@@HackerCSVideos hey good to see back I have a doubt what is sizeof NULL in c can you tell me what is actually means

i have the same doubt

Short answer: All vars declared (and thereby defined) outside funcs() are "globally visible" by default. 'Extern' was needed with 'int x;' (in this poor 'lesson') to DECLARE to the compiler that the LINKER will (should) find 'x' DEFINED (space allocated) somewhere in a related OBJ... Function prototypes are not function DECLARATIONS; they are simply 'promises' for the compiler to check parameters and return types. One could prototype a function "void foobar( int, float*, (char*)(int, int), int**);" and then never get around to writing the actual code for it... Just don't try to call it! Putting 'extern int x;' or the function prototype into caller.c is HORRIBLE practice. The compiler will not be able to confirm the func's parameters and return type (its "function signature") are correct. Likewise, 'x' may be declared an 'char *' in main.c, and 'float *' in caller.c... Good luck!! The compiler is helpless. This is why #include "foobar.h"; (header files) must be used to declare common (global) variables and functions. It is also why some version of "makefile" should be used for the compiler to know what OBJs and EXEs are stale after one or more source files has been altered. CAUTION: Do not declare static vars or static funcs in a header file (unless you know what you're doing). Each OBJ would otherwise receive its own private instance of declared variables... A better (minimal) example: FOO.h #include <stdio.h> extern int x; // prototype declaring there exists a variable named 'x' and that its type is 'int' void print_num(void); // prototype declaring there (may) exist a function (taking no params and returning nothing) void caller(void); // another function prototype DATASPACE.c #include "foo.h" // For compiler to 'see' and check that 'x' is a well-known integer data type... int x = 0; // definition of variable. (space allocated) CALLER.c #include "foo.h" // For compiler to 'see' there exists an 'x' and a 'print_num()' to link void caller(void) { print_num(); x = x + 1; } MAIN.c #include "foo.h" // For compiler to 'see' there exists an 'x' and what 'caller()' looks like... void print_num(void) { printf("%d ", x ); } int main(void) { caller(); // Will print "0" caller(); // Will print "1" }