Beej’s Guide to C Programming [pdf]

Post by bluetomcat » Fri, Apr 23, 2021, 8:42 AM UTC

shric wrote: post #23

bluetomcat wrote: post #14

Earlier quoted context omitted.

Just another integral data type which supports not only arithmetic (adding or removing an integral value), but also "dereferencing" which means getting a value of the pointed-to type at the pointed-to address. The quantities added or removed in arithmetic operations are the same as the size of the type that is being pointed to.

It's incorrect to call a pointer an integral type. It doesn't behave like an integer (you can't add two pointers together). You can't even subtract two pointers from each other unless they point into the same array (or one element past) without causing undefined behaviour. You can't even reliably convert a pointer generally to an integer and back again unless it's a void *. Edit: just remembered I wrote a rambling on…

Post by qsort » Fri, Apr 23, 2021, 8:50 AM UTC

unwind wrote: post #28

qsort wrote: post #16

Earlier quoted context omitted.

What's difficult to understand about pointers isn't the concept of a pointer itself, or even * and &, it's the fact that working with pointers requires you to simultaneously understand different abstraction levels. While it's not unique to pointers, and it's in fact the case for most nontrivial programming tasks, what's unique about C is that pointers are so pervasive you can't really do anything if you don't underst…

These are good points. I can sometimes feel that Python is more pointer-y (?) than people expect, with stuff like: a = {"one": 1, "two": 2 } b = a b["two"] = 99 print(a["two"]) The above prints 99, since "b = a" does not copy the value (the dictionary) but just the reference to the value ("the pointer", kind of). This is surprising to some people.

Post by aphrax » Fri, Apr 23, 2021, 8:59 AM UTC

DyslexicAtheist wrote: post #25

aphrax wrote: post #7

I've enjoyed this guide a number of times but each time I hit a brick wall trying to understand pointers. I'm still keen to learn but it just doesnt 'click' for me... Edit: poor grammar

around 25 years ago I got so frustrated because I hit the same brick walls over and over especially with advanced pointer stuff. It sounds silly today but what helped me are some really basic books about C. iirc the "for dummies" series and others. It took another couple of years until I understood what I lacked wasn't time spent reading another section on pointers but additional tooling. Using a debugger and steppin…

Post by bluetomcat » Fri, Apr 23, 2021, 9:07 AM UTC

qsort wrote: post #30

bluetomcat wrote: post #26

Earlier quoted context omitted.

In C, pointers require you to think deeply about the ownership and lifetime of any "allocated object" at runtime. How long does it live, who is responsible for the deallocation, how many pointers does your program hold to that object (dangling issues). Ultimately, it can lead to a cleaner design if these issues are taken seriously up-front.

I don't disagree with that, but most cases fall within a pretty clear pattern: - typedef struct { ... } foo - foo *foo_create() - void foo_destroy(foo *) - a bunch of functions that take foo* as their first arg which is kind of the same as a class and only more error-prone. I say this as someone who actually _likes_ C, but the manual memory management model is very often unnecessary, confusing, repetitive. There was…

    foo f;
    foo_init(&f);
    foo_destroy(&f);
    ...
    foo *g = malloc(sizeof(foo));
    foo_init(g);
    foo_destroy(g);
    free(g);

Post by shric » Fri, Apr 23, 2021, 9:11 AM UTC

bluetomcat wrote: post #31

shric wrote: post #23

Earlier quoted context omitted.

It's incorrect to call a pointer an integral type. It doesn't behave like an integer (you can't add two pointers together). You can't even subtract two pointers from each other unless they point into the same array (or one element past) without causing undefined behaviour. You can't even reliably convert a pointer generally to an integer and back again unless it's a void *. Edit: just remembered I wrote a rambling on…

Strictly it's not an integral type as you point out, but for the purpose of introducing a beginner who might be afraid that there is "deep magic" involved, it can be a helpful comparison. Subtracting two pointers yields ptrdiff_t which is not a pointer type in itself.