#undefined-behavior

[sevennet-blog]

How to: Is the "struct hack" technically undefined behavior? #computers #fix #dev

How to: Is the "struct hack" technically undefined behavior? #computers #fix #dev

Is the "struct hack" technically undefined behavior?

What I am asking about is the well known “last member of a struct has variable length” trick. It goes something like this:

struct T { int len; char s[1]; }; struct T *p = malloc(sizeof(struct T) + 100); p->len = 100; strcpy(p->s, "hello world");

Because of the way that the struct is laid out in memory, we are able to overlay the struct over a…

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[goodanswerbiz]

Fixed Undefined, unspecified and implementation-defined behavior #dev #it #asnwer

Fixed Undefined, unspecified and implementation-defined behavior #dev #it #asnwer

Undefined, unspecified and implementation-defined behavior

What is the difference between undefined, unspecified, and implementation-defined behavior in C and C++?

Answer: Undefined, unspecified and implementation-defined behavior

From the official C Rationale Document

The terms unspecified behavior, undefined behavior, and implementation-definedbehavior are used to categorize the result of…

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[goodanswerbiz]

Fixed What are all the common undefined behaviours that a C++ programmer should know about? #dev #it #asnwer

Fixed What are all the common undefined behaviours that a C++ programmer should know about? #dev #it #asnwer

What are all the common undefined behaviours that a C++ programmer should know about?

What are all the common undefined behaviours that a C++ programmer should know about?

Say, like:

a[i] = i++;

Answer: What are all the common undefined behaviours that a C++ programmer should know about?

The only type for which C++ guarantees a size is char. And the size is 1. The size of all other types is…

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[sevennet-blog]

How to: How did I get a value larger than 8 bits in size from an 8-bit integer?

How to: How did I get a value larger than 8 bits in size from an 8-bit integer?

How did I get a value larger than 8 bits in size from an 8-bit integer?

I tracked down an extremely nasty bug hiding behind this little gem. I am aware that per the C++ spec, signed overflows are undefined behavior, but only when the overflow occurs when the value is extended to bit-width sizeof(int). As I understand it, incrementing a char shouldn’t ever be undefined behavior as long as sizeof(…

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[sevennet-blog]

How to: Why does this loop produce "warning: iteration 3u invokes undefined behavior" and output more than 4 lines?

How to: Why does this loop produce "warning: iteration 3u invokes undefined behavior" and output more than 4 lines?

Why does this loop produce "warning: iteration 3u invokes undefined behavior" and output more than 4 lines?

Compiling this:

#include <iostream> int main() { for (int i = 0; i < 4; ++i) std::cout << i*1000000000 << std::endl; }

and gcc produces the following warning:

warning: iteration 3u invokes undefined behavior [-Waggressive-loop-optimizations] std::cout << i*1000000000 << std::endl; ^

I…

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[goodanswerbiz]

Fixed Why are these constructs (using ++) undefined behavior? #dev #it #asnwer

Fixed Why are these constructs (using ++) undefined behavior? #dev #it #asnwer

Why are these constructs (using ++) undefined behavior?

int main(int argc, char ** argv) { int i = 0; i = i++ + ++i; printf("%d\n", i); // 3 i = 1; i = (i++); printf("%d\n", i); // 2 Should be 1, no ? volatile int u = 0; u = u++ + ++u; printf("%d\n", u); // 1 u = 1; u = (u++); printf("%d\n", u); // 2 Should also be one, no ? register int v = 0; v = v++ + ++v; printf("%d\n", v); // 3 (Should be…

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[goodanswerbiz]

Fixed What is the strict aliasing rule? #dev #it #asnwer

Fixed What is the strict aliasing rule? #dev #it #asnwer

What is the strict aliasing rule?

When asking about common undefined behavior in C, souls more enlightened than I referred to the strict aliasing rule. What are they talking about?

Answer: What is the strict aliasing rule?

As addendum to what Doug T. already wrote, here is a simple test case which probably triggers it with gcc :

check.c

#include void check(short *h,long *k) { *h=5; *k=6; if (*h…

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[goodanswerbiz]

Fixed Undefined Behavior and Sequence Points #dev #it #asnwer

Fixed Undefined Behavior and Sequence Points #dev #it #asnwer

Undefined Behavior and Sequence Points

What are “Sequence Points”? What is the relation between Undefined Behaviour and Sequence Points?

I often use funny and convoluted expressions like a[++i] = i;, to make myself feel better. Why should I stop using them?

If you’ve read this, be sure to visit the follow-up question Undefined Behavior and Sequence Points Reloaded.

(Note: This is meant to be an…

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