templon@silver.bacs.indiana.edu (jeffrey templon) (08/23/89)
I would like to know one simple (I hope) thing: what happens if you set the value of a variable (local) in a subroutine, exit the subroutine, and come back later? Is it the same value, zero, or worse? I read all TFMs I could find and saw no answer to this one. I am using VAX Fortran, whatever version runs on VMS V4.6. Thanks for any help you can send. Jeff Templon
ghe@nucthy.physics.orst.edu (Guangliang He) (08/23/89)
In article <24891@iuvax.cs.indiana.edu> templon@silver.bacs.indiana.edu (jeffrey templon) writes: = =I would like to know one simple (I hope) thing: what happens if you set =the value of a variable (local) in a subroutine, exit the subroutine, and =come back later? Is it the same value, zero, or worse? I read all TFMs =I could find and saw no answer to this one. I am using VAX Fortran, =whatever version runs on VMS V4.6. Thanks for any help you can send. = = Jeff Templon As my understanding, Fortran 77 standard does NOT save the value of a local variable unless you use save in the subroutine. But I am not very sure about the detail. A lot of compilers do save the value of local variables. The Unix f77 does it. But I have heard the oppesite case. The Mac Fortran doesn't save them. You should read the manual carefully to see how VAX Fortran does these kind things. ----------------------------------------------------------------------- | USMAIL: Guangliang He | INTERNET: ghe@PHYSICS.ORST.EDU Department of Physics | BITNET: hegl@ORSTVM.BITNET Oregon State University | Corvallis, OR 97331 | PHONE: (503) 737-4631 | -----------------------------------------------------------------------
khb@road.Sun.COM (Keith Bierman - Advanced Languages - Floating Point Group ) (08/23/89)
In article <24891@iuvax.cs.indiana.edu> templon@silver.bacs.indiana.edu (jeffrey templon) writes: > >I would like to know one simple (I hope) thing: what happens if you set >the value of a variable (local) in a subroutine, exit the subroutine, and >come back later? Is it the same value, zero, or worse? I read all TFMs >I could find and saw no answer to this one. I am using VAX Fortran, >whatever version runs on VMS V4.6. Thanks for any help you can send. > > Jeff Templon Well, if you read the standards document (:> turgid prose at best), you will find that if you declare your variable subroutine sub(stuff) real your_var ! extended fortran, for long var name and underscore save your_var ! not to mention lower case and ! the standard assures you that your_var has the result from the last invocation. If you don't employ save, vendor gets to do whatever they want. DEC VMS FORTRAN happens to make all variables SAVEd for you anyway. UNIVAC (used to) gives you junk. Sun (well me) strongly advises use of the SAVE, if you really want your old value back. I know of no one who promises you zero ... many machines (dec, sun) give you zero the first time through ... second time zero is about the least likely number. -- Keith H. Bierman |*My thoughts are my own. !! kbierman@sun.com It's Not My Fault | MTS --Only my work belongs to Sun* I Voted for Bill & | Advanced Languages/Floating Point Group Opus | "When the going gets Weird .. the Weird turn PRO" Keith H. Bierman |*My thoughts are my own. !! kbierman@sun.com It's Not My Fault | MTS --Only my work belongs to Sun* I Voted for Bill & | Advanced Languages/Floating Point Group Opus | "When the going gets Weird .. the Weird turn PRO"
jkw@alpha.lanl.gov (Jay Wooten) (08/24/89)
In article <24891@iuvax.cs.indiana.edu>, templon@silver.bacs.indiana.edu (jeffrey templon) writes: > > I would like to know one simple (I hope) thing: what happens if you set > the value of a variable (local) in a subroutine, exit the subroutine, and > come back later? Is it the same value, zero, or worse? I read all TFMs > I could find and saw no answer to this one. I am using VAX Fortran, > I could find and saw no answer to this one. I am using VAX Fortran, > whatever version runs on VMS V4.6. Thanks for any help you can send. > > Jeff Templon Look in the "Programming in VAX FORTRAN" manual under the description of the SAVE statement (pp. 8-24 and 8-25 in the 4.0 version): "The SAVE statement causes the definition of data entities to be retained after execution of a RETURN or END statement in a subprogram." ... "NOTE: It is not necessary to use SAVE statements in VAX FORTRAN programs. The definitions of data entities are retained automatically by VAX FORTRAN, making the use of SAVE statements a redundant exercise [unless you want to port your program to a non-VMS system]." In my experience, variables in a VMS FORTRAN subprogram are always the same values as you left them the last time. ~~~~~~ Inquiring minds want to know ! ~~~~~~ Jay Wooten Los Alamos National Lab ARPA: jkw@lanl.gov
urjlew@ecsvax.UUCP (Rostyk Lewyckyj) (08/24/89)
In a similar vein to saving values of local variables in subroutines
across invocations. What does the standard and experience say
about saving of array shape parameters passed in through an ENTRY
statement
Subroutine xyz
dimension a(m,n)
......
x=a(i,j)
.....
return
ENTRY setup(a,m,n)
return
end
Setup is called once from some point in the program.
xyz is called many times from various points in the program,
not necessarily from the same routine that calls setup.
THis works on IBM 370 type systems, MVS, VM with VS Fortran
compiler, at least in programs where xyz is not overlaid/reloaded.
This is/was a way of improving performance by eliminating the
need to pass a,m,n through all the paths needed to include it
in the xyz call, and also avoiding the additional complexity of
getting it done correctly.
-----------------------------------------------
Reply-To: Rostyslaw Jarema Lewyckyj
urjlew@ecsvax.UUCP , urjlew@unc.bitnet
or urjlew@uncvm1.acs.unc.edu (ARPA,SURA,NSF etc. internet)
tel. (919)-962-9107mike@hpfcdc.HP.COM (Mike McNelly) (08/24/89)
If the compiler supports recursion (not in standard FORTRAN 77), it is unlikely that you can count on implicit value saving from one subprogram invocation to the next. Many (most?) implementations that support recursion use a separate stack frame for each function activation. Sometimes these stack frames align from one invocation to the next but there's certainly no guarantee that this will always be the case. If you need to save values use the standard SAVE statement. It will save you considerable grief should you ever port your program to a new architecture. Mike McNelly mike%hpfcla@hplabs.hp.com