the memory location of static and extern storage class in C -

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this question has answer here:

i have 2 files sharing global variables.

in main.c

#include<stdio.h> static int b; extern int b; main() { extern int a; printf("a=%d &a:%p\n",a,&a); printf("b=%d &b:%p\n",b,&b); fn(); }

in fun.c

#include<stdio.h> int b=25; int a=10; fn() { printf("in fna=%d &a:%p\n",a,&a); printf("in fnb=%d &b:%p\n",b,&b); }

if compile both files. i'm not getting compilation error. , fine.the output

a=10 &a:0x804a018 b=0 &b:0x804a024 in fna=10 &a:0x804a018 in fnb=25 &b:0x804a014

but, in main.c if alter lines extern int b , static int b this

#include<stdio.h> extern int b; static int b; main() { extern int a; printf("a=%d &a:%p\n",a,&a); printf("b=%d &b:%p\n",b,&b); fn(); }

at compilation, i'm getting error.

main.c:6:12: error: static declaration of ‘b’ follows non-static declaration main.c:5:12: note: previous declaration of ‘b’ here

here issue memory @ static , extern variables stored. couldn't able conclude exact reason why compilation error @ second time

note: i'm using gcc compiler.

these 2 quotes c standard (6.2.2 linkages of identifiers) understand problem.

4 identifier declared storage-class specifier extern in scope in prior declaration of identifier visible,31) if prior declaration specifies internal or external linkage, linkage of identifier @ later declaration same linkage specified @ prior declaration. if no prior declaration visible, or if prior declaration specifies no linkage, identifier has external linkage.

and

7 if, within translation unit, same identifier appears both internal , external linkage, behavior undefined.

in translation unit

#include<stdio.h> static int b; extern int b; main() { extern int a; printf("a=%d &a:%p\n",a,&a); printf("b=%d &b:%p\n",b,&b); fn(); }

indentifier b has internal linkage (read first quote). first declaration of b declares identifier having internal linkage due storage-calss specifier static. second declaration of b storage-class specifier extern has prior declaration of b (the first declaration) the storage class specifier static. linkage of identifer same linkage of prior declared identifier.

in translation unit

#include<stdio.h> extern int b; static int b; main() { extern int a; printf("a=%d &a:%p\n",a,&a); printf("b=%d &b:%p\n",b,&b); fn(); }

identifier b declared having external , internal linkage (read both quotes). compiler issues message.

at first identifier b declared having external linkage (because there no prior declaration given linkage) , due specifier static same identifier declared having internal linkage.


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