When to use extern in C

Question

I m reading  Think in C    and it just introduced the extern declaration  For example    extern int x  extern float y    I think I understand the meaning  declaration without definition   but I wonder when it proves useful    Can someone provide an example

User · Answer

This is useful when you want to have a global variable  You define the global variables in some source file  and declare them extern in a header file so that any file that includes that header file will then see the same global variable

User · Answer

It is useful when you share a variable between a few modules  You define it in one module  and use extern in the others   For example   in file1 cpp   int global int   1    in file2 cpp   extern int global int    in some function cout  lt  lt   global int      lt  lt  global int

User · Answer

It s all about the linkage   The previous answers provided good explainations about extern   But I want to add an important point   You ask about extern in C   not in C and I don t know why there is no answer mentioning about the case when extern comes with const in C     In C    a const variable has internal linkage by default  not like C    So this scenario will lead to linking error   Source 1    const int global   255    wrong way to make a definition of global const variable in C     Source 2    extern const int global    declaration   It need to be like this   Source 1    extern const int global   255    a definition of global const variable in C     Source 2    extern const int global    declaration

User · Answer

This comes in useful when you have global variables  You declare the existence of global variables in a header  so that each source file that includes the header knows about it  but you only need to  ldquo define rdquo  it once in one of your source files   To clarify  using extern int x  tells the compiler that an object of type int called x exists somewhere  It s not the compilers job to know where it exists  it just needs to know the type and name so it knows how to use it  Once all of the source files have been compiled  the linker will resolve all of the references of x to the one definition that it finds in one of the compiled source files  For it to work  the definition of the x variable needs to have what s called  ldquo external linkage rdquo   which basically means that it needs to be declared outside of a function  at what s usually called  ldquo the file scope rdquo   and without the static keyword   header    ifndef HEADER H  define HEADER H     any source file that includes this will be able to use  global x  extern int global x   void print global x      endif   source 1    include  header h      since global x still needs to be defined somewhere     we define it  for example  in this source file int global x   int main           set global x here      global x   5       print global x        source 2    include  lt iostream gt   include  header h   void print global x           print global x here      std  cout  lt  lt  global x  lt  lt  std  endl

[c++] When to use extern in C++

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