Thread: a couple of questions that have arrisen over the last 24 hours

Originally Posted by cooper1200

1) what is the -> operator and when and where should i use it.?

Given a pointer p to a struct type with a member foo, p->foo is equivalent to (*p).foo, so you would use it as syntactic sugar whenever you want to write (*p).foo.

Originally Posted by cooper1200

2) when and where should i use static it has been suggested by accomplished people on here to never use it but equally, other equally accomplished people have used it in their examples.

static is a storage class specifier. When using it to declare an object, it means that that object has static storage duration, i.e., its lifetime is the entire duration of the program, and it is actually initialised once prior to program startup. This can be problematic because it means that even though the object might be declared in a local context, it actually has global state, so you have to worry about the state being changed when you don't expect it (and consequently things like reentrancy and thread safety). A classic example would be trying to use strtok to parse a string, and while doing so, trying to use strtok again to parse a token extracted from the string, all in nested loops. Hence, it is wise to be cautious about declaring objects static, including static local variables.

Besides specifying static storage duration, the static keyword also specifies that the identifier in the declaration has internal linkage. Linkage has to do with how the identifier refers to entities with respect to the translation unit in which it is declared. For example, if you declare a variable at file scope (i.e., a global variable), it automatically has static storage duration, but it has external linkage rather than internal linkage, so you can use the same variable name to refer to the same object from another translation unit. If you added static to the declaration, then it would have internal linkage, i.e., another file scope variable in another translation unit would refer to a different object. This turns out to be particularly advantageous in declaring helper functions since you basically make the helper functions local to the source file such that if you declare another helper function with the same name in another source file, there will be no name collision.

Originally Posted by cooper1200

3)what is malloc/calloc is it asking for more memory than has been set aside for the program or is it taking it from the memory already allocated. If it is the latter what is the difference between writing int my_array[n] where n has been entered by the use before that deceleration to using the appropriate malloc call. if malloc/calloc gets it from a separate area of memory can you do real damage by accidentally overwriting something important. in which case something best avoided until i really understand pointers and addresses

I would echo what someone else replied to you: don't worry. Generally, modern operating systems would have some kind of memory protection that makes it practically impossible for you to trash your system by abusing malloc calls: your program will likely just crash. You should be able to easily find online explanations for terms like "stack versus heap" with respect to malloc, and search for variable length array to understand the int my_array[n] thing.