The worst problem with strtok() isn't that it modifies its input; it's that it's stateful. It retains a pointer to the input string internally, and returns subs

The worst problem with strtok() isn't that it modifies its input; it's that it's stateful. It retains a pointer to the input string internally, and returns subsequent words from it when called with an input of NULL. strsep() fixes this by using a char * argument instead.