Error: redefinition of “a static variable” in C header files


Error: redefinition of “a static variable” in C header files



I have some static variables (say, var1 and var2) declared in two different files. The variables have same name in both files. Some variables (say var1) are not initialized in their declaration and some are (var2), like following.



file1.h


static bool var1;
static bool var2 = false;



file2.h


static bool var1;
static bool var2 = false;



According to my understanding, static variables are only restricted to the c files(or h files) they're declared in, so I should be safe having same variable names in multiple header files. But when I compile code, I get error "redefinition of var2", only for the variables that have been initialized.



Edit:
since some mentioned to use extern keyword, I'd like to clarify that both var1 and var2 are supposed to have different values in different c files, and should only be restricted to their respective files,





Header files are copied into .c files. If you include both headers into the same file, you will have multiple definitions.
– DeiDei
49 mins ago


.c





There is only a tentative definition for var1.
– Antti Haapala
46 mins ago


var1





use extern in the header file: When to use extern in C++
– Thomas
45 mins ago


extern




3 Answers
3



No. The multiple declaration of var1 is okay, but the multiple definition of var2 is not. You can't initialize a variable twice...even if the values are the same.


var1


var2



I solve problems like this using preprocessor guards, such as:


#if !defined(MY_APP__VARS_DEFINED)
static int var1, var2=0;
#define MY_APP__VARS_DEFINED
#endif



Even then, I don't recommend duplicating the definitions in multiple header files. It's a maintenance problem. Sooner or later, someone is likely to change that initial value in one header and not find all other headers it's defined in. It also makes renaming ("refactoring") harder. (...and violates the DRY Principle.)



You might want to rethink your design, though. Global variables usually lead to brittle applications; harder to maintain and easy to break.





actually these variables are supposed to hold different values in different c files, but since the information is almost similar I named them same, they're supposed to be global inside one c file, but should not be global to the whole program, isn't that's why static keyword used for ? just like we use same name local variables in multiple functions(e.g., 'i' in for loops), I want same global variables in multiple compilation unit, is it possible ?
– Salman
14 mins ago



Why this error occurs only for var2 ? if you include file1.h and file2.h in same source file test.cpp then var2 will be having two definition and compiler is unable to choose which one to take and throws the error.


file1.h


file2.h


test.cpp


var2



To avoid this declare var2 as a extern in header .h file and define in respective source .cpp file.


var2


extern


.h


.cpp



While in case of var1 it's just declaration not definition so it won't throws the error i.e multiple declaration is possible.


var1


declaration


definition



Also the static declaration like


static



static bool var1;


static bool var1;



means that the variabe var1 is visible only in the current compilation unit.


var1



Side note, static variable having internal linkage i.e it hides static variable from other translation units. Though static variables can be defined in multiple translation units.


static


static


static


defined



And extern variable having external linkage, i.e extern variable will be visible to other translation units, but the extern variable should be define only in one translation unit.


extern


extern


extern





actually these variables are supposed to hold different values in different c files, but since the information is almost similar I named them same, they're supposed to be global inside one c file, but should not be global to the whole program, isn't that's why static keyword used for ? just like we use same name local variables in multiple functions(e.g., 'i' in for loops), I want same global variables in multiple compilation unit, is it possible ?
– Salman
7 mins ago



If a source file is including both file1.h and file2.h, it will have multiple definitions of var2. That's what causing the error.


var2



Rather than declaring / defining these static variables in header files, put them directly the necessary C source files. That way you won't have to deal with multiple definitions.






By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Comments

Post a Comment

Popular posts from this blog

paramiko-expect timeout is happening after executing the command

paramiko-expect timeout is happening after executing the command Can someone help me why timeout is happening after executing the command .I am trying to SSH to a machine and execute a command.I want to store the result of the executed command . I have referred paramiko-timeout but i didn't get my expected result. import traceback try: import paramiko except ImportError: raise Exception("Please install paramiko , pip install paramiko") import pexpect from paramiko_expect import SSHClientInteraction def main(): HOSTNAME = "minion5.net" USERNAME = "root" PASSWORD = "help@432" PROMPT = "root@*:~$s+" try: client = paramiko.SSHClient() client.load_system_host_keys() client.set_missing_host_key_policy(paramiko.AutoAddPolicy()) client.connect(hostname=HOSTNAME, username=USERNAME, password=PASSWORD) with SSHClientInteraction(client, timeout=20, display=True) as interact: ...
Read more

Export result set on Dbeaver to CSV

Export result set on Dbeaver to CSV Normally I use Dbeaver for windows and always export my result set like this: run my query --> select the result --> export the result set --> select export to clipboard --> done This step by step puts my result set in my clipboard and I can paste it wherever I want to work with it. The problem is that now I am using dbeaver for mac and this guide is not working. I can go on until the moment that I select my result set like in the image below: exporting data set But once I go further in the process, in the last step I get: no query Note that in "source" it was suppose to show the query that originated the result set, but instead it says just "select. As a result it does't select my result or anything (besides being "successful"). Normally my query would show up there automatically and I couldn't find any option that corrects this problem in the menus. Please help me! Thanks. Aft...
Read more

Opening a url is failing in Swift

Opening a url is failing in Swift breakpointing inside the callback and logging success shows false. I added itms-apps to my plist: LSApplicationQueriesSchemes and put this in my app delegate: func application(_ app: UIApplication, open url: URL, options: [UIApplicationOpenURLOptionsKey : Any] = [:]) -> Bool { return true } func moreTapped() { let url = NSURL(string: "itms-apps://itunes.com/developer/quantum-productions/id979315877") if #available(iOS 10.0, *) { UIApplication.shared.open(url! as URL, options: [:], completionHandler: {(success: Bool) in }) } else { UIApplication.shared.openURL(url! as URL) } } Are you testing on a real device? – rmaddy Jun 21 at 23:57 I try on a device, get "Cannot Connect to App Store". Device is online ...
Read more