Reading /proc and getrusage

Read your resource limits, your own /proc/self/status, and CPU/memory stats from getrusage. The /proc part is Linux-only; on macOS it prints a note and the rest still runs.

package main

import (
	"fmt"
	"os"
	"runtime"
	"strings"
	"syscall"
)

func main() {
	// How many open files this process is allowed.
	var lim syscall.Rlimit
	syscall.Getrlimit(syscall.RLIMIT_NOFILE, &lim)
	fmt.Printf("open-file limit: soft=%d hard=%d\n", lim.Cur, lim.Max)

	// /proc/self is a symlink to this process's own /proc entry (Linux).
	if status, err := os.ReadFile("/proc/self/status"); err == nil {
		for _, line := range strings.Split(string(status), "\n") {
			if strings.HasPrefix(line, "VmRSS") || strings.HasPrefix(line, "Threads") {
				fmt.Println(strings.Join(strings.Fields(line), " "))
			}
		}
	} else {
		fmt.Println("/proc not available (not Linux)")
	}

	// getrusage: CPU time split into user vs kernel, plus peak resident memory.
	var ru syscall.Rusage
	syscall.Getrusage(syscall.RUSAGE_SELF, &ru)
	fmt.Println("user CPU (s):", ru.Utime.Sec)
	fmt.Println("sys CPU (s):", ru.Stime.Sec)
	fmt.Println("peak RSS:", ru.Maxrss) // KB on Linux, bytes on macOS

	fmt.Println("NumCPU:", runtime.NumCPU(), "GOMAXPROCS:", runtime.GOMAXPROCS(0))
}

Save the code as main.go and run it:

go run main.go

Expected output on Linux:

open-file limit: soft=1024 hard=1048576
VmRSS: 6784 kB
Threads: 6
user CPU (s): 0
sys CPU (s): 0
peak RSS: 6784
NumCPU: 8 GOMAXPROCS: 8

Memory, thread, and CPU numbers vary by machine. On macOS there's no /proc, so the VmRSS/Threads lines are replaced by /proc not available (not Linux), and peak RSS is reported in bytes instead of kilobytes.