Variable definition vs. declaration

A variable is defined when the compiler generates instructions to allocate the storage for the variable.

A variable is declared when the compiler is informed that a variable exists along with its type. The compiler does not generate instructions to allocate the storage for the variable at that point.

Pure declaration example

#include <stdio.h>

extern int myVariable;      // Pure declaration of a variable

int main(){
	myVariable = 540;
	printf("The variable has the value of %d", myVariable);
	return 0;
}

Output

/usr/bin/ld: /tmp/ccnUJk4h.o: warning: relocation against `myVariable' in read-only section `.text'
/usr/bin/ld: /tmp/ccnUJk4h.o: in function `main':
main.c:(.text+0xa): undefined reference to `myVariable'
/usr/bin/ld: main.c:(.text+0x14): undefined reference to `myVariable'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status

In this example, the compiler does not allocate memory to this variable so it can't use it in the main( ) function. This is because it thinks that the variable myVariable is defined in another file.

Definition example

#include <stdio.h>

int myVariable;   // Definition (and declaration) of a variable

int main(){
	myVariable = 540;
	printf("The variable has the value of %d", myVariable);
	return 0;
}

Output

The variable has the value of 540

Here, the compiler does indeed make use of the variable myVariable because it allocated memory for that. It is both defined and declared in this file.