Socket

• Each socket = one endpoint of a single connection.

Socket IDs (File Descriptors) (fd)

• File descriptor :

  • The term file descriptor comes from Unix and Unix-like operating systems, and it’s deeply rooted in how these systems abstract I/O.

  • In Unix philosophy, "everything is a file".

    • Disk files

    • Pipes

    • Devices

    • Sockets

    • Terminals

    • Etc.

  • So:

    • When you open a file → you get a file descriptor.

    • When you open a socket → you also get a file descriptor.

  • They’re both handled through the same I/O API: read() , write() , close() , etc.

  • This abstraction is why sockets use file descriptors — they’re just "special files" from the OS’s point of view.

• Usage :

  • The socket ID is used by your program to interact with the OS-managed socket.

  • The OS routes TCP packets based on the socket's internal connection tuple, not your program directly.

    • Local Socket A: 127.0.0.1:6060

    • Local Socket B: 127.0.0.1:50543 -> 127.0.0.1:6061

  • The OS  stores and manages the mapping between:

    • Socket ID

    • The internal state of that connection

    • File/socket types, flags, permissions, etc.

  • You can think of the socket ID  as an index or key into a kernel-managed table of open resources, much like a hashmap, though it's typically implemented as a simple indexed array inside the OS.

• You can safely have one listening socket on 6060  and one connected socket to 6061  in the same program.