When I malloc more space for a string within an array of strings, the array of strings duplicates some strings

Question

I have an array of strings, and I am trying to malloc more space for one of those strings so I can change the value of the string.

int catenate_strings (char** arr, int index1, int index2) { 
    char *new_string;
    new_string = malloc(1000*sizeof(char));
    if (new_string == NULL) {
        printf("\nError allocating memory\n");
    }   

    strcpy(new_string, arr[index1]);
    strcat(new_string, arr[index2]);
    arr[index1] = new_string;
}

However when I run my code, it will work for some instances, but in others it will duplicate the string at index1 and put it at index1 + 1 as well.

Memory is leaking when you do `arr[index1] = new_string`, you are losing the previous string. — Selçuk Cihan, Jan 21 '16 at 08:40
I am trying to resize the space allocated for the string so I can put a longer string there. — John Doe, Jan 21 '16 at 08:40
Another possibility is overflowing the buffer of `new_string` — M.M, Jan 21 '16 at 08:48

score 1 · Answer 1 · answered Jan 21 '16 at 12:57

Your code has some problems:

Memory leak in arr[index1] = new_string because you are not freeing the old buffer.
Buffer overflow if the result string is longer than 1000 bytes.
Not returning any value from catenate_strings despite the function having return value int.

If all entries in arr are allocated using malloc then you can use realloc.

int catenate_strings (char** arr, int index1, int index2)
{ 
    // Resize buffer to hold old string + new string (+ terminating null byte!)
    char * const new_string = realloc(strlen(arr[index1]) + strlen(arr[index2]) + 1);
    if (new_string == NULL) {
        printf("\nError allocating Memory, cannot resize string\n");
        return -1;
    }   
    strcat(new_string, arr[index2]);
    arr[index1] = new_string;

    return 0;
}

The duplication to index+1 comes not from the shown code but from somewhere else in your code.

When I malloc more space for a string within an array of strings, the array of strings duplicates some strings

1 Answers1