How do I share variables between different c files

Question

beginner question about C declaration   In a  c file  how to use variables defined in another  c file

User · Answer

if the variable is    int foo    in the 2nd C file you declare   extern int foo

User · Answer

In 99 9  of all cases it is bad program design to share non-constant  global variables between files  There are very few cases when you actually need to do this  they are so rare that I cannot come up with any valid cases  Declarations of hardware registers perhaps    In most of the cases  you should either use  possibly inlined  setter getter functions   public    static variables at file scope   private    or incomplete type implementations   private   instead   In those few rare cases when you need to share a variable between files  do like this      file h extern int my var      file c  include  file h  int my var   something      main c  include  file h  use my var     Never put any form of variable definition in a h-file

User · Answer

Try to avoid globals  If you must use a global  see the other answers  Pass it as an argument to a function

User · Answer

In fileA c   int myGlobal   0    In fileA h  extern int myGlobal    In fileB c    include  fileA h  myGlobal   1    So this is how it works    the variable lives in fileA c fileA h tells the world that it exists  and what its type is  int  fileB c includes fileA h so that the compiler knows about myGlobal before fileB c tries to use it

User · Answer

Those other variables would have to be declared public  use extern  public is for C     and you would have to include that  c file  However  I recommend creating appropriate  h files to define all of your variables   For example  for hello c  you would have a hello h  and hello h would store your variable definitions  Then another  c file  such as world c would have this piece of code at the top    include  hello h    That will allow world c to use variables that are defined in hello h  It s slightly more complicated than that though  You may use  lt    to include library files found on your OS s path  As a beginner I would stick all of your files in the same folder and use the     syntax

User · Answer

The 2nd file needs to know about the existance of your variable  To do this you declare the variable again but use the keyword extern in front of it  This tells the compiler that the variable is available but declared somewhere else  thus prevent instanciating it  again  which would cause clashes when linking   While you can put the extern declaration in the C file itself it s common style to have an accompanying header  i e   h  file for each  c file that provides functions or variables to others which hold the extern declaration  This way you avoid copying the extern declaration  especially if it s used in multiple other files  The same applies for functions  though you don t need the keyword extern for them   That way you would have at least three files  the source file that declares the variable  it s acompanying header that does the extern declaration and the second source file that  includes the header to gain access to the exported variable  or any other symbol exported in the header   Of course you need all source files  or the appropriate object files  when trying to link something like that  as the linker needs to resolve the symbol which is only possible if it actually exists in the files linked

[c] How do I share variables between different .c files?

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