My function argument is being altered but why?

Code:

#define ARRSIZE 200

        void itoa(signed char, char[]);
        void reverse(char[]);

int main(void) {
        
        signed char num = -127;
        char string[ARRSIZE];

        printf("%d\n\n", num);

        itoa(num, string);

        printf("%d\n\n", num);

        printf("\n%s\n", string);

return 0;}
/*++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*/
/*ITOA converts signed int n into its character string                 */
/*equivalent in s                                                */
void itoa(signed char n, char s[]) {

        char i=0;
        signed sign;

        if ((sign = n) < 0)
                n=-n;
        
        do {
                s[i++] = n % 10 + '0';}
        while ((n/=10) > 0);
        
        if (sign < 0)
                s[i++] = '-';
        
        s[i]=0;

        reverse(s);
}
/*==============================================================*/
/*REVERSE reverses the ordering of values in string s                */
void reverse(char s[ARRSIZE]) {

        char i=0, j;
        char temp[ARRSIZE];
                
        while (s[i]!=0)
                ++i;

        for (j=i-1, i=0; s[i]!=0; --j, ++i) 
                temp[i] = s[j];

        for (; s[j]!=0; ++j) 
                s[j]=temp[j];
}

As you can see, I've placed 2 printfs around my 'itoa' function call in main. According to my text book, the value displayed in both cases for the value of 'num' shouldn't differ because in C "all function arguments are passed 'by value.'" However it is -127 and 4 respectively. I'm confused! Can someone please demystify this situation for me please?