Variable Modifiers − Auto & Extern
ฝัง
- เผยแพร่เมื่อ 12 ก.ย. 2024
- Programming & Data Structures: Auto and extern variable modifiers in C programming.
Topics discussed:
1. Auto modifier.
2. Extern modifier.
3. Extern modifier examples.
4. Procedure to create a project and different files within that project.
C Programming Lectures: goo.gl/7Eh2SS
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And Wrong:
1. auto is the opposite of register. Declaring a variable as auto, prevents compiler-optimisation to turn it into a register-variable. Typically you want register-variables in inner loop, but declare variables of the outer loop first, so delare the variables for the outer loop as auto.
2. Global variables are not initialized to zero. If your system/compiler does that - nice - but the C-standards says there is garbage.
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use of uninitialized local variables leads to undefined behaviour. It depends on the compiler, in the visual studio community if we print the value of uninitialized variable it gives compile-time error while in GCC compiler it prints 0
I'm the student from taiwan , this lesson help me a lot in learning c , Just keep going , you already change the way the cs student around the world how to study. Thank you !!!!
at 1:43 , I haven't initialized a variable in main function and by printing that variable,it gave me as 0 but not a garbage value.
Really good Explanations, god bless you!
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Chill bro it's bot,which was programmed by TH-cam
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1:44 sir what is garbage value and how it is printed?
Thank you so much for making this video. It was worth seeing this video 😀😃
GCC didn't compile program with auto keyword. It throws error:two or more data types in declaration of 'var'. While, cl(MSVC) printed 0.
sir I had tried to print output for an integer(not initialized) but instead of printing any random garbage value it prints 0 all the time. why so?
was it global?
same and it was not global
@@riturajpathak1905varies according to machine properties
@@riturajpathak1905 ya but it is unpredictible in latest version of gcc. see geeks for geeks
it happened for me too
Beautifully explained, thanks alot!
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sir extern int a; doesn't work in my code blocks what should I do?
my main.c
#include
extern int a;
int main()
{
printf("%d", a);
return 0;
}
and my other.c
int a = 56;
Very clear description. Thank you very much!
@2:35 int var; // It is just declared, not defined..
// definition is when you assign memory to it with = operator
Excellent way of Teaching !!!
helpful. Thanks NESO, I love you guys
Very clear explanation Sir, thank you.
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Thanks for sharing so many small small details , not only for this topic but all topics in all of your videos ...
I do have one question here for extern
What if we declare int a = something in two different files and try to refer in our main file using extern ..which one it will refer?
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I will just leave a comment for myself. Feel free to comment, thanks. Global variables are initialized to 0 by default. Extern is short for external. It declares the variable but no definition (though you can also define it), therefore no memory allocated to it. This is used when we want access a variable of the same name but located and defined in the nearest outer scope which can also include other files in the current workspace. It avoids redifinition or having to find another variable name for the same variable. Autos can't be declared multiple times, but externs can be. Declaration then definition. Initialize == define.
Sir, in my computer auto variable does not give garbage values it also initializes with 0 as same as global variable.
Thank you so much for making such videos. These are very helpful and easy to understand 😊😊
Sir then What is the use of auto modifier ,if it doesn't change anything in program.
At 8:41 on Video 14 (Variable Modifier - Static) you place a variable outside the scope of any function and specifically call it global. Thus it retains its value and should be accessible to main which is within a completely different file. If this is true then why the need for extern? It appears to me that you cannot compile when you delete the line extern within main here in this video which seems to suggest just the opposite of Video 14 which is to say that int a= 5; is NOT really global after all.
extern int a=10;
extern int a; //do not have this name variable in any other file
Above code do not produce error. Multiple declaration is allowed which is ok, but second line is linking to variable in same scope ?
Yes exactly. By writing extern int a=10; it means both declaration and definition
when i keep the variable global and uninitialized, its showing compilation error and for local variable (not initialized) its showing 0 and not garbage value....why is that?
Good video. But you are wrong about c giving garbage value to var which is declared but not defined under auto. I did it in an online compiler but it doesn't give any random value as you said garbage value. Please correct me if I am wrong.
What I think is, the extern variable works like the complement set. 'A' complement is equal to universal set minus 'A' set.
Short and to the point videos, finally I found the course I needed. Thank you so much for all your effort. _/\_
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Good Explanation
What if there are multiple files with int a declaration and defination and a single extern int a in main file?
If memory is not provided to extern int a, so when does the exact value and memory for variable a is filled?
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at 9.52 even if we dont write extern in main then also it will print 9 as we have already defined the global variable then why to use extern.
9:41 , do we still need to use extern int for accessing global variable?
Hi thanks for the lessons I appreciate it very much. However, I have a question.
I tried not intializing my variable with int var; and also with auto int var; where both are actually the same in the local scope.
The result I obtained was zero instead of indeterminate/random/garbage value. Why is that so?
This means the int var; in local is the same as if I declare it globally since int var; in global is always zero.
Thank you and hope to hear your explanation when you're free. Cheers!
int var in local getting zero is not the same as int var in global because when you declare a global variable the initial or the default value is zero but a local variable stores a random number so if you get a local variable equal zero then it is just a coincidence maybe if u give local int var another time it will give a diff ans. hope it helps
if you are a nerdy nerd like my self and you are using vim or emacs you can access the variable by compiling the two files, like this "gcc main.c other.c -o main". where main is the executable.
Thank you very much. You are a genius. 👍👍🔝🔝
Awesome tutorial to learn c programming
Hi sir, Thanks for this useful content. I have one query in this , at 10:03 you told if extern variable don't have definition in current file, it will go and search other files to retrieve the definition..my question here is which file it takes first to search the definition of this extern variable...because in your case you have created only one "other.c" file...but in reality there are lot of files will be there.If I am varying that extern variables in multiple files which file definition it will consider first?
Very helpful video
Awesome explanation!!
@7:28 -> external variable declared is a global variable for that file right? All the global variables are by default initialized to 0, isn't it? why don't it give the output as 0 and instead giving an error?
Will these concepts of system level features be used in other programing language like python?????????
Are the notes of these lectures available somewhere...? And how can i access them...?
Thanks neso academy
Thank you for a wonderful video and a great explanation.
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some people gets that value is zero. Instead of any garbage value. because of it depends on what was stored at the location assigned to variable. Some compliers may also initialize it to zero
thanks alot
Great one sir!
informative lecture....thanx sr...
amazing
Does [extern int var] always need to be a global variable???????
sir auto is not giving any garbage value? in my pc
Latest version compiler will not give garbage
Same problem bro
now, what is the pros of external keyword, if we can just include one file in another in order to access his declared globale variable instead of those geeky flashy gamic extern keyword?
simply awesomeeeee.....
If I define variable int a =3; inside some function say func() then , I write extern int a; in main() and printf ("%d",a);
Whether it will show 3 or error.
Error
You are excellent !!!
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Nice explanation
dear sir i try it what i notice is when i auto char auto float is value is expect 0 also unsigned int
What compiler u have use sir...
What if there are three file in project and all other 1,other 2 have the value of a ,then which value is assigned to a.?
Linker throws error. Only one time definition allowed. Linker get confused while binding the modules.
@@abhishekmazumdar2072 if she declares it to be static global variable, there will be internal linkage, hence no error
At 10:23 in this video you just initialized the extern variable a in main.c file , but at that time in othre.c file has a commented variable definition what if i would remove out that comment and define the variable like " int a=9; " at that other.c file and also write " extern int a=10; "in main.c file so what it would give as an output? is it producing an error because of multiple definition of a single variable or it will going to give priority to that externed initialized variable and give output=10?? help me as soon as possible..
Compile in compiler yaar...
Excellent lecture
For me 'auto int var' giving 0 as output, when I won't initialize var, is garbage value?
what will happen if there are multiple files that have defined variable 'a'
Which file's value of a will the compiler access?
The output when i use var in form of global variable as well as in the scope gives the output as same as 0. What would be the reason behind it????????
What happen if we define int a=5,int a=6,in two different other file .what will be the output for extern variable.?
If you define them in both files as global variables,this will lead to name collision,you can compile both you c files in this case ,but when it comes to linking peiod,error happend.
possible to compile all three files separatly. but when linking ,error will raise that " 'a' variable redeclared multiple times"
what if we create more than 2 files like i created 3 files and declaring int a = 2 and int a = 3 , and then i write extern int a in my 1st file , so when i write printf(a) what will print?
I tried testing
#include
int main()
{
auto int var;
printf("%d", var);
return 0;
}
but
0
will be the output always to me, kindly check if I ever happen to have errors
same happened with me
Masterpiece
When using another compiler (gpp in atom) it didn't work at all. Perhaps no linker?
Undefined symbols for architecture x86_64:
"_dd", referenced from:
_main in main-f1adc3.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Thanks
Hey, I m not initializing var value still it is printing 0 as garbage value though I m not using global variable, why?