For example:
code1.c / .cpp
int a;
// ... and so on
code2.c / .cpp
int a;
int main(void) {
return 0;
}
go to compile:
$gcc code1.c code2.c # this is fine
$
$g++ code1.cpp code2.cpp # this is dead
/tmp/ccLY66HQ.o:(.bss+0x0): multiple definition of `a'
/tmp/ccnIOmPC.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status
Is there any global variable linkage difference between C & C++?
It's not strictly legal. int a;
is a tentative definition in C. You are allowed multiple tentative definitions and at most one non-tentative definition per translation unit of each object with external linkage in C, but only one definition across all translation units in a program.
It is a commonly implemented extension to allow tentative definitions across multiple translation units in C so long as not more than one translation unit contains a non-tentative definition, but it's not strictly standard.
In C++ int a;
is just a definition - there's no concept of tentative - and it's still illegal to have multiple definitions of an object across the translation units of a program.
For the C case, you may wish to look at this question.
It's illegal in both, but C compilers generally implement an extension. See this answer.
There are three ways for resolution of problem:
If variable
a
is the same in both files, you must declare it asextern
in all files except one.extern
keyword says to linker that this name is located in another files.You may use
static
keyword to limit scope of variable to one file. In which it is declared.Or you may use nameless namespace.
g++ compiler is more strict then gcc compiler. It also depends on version of gcc, may be higher version of gcc i.e. 4.X onwards it can give same error.
Use extern
to avoid
来源:https://stackoverflow.com/questions/6371048/c-c-default-global-variable-linkage-multiple-declaration-definition-probl