Question

I have a loop running to add spaces between some special characters (&,|,<,>). The following code successfully adds spaces before and after a special character:

    char keys[] = "<>&|";
    int i = strcspn (input,keys); 
    appenda(input, " " , i);
    appenda(input, " " , i+2);

The above code converts input “asdf&asdf” to “asdf & asdf”.

However, my goal is to do that for every special character in the entire input even with multiple special characters (like “asdf&asdf&asdf”). So I made a while loop:

    char keys[] = "<>&|";
    int i = strcspn (input,keys);
    while(i < strlen(input)){
        appenda(input, " " , i);
        appenda(input, " " , i+2);
        i = strcspn (input,keys);
    }

Yet, when I run my code now, it returns with “*** stack smashing detected ***“

Any Ideas on what this means and how to get around it?

EDIT
Appenda inserts a string into another string at a designated point. it takes in 3 arguments: 1st is a string which i insert into, 2nd is a string which i want to insert and 3rd is the index. so appenda(ABCD, X, 2) returns AXBCD

Answer 1

You’re always calling strcspn on the same input, and you’re never actually removing the characters that it’s finding, so it is always finding the same one.

For example, if your string is

asdf&asdf&asdf

The first call to

int i = strcspn (input,keys)

Returns i = 3 since the & is in position 4. Then you insert the spaces, which works fine, and the string becomes:

asdf & asdf&asdf

Now you again call

i = strcspn (input,keys)

And this returns i = 4 because now it finds the first & in position 5. So when you insert spaces again the string becomes:

asdf  &  asdf&asdf

And so on. It keeps inserting more and more spaces around the first &, and the loop never ends until you overrun the buffer inside appenda and your program dies.

Instead, once you’ve inserted spaces, you need to tell strcspn to start looking for the next special character past the place where you found the previous one. This should work:

char keys[] = "<>&|";
int i = strcspn (input,keys);
while(i < strlen(input)){
    appenda(input, " " , i);
    i += 2;
    appenda(input, " " , i);
    i += strcspn (input + i,keys);
}

This “moves” the value of i such that when you call strcspn(input + i, keys), the value input + i always refers to the next position that has not yet been looked at for special characters.