Segmentation fault when trying dynamically allocate 2d char array

Question

I have written the next similar example when I get a segmentation fault error

{
    const char** var = NULL;
    const char* tmp = "Hello! How are you?";
    var = (const char**)malloc(5 * sizeof(char*));

    for (int i = 0; i < 5; i++)  
    {
        var[i] = (char*)malloc(50* sizeof(char));
        strcpy((char*)var[i], tmp);
    }
    for (int i = 0; var[i]; i++)
    {
        std::cout << (long int)var[i] << std::endl;
        std::cout << var[i] << std::endl;
    }
    // Free memory
    ....
    
}

And on the 6th iteration, for loop doesn't stop (I expected then var[i]==NULL) and I get the "Segmentation fault" error. Can you explain what I am doing wrong, please?

This isn't C, either. It appears to be C++, so retagged accordingly. — tadman
– tadman, Commented Dec 15, 2020 at 9:10
As pointed by Peter, out of your belongings memory can be anything, not necessarily NULL. — Gabriel Pellegrino
– Gabriel Pellegrino, Commented Dec 15, 2020 at 9:11
Is the intent of (long int)var[i] to perform a string to integer conversion, or to represent that pointer as a number just to see what it happens to be? — tadman
– tadman, Commented Dec 15, 2020 at 9:15

tadman · Accepted Answer · 2020-12-15 09:16:48Z

4

This walks off the end of the array:

for (int i = 0; var[i]; i++)

Accessing var out of bounds is undefined behaviour, which can lead to a crash. You won't get a nice, neat value when you exceed the bounds like you might in JavaScript, Python or Ruby, you get problems. Unfortunately there's no checks for this, so you must be vigilant about not doing it inadvertently, the responsibility is on you.

Since you're apparently using C++ there's a lot of things you can fix here:

Don't use C arrays if you can avoid it, use std::vector
Don't use C strings if you can avoid it, use std::string

Combining this advice you get:

const std::string tmp = "Hello! How are you?";

std::vector<std::string> var;

for (int i = 0; i < 5; i++) {
  var.push_back(tmp);
}

for (auto& v : var) {
  std::cout << v << std::endl;
}

// No need to free memory, it's already handled for you.

When you're writing C++, try and enjoy the Standard Library. It will make your life significantly easier than leaning heavily on error-prone C techniques.

edited Dec 15, 2020 at 9:16

answered Dec 15, 2020 at 9:10

tadman

212k23 gold badges237 silver badges266 bronze badges

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

3 Comments

Anton Golovenko Over a year ago

But how I can do it in C-style?

tadman Over a year ago

You do it very carefully and make a point to never, ever walk off the end of an array. Always keep track of the maximum index you can use. Normally this is done by having a size variable, though there are other ways, like const values. It really depends on what the context is.

tadman Over a year ago

This is like driving on a mountain road with a very steep drop on the one side and absolutely no barrier. How do you not drive off the mountain? You pay very careful attention to what you're doing.

Collectives™ on Stack Overflow

Segmentation fault when trying dynamically allocate 2d char array

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related