How to reverse an array of strings and reverse each string in place using pointers?

Question

I am learning C and I'm trying to reverse each string in an array in place with pointers. When i run the code below, I get the warnings that passing argument 1 and 2 of 'swapChars' makes pointer from integer without a cast. When I run it, I get a "bus error". Does anyone know why?

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

void swapChars(char *a, char *b) {
    char temp = *a; 
    *a = *b; 
    *b = temp; 
}


void reverse(int size, char* arr[]) { 
    for(int w = 0; w < size; w++) {
        int length = strlen(arr[w]); 
        for(int i = 0; i < length/2; i++) {
            swapChars(arr[w][i], arr[w][length-i]);
        }
    }
}

EDIT: thank you! I don't get any errors/warnings anymore, but do you know why it doesn't print anything when i run it with "hello world" ? this is my main():

int main(int argc, char* argv[]) {
    int numWords = argc-1;
    char** words = argv+1;
    reverse(numWords,words);
    for (char** word = words; word < words+numWords; word++) {
        printf("%s ",*word);
    }
    printf("\n");
    return 0;
}

Answer 1

swapChars( &(arr[w][i]), &(arr[w][(length-1)-i]));

You need to pass in the address of ( or "pointer to" ) the array element, not the array element itself. Without this the compiler is warning you that it will try to convert the array element's value into a pointer - which it suspects is the wrong thing to do. So it warns you about this. And indeed - it is not what you intended.

@Pi is more right than I am. Fixed it. Add the -1 in the second argument because otherwise the null at the end of the string gets reversed also. That is why your string doesn't print. Probably.

Answer 2

When you are passing your arguments, you are dereferencing both values that you are passing in.
To correct this, the line should read

swapChars(&arr[w][i], &arr[w][length-1-i]);

This will pass in the addresses, rather than the values.