Segmentation fault with memmove and strcat

Question

#include <stdio.h>
#include <string.h>


char* longestConsec(char* strarr[], int n, int k)
{
    char* longest_arr[k];
    unsigned int longest_len = 0;
    if (n == 0 || k > n || k <= 0)
        return "";


    for (int i = 0; i < (n - (k - 1)); i++)
    {
        /*
        if(( strlen(strarr[i]) + strlen(strarr[i + 1]) ) > longest_len)
        {
            longest_len = strlen(strarr[i]) + strlen(strarr[i + 1]);
            memmove(longest_arr1, strarr[i], 10);
            memmove(longest_arr2, strarr[i + 1], 10);
        }
        */
        unsigned int cmp_len = 0;


        // get the length of every k consecutive string in a list and add them up to cmp_len
        for (int j = 0; j < k; j++)
            cmp_len += strlen( strarr[i + j] );


        // compare if cmp_len is greater than longest_len then add that string into longest_arr overlap the previous one
        if (cmp_len > longest_len)
        {
            for (int m = 0; m < k; m++)
                memmove(longest_arr[m], strarr[i + m], strlen(strarr[i + m]) + 1); // the first suspect
            longest_len = cmp_len;
        }
    }

    // if there is more than 1 consecutive string then the output is combine of k consecutive string in the list
    if (k > 1)
    {
        for (int l = k - 1; l >= 0; l--)
            strcat(longest_arr[l - 1], longest_arr[l]); // the second suspect
    }
    // return the longest_arr[0] the string that have been combine
    return longest_arr[0];
}


int main(void)
{
    // test subject
    char* a1[] = {"zone", "abigail", "theta", "form", "libe", "zas", "theta", "abigail"};
    char* newstring = longestConsec(a1, 8, 2);
    printf("%s\n", newstring);
    return 0;
}

when i try to run it got Segmentation fault. I suspect it is the memcpy and strcpy function but don't know how to fix it. The code is suppose to do get in a list of string and then check k consecutive string in that list, and print out the concatenation of those string.

You're doing string concatenation, but I don't see anywhere where you allocate space for the larger string. Thus you're almost certainly overrunning an array, which is undefined behavior. — TypeIA, Nov 26 '18 at 15:27
sorry but i am new to C so how can u allocate space for larger string in this situation? @TypeIA — haxerl, Nov 26 '18 at 15:49
That's a broad topic but reading the answers to the linked question would be a good start. You will need to learn and understand how memory management works in C. — TypeIA, Nov 26 '18 at 16:05

score 0 · Answer 1 · answered Nov 26 '18 at 15:33

The problem is here

char* longest_arr[k];
for (int m = 0; m < k; m++)  
    memmove(longest_arr[m], strarr[i + m], strlen(strarr[i + m]) + 1); /* there is no memory allocated for longest_arr[m] */

Since longest_arr is array of k character, you should allocate memory for each longest_arr[m]. for e.g

for (int m = 0; m < k; m++) {
        longest_arr[m] = malloc(SIZE); /* Allocate memory here, define the SIZE */
        memmove(longest_arr[m], strarr[i + m], strlen(strarr[i + m]) + 1);
}

And don't forget to free the dynamically allocated memory by calling free() function to avoid memory leakage.

Segmentation fault with memmove and strcat

1 Answers1