golang os *File.Readdir using lstat on all files. Can it be optimised?

I am writing a program that finds all sub-directories from a parent directory which contains a huge number of files using os.File.Readdir, but running an strace to see the count of systemcalls showed that the go version is using an lstat() on all the files/directories present in the parent directory. (I am testing this with /usr/bin directory for now)

Go code:

package main
import (
    	"fmt"
	"os"
)
func main() {
	x, err := os.Open("/usr/bin")
	if err != nil {
		panic(err)
	}
	y, err := x.Readdir(0)
	if err != nil {
		panic(err)
	}
	for _, i := range y {
	fmt.Println(i)
	}

}

Strace on the program (without following threads):

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 93.62    0.004110           2      2466           write
  3.46    0.000152           7        22           getdents64
  2.92    0.000128           0      2466           lstat // this increases with increase in no. of files.
  0.00    0.000000           0        11           mmap
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0       114           rt_sigaction
  0.00    0.000000           0         8           rt_sigprocmask
  0.00    0.000000           0         1           sched_yield
  0.00    0.000000           0         3           clone
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2           sigaltstack
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         1           gettid
  0.00    0.000000           0        57           futex
  0.00    0.000000           0         1           sched_getaffinity
  0.00    0.000000           0         1           openat
------ ----------- ----------- --------- --------- ----------------
100.00    0.004390                  5156           total

I tested the same with the C's readdir() without seeing this behaviour.

C code:

#include <stdio.h>
#include <dirent.h>

int main (void) {
    DIR* dir_p;
    struct dirent* dir_ent;

    dir_p = opendir ("/usr/bin");

    if (dir_p != NULL) {
        // The readdir() function returns a pointer to a dirent structure representing the next
        // directory entry in the directory stream pointed to by dirp.
        // It returns NULL on reaching the end of the directory stream or if an error occurred.
        while ((dir_ent = readdir (dir_p)) != NULL) {
            // printf("%s", dir_ent->d_name);
            // printf("%d", dir_ent->d_type);
            if (dir_ent->d_type == DT_DIR) {
                printf("%s is a directory", dir_ent->d_name);
            } else {
                printf("%s is not a directory", dir_ent->d_name);
            }

            printf("\n");
        }
            (void) closedir(dir_p);

    }
    else
        perror ("Couldn't open the directory");

    return 0;
}

Strace on the program:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000128           0      2468           write
  0.00    0.000000           0         1           read
  0.00    0.000000           0         3           open
  0.00    0.000000           0         3           close
  0.00    0.000000           0         4           fstat
  0.00    0.000000           0         8           mmap
  0.00    0.000000           0         3           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         3         3 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         4           getdents
  0.00    0.000000           0         1           arch_prctl
------ ----------- ----------- --------- --------- ----------------
100.00    0.000128                  2503         3 total

I am aware that the only fields in the dirent structure that are mandated by POSIX.1 are d_name and d_ino, but I am writing this for a specific filesystem.

Tried *File.Readdirnames(), which doesn't use an lstat and gives a list of all files and directories, but to see if the returned string is a file or a directory will eventually do an lstat again.

• I was wondering if it is possible to re-write the go program in a way to avoid the lstat() on all the files un-necessarily. I could see the C program is using the following syscalls. open("/usr/bin", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFDIR|0755, st_size=69632, ...}) = 0
brk(NULL) = 0x1098000
brk(0x10c1000) = 0x10c1000
getdents(3, /* 986 entries */, 32768) = 32752
• Is this something like a premature optimisation, which I shouldn't be worried about? I raised this question because the number of files in the directory being monitored will be having huge number of small archived files, and the difference in systemcalls is almost twice between C and GO version, which will be hitting the disk.

The package dirent looks like it accomplishes what you are looking for. Below is your C example written in Go:

package main

import (
	"bytes"
	"fmt"
	"io"

	"github.com/EricLagergren/go-gnulib/dirent"
	"golang.org/x/sys/unix"
)

func int8ToString(s []int8) string {
	var buff bytes.Buffer
	for _, chr := range s {
		if chr == 0x00 {
			break
		}
		buff.WriteByte(byte(chr))
	}
	return buff.String()
}

func main() {
	stream, err := dirent.Open("/usr/bin")
	if err != nil {
		panic(err)
	}
	defer stream.Close()
	for {
		entry, err := stream.Read()
		if err != nil {
			if err == io.EOF {
				break
			}
			panic(err)
		}

		name := int8ToString(entry.Name[:])
		if entry.Type == unix.DT_DIR {
			fmt.Printf("%s is a directory\n", name)
		} else {
			fmt.Printf("%s is not a directory\n", name)
		}
	}
}

Starting with Go 1.16 (Feb 2021), a good option is os.ReadDir:

package main
import "os"

func main() {
   files, e := os.ReadDir(".")
   if e != nil {
      panic(e)
   }
   for _, file := range files {
      println(file.Name())
   }
}

os.ReadDir returns fs.DirEntry instead of fs.FileInfo, which means that
Size and ModTime methods are omitted, making the process more efficient.

https://golang.org/pkg/os#ReadDir