Uses for multiple levels of pointer dereferences?

Question

When does using pointers in any language require someone to use more than one, let's say a triple pointer. When does it make sense to use a triple pointer instead of just using a regular pointer?

For example:

char  * * *ptr; 

instead of

char *ptr; 

Answer 1

each star should be read as "which pointed to by a pointer" so

char *foo; 

is "char which pointed to by a pointer foo". However

char *** foo; 

is "char which pointed to by a pointer which is pointed to a pointer which is pointed to a pointer foo". Thus foo is a pointer. At that address is a second pointer. At the address pointed to by that is a third pointer. Dereferencing the third pointer results in a char. If that's all there is to it, its hard to make much of a case for that.

Its still possible to get some useful work done, though. Imagine we're writing a substitute for bash, or some other process control program. We want to manage our processes' invocations in an object oriented way...

struct invocation {     char* command; // command to invoke the subprocess     char* path; // path to executable     char** env; // environment variables passed to the subprocess     ... } 

But we want to do something fancy. We want to have a way to browse all of the different sets of environment variables as seen by each subprocess. to do that, we gather each set of env members from the invocation instances into an array env_list and pass it to the function that deals with that:

void browse_env(size_t envc, char*** env_list);