Where should I use a type char ** in some case as the input parameter?

Pointer to pointer is used where the passed pointer is need to be modified in the called function and that modification need to be seen in the caller function. This is required because in C arguments are passed by value. So, when an argument is passed to a function then it simply copied to the function's parameter and created a local copy having block scope. Any change to that variable will not be seen in the argument that has been passed.

void foo(in x){    x = 10;}int main(void){     int x = 5;     foo(x)     printf("%d\n", x);  // x will be 5}

Same happens with pointers

void bar(char *p){    p = "foobar";}int main(void){     char *p = NULL;     bar(p);     if(p)         printf("p is pointing to %s\n", p);     else          printf("p is NULL\n");   // This will }

Using a pointer to pointer will do the desired job (pointing p to the string "foobar"

void bar(char **p){ *p = "foobar"; }

int main(void){     char *p = NULL;     bar(&p);     if(p)         printf("p is pointing to %s\n", p);     else          printf("p is NULL\n");   // This will }  

Another use is when an array of string is need to passed to a function. Like

int main(int argc, char **argv)  

or

void print_fruits(char **fruits, size_t len){    for(int i = 0; i < len; i++)        printf("%s\n", fruits[i]);}int main(void){    char *fruits[5] = {"Apple", "Banana", "Cherry", "Kiwi", "Orange"};    print_fruits(fruits, sizeof(fruits)/sizeof(fruits[0]));}

Note that in function call print_fruits, fruits in the argument list will decay to pointer to its first element and the expression fruits will become of type char ** after the conversion.