C++ segmentation fault

Question

I'm new to C++ and the memory nuances that are needed to write and debug the language. Can anyone tell me why the following code is giving me a segmentation fault?

string Polynomial::toString(){
    int i, exponent;
    stringstream result;

    for (i = 0; i < coeffs.size(); i++){

        // For first non-zero coefficient
        if (result.str().empty()){
            if(coeffs[i] < 0)
                result << "-";
            if(coeffs[i] != 0)
                result << coeffs[i];
        }
        else{
            if(coeffs[i] < 0)
                result << " - " << abs(coeffs[i]);
            else if(coeffs[i] > 0)
                result << " + " << coeffs[i];
        }

        exponent = (coeffs.size() - i - 1);
        if (coeffs[i] != 0){
            if (exponent > 1)
                result << coeffs[i] << "x^" << exponent;
            else if(exponent == 1)
                result << coeffs[i] << "x";
        }
    }

    result.str();
}

Can you show us where/how is coeffs defined, and how do you populate it with values? — SingerOfTheFall
– SingerOfTheFall, Commented Oct 17, 2012 at 5:47
Please show more code (notably declarations of all relevant classes), compile with all warnings and debugging information (e.g. g++ -Wall -g on Linux) and learn to use the debugger (gdb on Linux). — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 17, 2012 at 5:47
Maybe it's your lack of a return statement. That causes undefined behaviour, which can certainly crash. — Qaz
– Qaz, Commented Oct 17, 2012 at 5:48

juanchopanza · Accepted Answer · 2012-10-17 06:05:06Z

3

You are probably calling the function and assigning the result to something:

Polynomial p = ....;
std::string s = p.toString();

Since you have no return statement in Polynomial::toString(), which in itself is undefined behaviour, this can easily result in a segmentation violation. You can easily fix this by returning the stringstreams's `string:

return result.str();

answered Oct 17, 2012 at 6:05

juanchopanza

228k35 gold badges421 silver badges493 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Qaz · Accepted Answer · 2012-10-17 06:11:12Z

Your function is missing a return statement (probably just the return part). According to the standard, reaching the closing brace on any function besides main that has a return type is undefined behaviour, as per §6.6.2/2 of the C++11 standard, shown below, which usually results in a crash, though it might not always do so.

Other than that, there's nothing that could cause undefined behaviour or a crash. What you want to do is add the return part:

return result.str();

For what it's worth, GCC 4.7.2 gives the following warning:

warning: no return statement in function returning non-void [-Wreturn-type]

Always take advantage of the warnings compilers can and will give you.

Standard reference:

Flowing oﬀ the end of a function is equivalent to a return with no value; this results in undeﬁned behavior in a value-returning function.

Collectives™ on Stack Overflow

C++ segmentation fault

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related