, presumably, the format (FORMATTED/UNFORMATTED) of the sequential file, since that is really just an interpretation of the data in the file. Since I have "inherited" a FORMATTED (ASCII) file that won't open (with Lahey ELF90) by Direct access, I suppose it must be Sequential access. But I just don't see how the compiler or run-time module etc. can tell the difference. - Bob Greer Bear Mountain Cartography ------=_NextPart_000_01BCADA4.5DEBAF60 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable <html><head></head><BODY bgcolor=3D"#FFFFFF"><p><font size=3D2 = color=3D"#000000" face=3D"Bookman Old Style">According to =  "FORTRAN for Scientists and Engineers" by David = Weinman:<br><br>"In standard Fortran 77, it is not possible to = create a sequential file that can later be accessed = directly."<br><br><br>Why would this be so?   I'm more = interested in the mechanism that prevents this access, than in the = rationale behind it.  So far as I know, the sequential and direct = access files do not include, for example, header records that define = their types,  record size (in the case of Direct access) etc.. =   The reason isn't, presumably, the format = (FORMATTED/UNFORMATTED) of the sequential file, since that is really = just an interpretation of the data in the file.<br><br>Since I have = "inherited" a FORMATTED (ASCII) file that won't open (with = Lahey ELF90) by Direct access, I suppose it must be Sequential access. =  But I just don't see how the compiler or run-time module etc. can = tell the difference.<br><br>-  Bob Greer<br>   Bear = </font></body></html> ------=_NextPart_000_01BCADA4.5DEBAF60--
Mon, 07 Feb 2000 03:00:00 GMT

James Gile #3 / 3	File access question (Fortran 77) - please e-mail answer Quote: > According to "FORTRAN for Scientists and Engineers" by David Weinman: > "In standard Fortran 77, it is not possible to create a sequential file > that can later be accessed directly." > Why would this be so? I'm more interested in the mechanism that prevents > this access, than in the rationale behind it. [...] There is no mechanism as such. The problem is that sequential files may contain records of varying length. Trying to access those records directly would be difficult. The way direct access works is to position the file by multiplying the record length by the record number given in the I/O request. If the first N records are not all of the same length, the direct I/O positioning to the Nth record will be out of sync. Rather that try to hash out all the conditions and proper responses to this possibility, the committee just chose to disallow it. Having actually written an I/O support library for Fortran which allowed direct and sequential accesses to be mixed, I can attest that it's not a trivial task. -- J. Giles Ricercar Software
Mon, 07 Feb 2000 03:00:00 GMT

Page 1 of 1

[ 3 post ]