From 9198465745658212016 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 10f5bc,6f6e97a6fdbbf44c X-Google-Attributes: gid10f5bc,public X-Google-Thread: 109fba,6f6e97a6fdbbf44c X-Google-Attributes: gid109fba,public X-Google-Thread: 102b75,6f6e97a6fdbbf44c X-Google-Attributes: gid102b75,public X-Google-Thread: f78e5,6f6e97a6fdbbf44c X-Google-Attributes: gidf78e5,public X-Google-Thread: 1014db,6f6e97a6fdbbf44c X-Google-Attributes: gid1014db,public X-Google-Thread: 110292,6f6e97a6fdbbf44c X-Google-Attributes: gid110292,public From: tanmoy@qcd.lanl.gov (Tanmoy Bhattacharya) Subject: Re: And you thought that the Pentium bug was bad! (Get a load of this!) Date: 1995/06/20 Message-ID: <3s5pke$iet@newshost.lanl.gov>#1/1 X-Deja-AN: 104726347 distribution: world references: <3s2las$1br@hustle.rahul.net> <3s4h92$hpf@hacgate2.hac.com> followup-to: comp.lang.c organization: Los Alamos National Laboratory newsgroups: comp.std.c++,comp.lang.c,comp.lang.c++,comp.sys.intel,comp.arch,comp.software.testing Followup set to comp.lang.c: comp.std.c omitted from the huge list of groups because it has already been discussed there. In article <3s4h92$hpf@hacgate2.hac.com>, collins@thor.tu.hac.com (Ron Collins) writes: |> : double d = 0.9; |> : |> : int |> : main () |> : { |> : double d = 0.8; |> : { |> : extern double d; |> : |> : printf ("%f\n", d); |> : return 0; |> : } |> : } |> : ------------------------------------------------------------------------- |> |> : This program *should* print 0.900000, and with most compilers, it does. |> |> Maybe I'm missing the point here... but it seems to me that the program |> *should* print 0.8 -- since the "d" defined outside the block is initialized |> to 0.8. I don't have any books with me, but is "extern" *required* to |> scope outside of the function? (I thought it just had to scope outside |> the current block). The code should print 0.9 because the d relevant to the printf is the declaration that says `extern double d;'. When the compiler meets a definition that specifies extern, it asks if a previous declaration is visible. (In this case, the answer is yes: the `double d = 0.8;' is visible at that moment.) If a declaration is visible, it next asks if that declaration has internal or external linkage. (In this case, the answer is no: that declaration is block scope declaration that does not specify extern: so it is a declaration of an object with no linkage). If it has external or internal linkage, the new declaration specifies the same linkage; otherwise it specifies external linkage. (So this d has external linkage). The file scope `double d = 0.9;' does not specify static: at file scope absence of static is the same as specifying extern. Following the logic mentioned above, this d also has external linkage. Now every declaration of a variable with no linkage specifies a unique object: different from all others during its lifetime. All external declarations of the same name specify the same object. All internal declarations (none here) of teh same name _in a given translation unit_ (roughly source file) specify the same object. So, in this case the d being printed is the same as the d being initialized to 0.9; not the d being initialized to 0.8. (The details of the rules for interpretation of extern has been changed by the Technical Corrigendum: I described the new rules. For this code, it does not matter anyway.) Cheers Tanmoy -- tanmoy@qcd.lanl.gov(128.165.23.46) DECNET: BETA::"tanmoy@lanl.gov"(1.218=1242) Tanmoy Bhattacharya O:T-8(MS B285)LANL,NM87544-0285,USA H:#3,802,9 St,NM87545 Others see , or. -- fax: 1 (505) 665 3003 voice: 1 (505) 665 4733 [ Home: 1 (505) 662 5596 ]