Strlen returns undefined behaviour with C++

Question

int main ()
{
     char* tab=new char[14] ;
     cout << " lenght with sizeof: "<<sizeof(tab)<<endl;
     cout << " length with strlen: "<<strlen(tab)<<endl;
     
     system(" pause");
     return 0;
}

I got the output:

length with sizeof: 4

length with strlen: 30

I expect the result of sizeof but not what return strlen!

For those who will hurry to publish that it's a duplicate question. I want to say that it's not the opportunity at all. Because I know about compile time and run-time and many other things concerning strlen and sizeof however I cannot find explanation to this result.

Thank you for help in advance.

`sizeof(tab)` gives you the size of a `char*` pointer, not the allocated spaces size, or the size of a null terminated c-string stored in there. — πάντα ῥεῖ, Sep 26 '14 at 10:45
Your `char` array is not initialized, thus the result of `strlen` is undefined as it will count all characters until it founds a null character. — Jaffa, Sep 26 '14 at 10:47
@ πάντα ῥεῖ Thanks friend. I'm agree with you but you don't understand my question, I'm asking about the result returned by strlen wich is 30 !! — xtensa1408, Sep 26 '14 at 10:48
@SADOK Have a look at _@geoffroy_'s comment, that explains why. — πάντα ῥεῖ, Sep 26 '14 at 10:50
@Deduplicator, I thought that my problem is different because I allocated espace memory usinh new char ! — xtensa1408, Sep 26 '14 at 10:53
@πάντα ῥεῖ don't understand you! what a BTW duplicates ? — xtensa1408, Sep 26 '14 at 10:55
@SADOK Reads: _'And what's wrong with duplicates by the way?'_ — πάντα ῥεῖ, Sep 26 '14 at 10:56
Anyway, I know that my question is not like any other. Thank you all :) — xtensa1408, Sep 26 '14 at 11:01

score 4 · Accepted Answer · answered Sep 26 '14 at 10:47

4

Since you're allocating a char array but do not initialize it, strlen() will count from the beginning of the tab pointer to the first NUL character. So the result depends on the contents of your program's heap.

answered Sep 26 '14 at 10:47

Tore Olsen

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Should probably mention that this is undefined behaviour. – Rapptz Sep 26 '14 at 10:48
As is, this is an upvoted bad answer. – Deduplicator Sep 26 '14 at 10:55
Feel free to explain what's bad about it, or provide a better answer. – Tore Olsen Sep 26 '14 at 11:04

Strlen returns undefined behaviour with C++

1 Answers1