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On 11 feb, 15:57, Ben Bacarisse <ben.use...@bsb.me.uk> wrote:
> David RF <davran...@gmail.com> writes:
> > On 11 feb, 10:51, Mateusz_madi <madi.cz...@gmail.com> wrote:
> >> On 11 Lut, 10:17, David RF <davran...@gmail.com> wrote:
> >> > On 11 feb, 09:58, Mateusz_madi <madi.cz...@gmail.com> wrote:
> <snip>
> >> > > a.c
> >> > > ---------
> >> > > #include<stdio.h>
>
> >> > > int a=3;
>
> >> > > void drukuj(){
> >> > > Â* Â* Â* Â* printf("%d", a);
>
> >> > > }
>
> >> > > -----------
> >> > > b.c
> >> > > -----------
>
> >> > > #include<stdio.h>
>
> >> > > int a;
>
> >> > > int main()
> >> > > {
> >> > > Â* Â* Â* Â* a=8;
> >> > > Â* Â* Â* Â* drukuj();}
>
> >> > > ---------
>
> >> > > Why it prints 8 not 3.
>
> >> > Because a have global scope
>
> >> > Declare a as static in file b.c
> >> > static int a;- Ukryj cytowany tekst -
>
> >> > - Pokaż cytowany tekst -
>
> >> There is something i don't understand about global variables. I found
> >> in many pages that:
> >> Â*"Global - All the modules in the project - Â*The life of the program
> >> execution "
> >> buth if i declare global let's say g in one module and would like to
> >> increment it in second module it is impossible, so where is that
> >> global (All the modules in the project) scope?
>
> > Ok, I try to explain with my poor english
>
> > both a in a.c and b.c point to same location and have global scope, a
> > in b.c is redeclared, all variables declared without static duration
> > outside a function have global scope
>
> That's not really the problem. Â*The program is undefined because it has
> two external definitions of an object with the same name.
>
> Unfortunately your terms are rather confused. Â*Normally this does not
> matter, but in this case using the right terms helps to get everything
> clear.
>
> In C, a named object is characterised by three properties: the scope of
> the name, the lifetime of the object and the linkage of the name. Â*These
> are related but also to some extent independent.
>
> An external definition of an object (one that sits outside of any
> function) gives it static duration -- even if it does not use the word
> static! Â*If an external declaration does use the keyword "static", this
> alters the linkage but not the lifetime.
>
> C does not have global variables in the same sense that some other
> languages do. Â*Normally, when people talk about "glabals" in C they mean
> objects with external linkage, static duration and file scope. Â*Using the
> term "global" (or "global scope") is rarely a problem, but in this case
> it lies at the heart of the OP's confusion so I think it pays to use the
> right language. Â*Here are some examples:
>
> ----------------
> int a = 3; /* The name has file scope and external linkage.
> Â* Â* Â* Â* Â* Â* Â* The object named has static duration */
>
> static int b = 42; /* The name has file scope but internal linkage.
> Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* The object has static duration */
>
> void f(int a) /* f has file scope and external linkage but functions
> Â* Â* Â* Â* Â* Â* Â* Â* Â*don't have any storage duration.
> Â* Â* Â* Â* Â* Â* Â* Â* Â*a has block scope, no linkage and automatic duration. */
> {
> Â* Â* int b; /* block scope, no linkage, automatic duration. */
> Â* Â* static int c; /* block scope, internal linkage and static duration */
> Â* Â* extern int d; /* block scope, external linkage and static duration */
>
> }
>
> void g(int x); /* x has function prototype scope, no linkage and
> Â* Â* Â* Â* Â* Â* Â* Â* Â* (insofar as it makes any sense) automatic duration. */
>
> int t; /* A tentative definition of a name with file scope and external
> Â* Â* Â* Â* Â* linkage naming an object with static duration. *.
>
> int t2; /* Another tentative definition. */
>
> int t2 = 42; /* Not a re-definition since the previous one was
> Â* Â* Â* Â* Â* Â* Â* Â* tentative. */
>
> /* Here, at the end of the translation unit, the tentative definition of
> Â* Â*'t' is treated as if it had been "int t = 0;". Â*I.e. the tentative
> Â* Â*definition becomes a real one. */
> ----------------
>
> There are two rather curious details. Â*An object with automatic duration
> that is not a variably modified array exists from entry into the block
> until the block is exited[1]. Â*A variably modified array only exists
> from the point of it's definition:
>
> Â* int peculiar(unsigned int s)
> Â* {
> Â* Â* Â* Â*int *ip = 0;
> Â* top: if (ip) return *ip;
> Â* Â* Â* Â*int a[1];
> Â* Â* Â* Â*a[0] = 42;
> Â* Â* Â* Â*ip = a;
> Â* Â* Â* Â*goto top;
> Â* }
>
> is well-defined but if 'a' were changed to be "int a[s];" then the
> behaviour would be undefined.
>
> [1] Even this needs care to define properly but I'll leave it as
> self-evident for the moment.

Thanks for the correction