Segmentation fault when searching for the end of a string in main, but not in a function C++

Question

I'm trying to implement a simple string length function. If I write everything in main, the code segfaults. However, if I declare a strlen function, then it does not. Why is this happening?

Why does the following code work

using namespace std;
#include <iostream>

int strlen(char *s)
{
    char *p = s;
    while (*p != '\0')
        p++;
    return p - s;
}


int main()
{
    char *s;
    cin >> s;
    cout << strlen(s) << endl;
}

While this code segfaults?

using namespace std;
#include <iostream>

int main()
{
    char *s;
    cin >> s;
    char *p = s;
    while (*p != '\0')
        p++;
    cout << p-s << endl;
}

Both programs exhibit UB as s is a dangling (unallocated) pointer in both cases. It just so happens that one example appears to work while the other crashes. — Paul R
– Paul R, Commented Jan 9, 2014 at 7:49
Also, be careful with names like strlen. There is strlen from string.h or cstring, std::strlen from cstring, any of which might get included by other std library headers. So better put your function inside a namespace. — juanchopanza
– juanchopanza, Commented Jan 9, 2014 at 7:52

dandan78 · Accepted Answer · 2014-01-09 07:50:11Z

4

You haven't allocated any memory for s. The reason it works in one case and not the other is likely because of the wonders of undefined behavior in C++.

answered Jan 9, 2014 at 7:50

dandan78

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1 Comment

Lightness Races in Orbit Over a year ago

strikethrough "likely"

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