Question

I come across the functions like strtok_s where you will need to pass a pointer to pointer argument.

strtok_r(char *restrict str, const char *restrict sep, char **restrict lasts);

The way to use it is:

char *foo;
char *str = ...;
char *delimiter = ...;
strtok_r(str, delimiter, &foo);

Wondering why pass the address of pointer foo into the function?

Answer 1

It’s so that strtok can resume where it left off. This version of strtok is thread safe (since it uses a pointer you supply and not the internal pointer of the other version).

It saves the address of the last token read in a char * so you need to pass a pointer to that pointer so it can change the value and return it to you.