CODEWITHSUNDEEP
cglobal-variables
user10109647
user10109647

Reputation: 76

How is the value of global variable changing in my code?

I have declared global variable again after the main function but It still affects main function. I know that C allows a global variable to be declared again when first declaration doesn’t initialize the variable(It will not work in c++). If I assign the value after the main function it still works with two warning in c but gives error in c++.

I have debugged the code but it never reaches the line int a=10;.

#include <stdio.h>
#include <string.h>

int a;

int main()
{
    printf("%d",a);
    return 0;
}
/*a=10  works fine with following warnings in c.
        warning: data definition has no type or storage class
        warning: type defaults to 'int' in declaration of 'a' [-Wimplicit-int]|

        but c++ gives the following error
        error: 'a' does not name a type|
*/
int a=10;

The output is 10.

Upvotes: 4

Views: 1604

Answers (4)

H.S.
H.S.

Reputation: 12669

From C Standard#6.9.2p2

2 A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition.....

So, this

int a;

is tentative definition of identifier a.

Couple of points about tentative definition:

  • If there are no definitions in the same translation unit, then the tentative definition acts as an actual definition with the initializer = 0.
  • If an actual external definition is found earlier or later in the same translation unit, then the tentative definition just acts as a declaration.

In your program, compiler found the definition of a in the same translation unit:

int a=10;

Hence, you are getting the output 10 when compiling with C compiler.

Now, regarding the error when compiling with C++ compiler:

If you have this statement in your program:

a=10;

This will give error when compile with C++ compiler because you are missing the type specifier which is required. But this code will compile with C complier because, in older version of C (C89/90), if the type specifier is missing then it will default set to int. Of course, you will get warning message when compile with C99 & C11 compiler because this implicit declaration is no longer supported.

If you have this statement in your program:

int a=10;

C++ does not have concept of tentative definitions and int a; is definition in C++. Hence, due to concept of One Definition Rule the C++ compiler will give error - redefinition of 'a'.

Upvotes: 2

seftikara al mayasir
seftikara al mayasir

Reputation: 1

all i know, today's c++ compiler cannot run the code:

int a;
int main()
{
    printf("%d",a);
    return 0;
}
int a=10;

nor

int a;
int main()
{
    printf("%d",a);
    return 0;
}
a=10;

because c++ detects double declaration of variable.
and
because it cannot initialize a variable outside a method.

error "'a' does not name a type" is because of that (the second) error, c++ expect the first word to be a type for declaration (ex: int, long, char, -etc-), and variable is given.

Upvotes: 0

John Bode
John Bode

Reputation: 123448

Several things:

  • The first int a; is a tentative declaration; the second int a = 10; is a defining declaration.

  • a is declared at file scope, so it will have static storage duration - this means that storage for it will be set aside and initialized at program startup (before main executes), even though the defining declaration occurs later in the source code.

  • Older versions of C allow for implicit int declaration - if a variable or function call appears without a declaration, it is assumed to have type int. C++ does not support implicit declarations, so you will get an error.

Upvotes: 7

Achal
Achal

Reputation: 11921

Here

int a; /* global declaration */

compiler treats above statement as just a declaration not definition. It looks for definition of a in other translation units, it finds below main() as

int a=10;

Hence the output 10.

To avoid warnings, declare a with extern storage class for e.g

extern int a;

Upvotes: 3

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