treats diverse system resources—regular files, directories, devices, network sockets, and inter-process communication channels—using a unified file interface which is a file descriptor.
When you open a file, device, socket, or pipe in Unix, the kernel returns a file descriptor
file descriptor :- A file descriptor is a non-negative integer that the operating system assigns to a process as a unique identifier(or "handle").
example:
open("/etc/passwd", ...) → kernel sees a path, looks it up in the filesystem, returns fd (file descriptor integer like 3,4,5) pointing to a regular file on disk
socket(AF_INET, SOCK_STREAM, 0) → kernel allocates a network socket (no disk involved), returns fd pointing to that socket
now we can give file descriptor the integer 4 and ask for the data bytes it will check which process is linked to fd and gives the data back ez.
if network buffer is limited how we download big files?
- Server starts sending the 100 MB file
- Your kernel's TCP receive buffer (let's say 87 KB default) fills up with the first ~87 KB
- TCP flow control tells the server "buffer full, slow down"
-io.ReadAll internally runs a loop calling read(fd, buffer, size) repeatedly - Each read() call copies some bytes (typically 4-32 KB chunks) from the kernel buffer into your application's memory
- As space frees up in the kernel buffer, TCP tells the server "send more data"
