Wiki says:
The
externkeyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition. It is also possible to explicitly define a variable, i.e. to force a definition. It is done by assigning an initialization value to a variable.
That means, an extern declaration that initializes the variable serves as a definition for that variable. So,
/* Just for testing purpose only */
#include <stdio.h>
extern int y = 0;
int main(){
printf("%d\n", y);
return 0;
}
should be valid (compiled in C++11). But when compiled with options -Wall -Wextra -pedantic -std=c99 in GCC 4.7.2, produces a warning:
[Warning] 'y' initialized and declared 'extern' [enabled by default]
which should not. AFAIK,
extern int y = 0;
is effectively the same as
int i = 0;
What's going wrong here ?
All three versions of the standard — ISO/IEC 9899:1990, ISO/IEC 9899:1999 and ISO/IEC 9899:2011 — contain an example in the section with the title External object definitions (§6.7.2 of C90, and §6.9.2 of C99 and C11) which shows:
EXAMPLE 1
int i1 = 1; // definition, external linkage static int i2 = 2; // definition, internal linkage extern int i3 = 3; // definition, external linkage int i4; // tentative definition, external linkage static int i5; // tentative definition, internal linkage
The example continues, but the extern int i3 = 3; line clearly shows that the standard indicates that it should be allowed. Note, however, that examples in the standard are technically not 'normative' (see the foreword in the standard); they are not a definitive statement of what is and is not allowed.
That said, most people most of the time do not use extern and an initializer.