Hi everyone,

I'm currently working through the C programming tutorial. I've reached the infamous chapter on pointers and I have to admit it, a few parts have kind of tripped me up.

Here's the code that my questions are based on:

#include 
void foo(int *a)

{

 *a = 145; // We change the value at the address pointed to by a

 printf("Foo: Variable a = %d\n", *a); // *a == 145

}
int main(void)

{

 int a;
a = 10;

 printf("Main: Variable a = %d\n", a); // a == 10

 foo(&a); // We pass the address of a

 printf("Main: Variable a = %d\n", a); // a == 145

 return (0);

}

Here are my questions:

• In the line foo(&a);, does the & indeed give the address of the variable a?
• What's the purpose of the * in void foo(int *a)?
• What does int mean in int main(void)?
• And finally, is the void in int main(void) strictly necessary?

Thanks in advance!

Hey there,

• Yep, the & operator gives you the address of the variable that follows it.
• The * is used to access the value that the pointer is pointing to — in other words, it dereferences the pointer. If you just wrote a = 145; inside foo(), you'd be changing the pointer itself, not the value it points to. But by writing *a = 145;, you're modifying the value at the address stored in a.
• A C program returns a value to the operating system when it finishes running — that return value can be used by whatever called the program (like a shell script, for instance).
• No, void isn't strictly required anymore. But I'd still recommend including it when your function takes no parameters — it makes things clearer for both the compiler and anyone reading your code.

Edit: To dig a bit deeper into point 3, here's a basic example:

int main(void) {

    return 1;             // try return 0 too

}

Now if you call x.exe from a batch script y.bat:

@echo off

x

if errorlevel 1 goto err

echo x ran fine

goto eob

:err

echo x returned an error code: %errorlevel%

:eob

Just to add to the answer above:

• When you use * in a function parameter like int *a, it means the function is expecting a pointer. And when you use * in a variable declaration, it means you're declaring a pointer.

As for why you'd use a pointer in a parameter: C uses pass-by-value, which means function parameters are read-only copies by default. But by passing a pointer, you can actually modify the original variable from inside the function.

Also, the * is what lets you dereference the pointer — to access or modify the value at the given memory address — like in *a = 145;.

• So, since the function expects a pointer, you pass the address of a using the & operator: foo(&a);.
• Regarding main's return value, the stdlib.h library defines two constants you can use: EXIT_SUCCESS and EXIT_FAILURE.
• The void in main(void) just tells the compiler that this function takes no arguments. It also prevents someone from accidentally calling main() with parameters.
• That said, the real, standard signature of main is:

int main(int argc, char* argv[])

— which lets your program accept command-line arguments when it's run.

Thanks for your (clear) answers. It's tough figuring out all these little nuances when you're learning on your own. That's why forums like this are so helpful :D

Have a great weekend, everyone.