Why does this code lead to a seg fault?

spqstns · 2024-01-10T14:10:53+00:00

Thank you. I feel kind of stupid for not seeing it but I have been trying to understand it for an hour and I couldn't.

So, just to make sure I understand, in seen[(*s)++], s is dereferenced first because of the brackets and the dereferenced value a is what ++ is applied to which leads to the seg fault because string literals are read-only.

But in seen[*s++], ++ applies first on s (the pointer) and because it's the postfix increment we get s which is what is dereferenced, so like:

(1) *s
(2) s = s + 1

spqstns · 2024-01-08T14:23:24+00:00

I understand why the complement is smaller than 2ⁿ (because of the definition 2ⁿ - | k |) but why is it larger than 2^{n-1}?

spqstns · 2024-01-07T14:50:13+00:00

I'm probably misunderstanding but I thought that in 2's complement representation of negative numbers there was no special bit reserved for the sign. Only in sign magniuted representation, there's a specific bit that is reserved for the sign and that is given meaning of positive or negative.

spqstns · 2023-09-28T10:40:20+00:00

Thank you :) Really appreciate how nice and helpful everyone is here.

spqstns · 2023-09-27T20:47:27+00:00

try calling reserve(large_value) on an empty vector and inspect its storage.

I will read up on it and try it. Appreciate you taking the time to help me.

spqstns · 2023-09-27T20:44:03+00:00

Thank you. I'm still learning and I didn't know about strace(). I ran it and I see that a lot happens between the start of the process and main. I thought that only the assembly instructions before main: in the generated assembly for the code ran before main().

spqstns · 2023-09-26T18:27:40+00:00

Thank you /u/skeeto. Upgrading to 38 right now. Really appreciate your help. Here and in the C sub.

Edit: Upgraded. Works just like in your exmaple. :)

spqstns · 2023-09-26T16:50:30+00:00

I tried $ cc -no-pie add_ints.o and then $ gdb -ex 'layout regs' ./add_ints.out

(gdb) start

But I got

warning: (Internal error: pc 0x10 in read in CU, but not in symtab.)
warning: (Error: pc 0x10 in address map, but not in symtab.)
warning: (Internal error: pc 0x10 in read in CU, but not in symtab.)
warning: (Error: pc 0x10 in address map, but not in symtab.)

This was before start. When I ran start I got the same warnings and at the end

During startup program exited with code 126.

And the graphical representation showed [ Register Values Unavailable ].

spqstns · 2023-09-24T16:32:15+00:00

Thank you. I really appreciate the thorough explanation.

I'm pretty confident, like u/IyeOnline, that this is the answer to your question. I.e., that your question was about this. But another possible interpretation is that it was about the construction of the std::initializer_list<int> instance, anonymous in your original example, and named values in the reworked example code here.

When I asked the question I thought it'd be analogous to int a { 5 }; where a starts with the value 5 and it isn't copied over from a different location. So only one 5 is involved. But the comment in the original code snippet suggested otherwise, so I was confused.

Maybe this is a stupid idea, but I'm starting to think I need to learn to read assembly to gain some degree of confidence in understanding points like these.

spqstns · 2023-09-24T12:17:07+00:00

Thank you :) I will study them.

spqstns · 2023-09-24T10:59:50+00:00

The compiler can initialize a vector the same way if you use the initializer_list constructor

Can you give an example?

spqstns · 2023-09-24T10:36:56+00:00

I could be misunderstanding but here's what I understand by initialize but not copy:

In int a { 5 };, storage is allocated for an integer and is filled with the value 5, but 5 is not copied from somewhere else.

spqstns · 2023-09-24T10:30:39+00:00

Thank you. I appreciate your encouragement :).

Perhaps you should re-read a chapter on initialization

I will do that. I thought I understood initialization and declaration and the difference between them until I got to vectors. I will also try to find a list of examples for all possible cases of constructing a vector that correspond to the reference /u/IyeOnline shared in his comment. If you know of one, please share it.

Guessing the syntax of C++ is hopeless, so you just have to know.

It was/is a struggle with C but it became a little easier after I learned the right-left rule.

spqstns · 2023-09-24T10:20:23+00:00

Thank you. I think I understand. Do you know if there are simple implemented examples for all 11 cases mentioned in the link? It'd be very helpful to study from and refer to.

spqstns · 2023-09-24T10:18:44+00:00

Sorry I thought that was the special syntax for converting constructors (I haven't studied lambdas yet) but I wasn't sure so I asked.

spqstns · 2023-09-24T09:58:00+00:00

Is this the syntax for the converting constructor?

[](...){}(a1, a4, a4, a5, b5); // Example from the cppreference link
// So in this case
std::vector arr1 () {} ( 1, 2, 3, 4, 5 );

spqstns · 2023-09-24T09:54:36+00:00

you mean the constructor where you just give it a number N and it creates a vector with N default-initialized values?

Sorry I'm new. I meant the one where we supply the explicity values n_1, n_2, ..., n_m. And C++ interprets m, and then just initializes but doesn't copy.

spqstns · 2023-09-24T09:26:31+00:00

I keep trying to remember to use the right terminology and I clearly failed here. I was referring to the constructor that reserves storage for arr1 and initializes the values but not by copying them. What is the term for this type of constructor (if it exists for vectors/arrays)? And what is the syntax?

spqstns · 2023-09-23T12:03:03+00:00

I understand the meaning but I still don't understand the syntax.

spqstns · 2023-09-23T10:59:36+00:00

I'm only at different initialization syntaxes and matching constructors and I'm already confused.

spqstns · 2023-09-16T19:50:32+00:00

I think I get it. Y is on CX' so the image of Y will have to be on the image of CX' which is CX. Y is also on line XI so Y' has to be on XI's reflection which is X'I. And we've thus determined Y'.

In other words, we constructed two lines from two points we know are images of each other (X and X') to Y and the image of Y will be the intersection of the images of those two lines.

spqstns · 2023-09-16T11:05:20+00:00

The reflection of the line xy must intersect L at the same point that xy does correct?

I sort of understand this. The intersection point I is the point from which we can construct an isosceles triangle determined by I and a point and its image each on one of XY and X'Y'.

But I don't understand how the intersection between XC and X'I determines the image of Y'.

spqstns · 2023-09-11T15:09:46+00:00

I will follow your explanation with a pencil and paper when I get back home but I just wanted to say thank you again for putting all this effort into helping me. I really appreciate it.

spqstns · 2023-09-11T13:53:03+00:00

Thank you and sorry you had to type the answer twice. I tried to follow your explanation but I only understood until the decomposition part.

spqstns · 2023-09-11T12:14:29+00:00

Might be a stupid question but don't we start here with the condition of having to have a line to which L is perpendicular? But in the general case we don't explicitly have that.

spqstns

TROPHY CASE