Extern storage class - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the <code>extern</code> storage class in C?

The <code>extern</code> storage class tells the compiler that a variable or function is defined in another file or location. It allows sharing variables or functions across multiple files.

beginner

How does extern differ from static in C?

extern makes a variable or function visible across multiple files, while static limits visibility to the current file only.

intermediate

What happens if you declare a variable with extern but do not define it anywhere?

The linker will give an error because it cannot find the actual definition of the variable. extern only declares, it does not allocate memory.

beginner

Example: How to declare and use an extern variable across two files?

File1.c: <br> int count = 10; <br> File2.c: <br> extern int count; // refers to count in File1.c <br> This allows File2.c to access the variable count defined in File1.c.

beginner

Can extern be used with functions? If yes, how?

Yes. Functions have external linkage by default. You can explicitly declare a function with extern to indicate it is defined elsewhere, allowing calls across files.

What does the extern keyword do in C?

A Declares a variable defined in another file

B Defines a new variable in the current file

C Makes a variable private to the current file

D Allocates memory for a variable

extern declares a variable that is defined elsewhere, allowing cross-file access.

If you use extern int x; but never define x, what happens?

A Program compiles and runs fine

B Compiler error

C Variable <code>x</code> is initialized to zero automatically

D Linker error

The linker cannot find the definition of x, causing a linker error.

Which storage class limits a variable's visibility to the current file?

A extern

B auto

C static

D register

static limits visibility to the current file, unlike extern.

Are functions extern by default in C?

A Yes

B No

C Only if declared with extern

D Only if declared static

Functions have external linkage by default, so they are extern unless declared static.

How do you share a variable between two files in C?

A Declare it as static in both files

B Define it in one file and declare it as extern in the other

C Define it as auto in both files

D Use register keyword

Define the variable in one file and declare it with extern in the other to share it.

Explain how the extern storage class helps in sharing variables across multiple files in C.

Think about how you tell the compiler a variable exists somewhere else.

Describe the difference between extern and static storage classes in terms of variable visibility.

Consider where the variable can be accessed from.