How does defer work in Go and what are the common pitfalls?

defer schedules a function call to run when the surrounding function returns. Deferred calls run in LIFO order and are typically used for cleanup such as closing files and unlocking mutexes.

f, err := os.Open("data.txt")
if err != nil {
    return err
}
defer f.Close()

mu.Lock()
defer mu.Unlock()

Pitfalls to know:

• Arguments are evaluated when the defer statement runs, not when the deferred call runs. defer fmt.Println(i) captures the current i.
• Deferred functions can read and modify named return values, which enables error wrapping patterns.
• A defer inside a long loop accumulates until the function returns, potentially leaking file descriptors. Wrap the body in a closure or close explicitly.

for _, p := range paths {
    func() {
        f, _ := os.Open(p)
        defer f.Close()
    }()
}

Deferred calls run even when the function panics, which makes them reliable for releasing resources.