indirections

An indirection is a concept in computing and logic where a reference, pointer, or address is used to access data or a function indirectly. Instead of directly accessing the target, one first accesses an intermediate entity that holds the location of the target. This intermediate entity is the indirection.

In programming, indirections are fundamental to many data structures and algorithms. Pointers, for example, are a form of indirection; a pointer variable doesn't hold the data itself but rather the memory address where the data is stored. This allows for flexible data manipulation, such as dynamic memory allocation and linked lists. Function pointers are another type of indirection, enabling the execution of code at a specified memory address without knowing the function's name at compile time.

Virtual memory systems heavily rely on indirections. When a program accesses a virtual address, the operating system uses page tables to translate this virtual address into a physical memory address. The page table acts as the indirection, mapping the program's view of memory to the actual hardware memory locations. This allows for memory protection, sharing, and the illusion of a larger address space than physically available.

In networking, indirections are used in protocols like DNS (Domain Name System). When a user requests a website by its domain name (e.g., example.com), the DNS system provides an indirection by resolving the domain name to an IP address, which is the actual address used to connect to the server. This allows users to interact with memorable names instead of complex numerical IP addresses.

The use of indirections can introduce a small performance overhead due to the extra step of lookup or translation, but the flexibility, abstraction, and advanced capabilities they enable are often crucial for modern software and systems.