C implicit extern for global variable, when does it happen, how does it work -


i'm trying understand ways in c global variable can shared between multiple files (compilation units). i've read excellent question , answer here. after doing few tests i'm still left stuff don't get:

basically question be: if there's variable declared (but not defined) in header without extern keyword, ok include header in various compilation units in order make available variable compilation units? in scenario it's implied 1 (and one) compilation unit contains code initializing (defining?) variable, , called first before other compilation units try variable. if true, procedure what's called "implicit extern" ?

i'll illustrate question example:

header "mycommonheader.h" contains:

//mycommonheader.h int* i; //pointer int

file myfirstheader.h contains:

//myfirstheader.h void changeit(int newvalue);

file myfirstsource.c contains:

//myfirstsource.c #include "myfirstheader.h"   void changeit(int newvalue) {     *i = newvalue;  }

file mysecondsource.c contains:

//mysecondsource.c #include "mycommonheader.h" #include "myfirstheader.h"  void main() {    = malloc(sizeof(int));    changeit(10);    *i = 23; }

does above code operates same variable everywhere? need add externanywhere?

/* file.h */ int* i;

is tentative definition of i variable. means if there no other (external) definition variable in translation unit, defined once (initialized 0). if there 1 matching (external) definition of i elsewhere in translation unit, definition used, , tentative definition above behave declaration.

as common extension, compilers extend behavior across translation units. means, such compilers, can safely include header file in many translation units want, , there still 1 definition of i.

it have been different if had explicitly initialized i in header file :

/* file.h */ int* = 0;

this actual definition (not tentative), , can include header file in 1 compilation unit, or you'd multiple definition error.

the better way, define variable in .c file, , use extern in header file :

/* file.h */ extern int* i;  /* file.c */ int* = 0;

this makes absolutely clear there 1 definition (the 1 in .c file), , every compilation unit header file included refer definition.


Comments

Post a Comment

Popular posts from this blog

jquery - How can I dynamically add a browser tab? -

since using iframes house external pages are, accounts "not preferred method" , disallowed sites (flickr, one), want dynamically generate browser tabs. e.g., when user submits form, want dynamically generate tab on browser in response submitted. e.g., might want add new tab url like: http://www.bigsurgarrapata.com/contact how in jquery? you want form target attribute action attribute. http://www.w3schools.com/tags/att_form_target.asp browsers have ultimate control on result of target attribute open new tab value of _blank. <form action="contact_submit.php" target="_blank" method="post">
Read more

keyboard - C++ GetAsyncKeyState alternative -

i using windows.h library's getasynckeystate function define key pushed on keyboard . here program returns int value of pushed key example. #include <iostream> #include <windows.h> using namespace std; int main (){ char i; for(i=8;i<190;i++){ if(getasynckeystate(i)== -32767){ cout<<int (i)<<endl; } } return 0; } but trying make opengl simple game , function appeared slow . there function not need cycle 180 times , if statement . thanks you're trying make opengl game, you're going use glfw. if that's case can away using glfwgetkey() such: if (glfwgetkey(glfw_key_up) == glfw_press) {...} if (glfwgetkey(glfw_key_down) == glfw_press) {...} that method nicely explained in tutorial 6 opengl-tutorial.org . should start first tutorial of beginners tutorials , later intermediate tutorials . if you're going make win32 window can listen wm_keydown or wm_keyup messages , virtua...
Read more

android - java.net.UnknownHostException(Unable to resolve host “URL”: No address associated with hostname) -

i developing app testing asp.net webservice in android.for simple webservice adding 2 numbers used.where numbers passed parameters.the result in dialogue.the webserver works in local. when application runs following result exception.i provided internet permission in manifest. java.net.unknownhostexception(unable resolve host “ http://url.com/ ”: no address associated hostname) my code given below main activity public static string rslt=""; /** called when activity first created. */ @override public void oncreate(bundle savedinstancestate) { super.oncreate(savedinstancestate); setcontentview(r.layout.activity_main); button b1=(button)findviewbyid(r.id.button1); final alertdialog ad=new alertdialog.builder(this).create(); b1.setonclicklistener(new onclicklistener() { @override public void onclick(view arg0) { // todo auto-generated method stub try { ...
Read more