Check if a file is not open nor being used by another process

Question

I my application, i have below requests: 1. There has one thread will regularly record some logs in file. The log file will be rollovered in certain interval. for keeping the log files small. 2. There has another thread also will regularly to process these log files. ex: Move the log files to other place, parse the log's content to generate some log reports.

But, there has a condition is the second thread can not process the log file that's using to record the log. in code side, the pseudocode similars like below:

#code in second thread to process the log files
for logFile in os.listdir(logFolder):
     if not file_is_open(logFile) or file_is_use(logFile):
          ProcessLogFile(logFile) # move log file to other place, and generate log report....

So, how do i check is a file is already open or is used by other process? I did some research in internet. And have some results:

try:
   myfile = open(filename, "r+") # or "a+", whatever you need
except IOError:
    print "Could not open file! Please close Excel!"

I tried this code, but it doesn't work, no matter i use "r+" or "a+" flag

try:
   os.remove(filename) # try to remove it directly
except OSError as e:
    if e.errno == errno.ENOENT: # file doesn't exist
        break

This code can work, but it can not reach my request, since i don't want to delete the file to check if it is open.

Answer 1

An issue with trying to find out if a file is being used by another process is the possibility of a race condition. You could check a file, decide that it is not in use, then just before you open it another process (or thread) leaps in and grabs it (or even deletes it).

Ok, let's say you decide to live with that possibility and hope it does not occur. To check files in use by other processes is operating system dependant.

On Linux it is fairly easy, just iterate through the PIDs in /proc. Here is a generator that iterates over files in use for a specific PID:

def iterate_fds(pid):
    dir = '/proc/'+str(pid)+'/fd'
    if not os.access(dir,os.R_OK|os.X_OK): return

    for fds in os.listdir(dir):
        for fd in fds:
            full_name = os.path.join(dir, fd)
            try:
                file = os.readlink(full_name)
                if file == '/dev/null' or \
                  re.match(r'pipe:\[\d+\]',file) or \
                  re.match(r'socket:\[\d+\]',file):
                    file = None
            except OSError as err:
                if err.errno == 2:     
                    file = None
                else:
                    raise(err)

            yield (fd,file)

On Windows it is not quite so straightforward, the APIs are not published. There is a sysinternals tool (handle.exe) that can be used, but I recommend the PyPi module psutil, which is portable (i.e., it runs on Linux as well, and probably on other OS):

import psutil

for proc in psutil.process_iter():
    try:
        # this returns the list of opened files by the current process
        flist = proc.open_files()
        if flist:
            print(proc.pid,proc.name)
            for nt in flist:
                print("\t",nt.path)

    # This catches a race condition where a process ends
    # before we can examine its files    
    except psutil.NoSuchProcess as err:
        print("****",err) 

Answer 2

I like Daniel's answer, but for Windows users, I realized that it's safer and simpler to rename the file to the name it already has. That solves the problems brought up in the comments to his answer. Here's the code:

import os

f = 'C:/test.xlsx'
if os.path.exists(f):
    try:
        os.rename(f, f)
        print 'Access on file "' + f +'" is available!'
    except OSError as e:
        print 'Access-error on file "' + f + '"! \n' + str(e)

Answer 3

You can check if a file has a handle on it using the next function (remember to pass the full path to that file):

import psutil

def has_handle(fpath):
    for proc in psutil.process_iter():
        try:
            for item in proc.open_files():
                if fpath == item.path:
                    return True
        except Exception:
            pass

    return False

Answer 4

I know I'm late to the party but I also had this problem and I used the lsof command to solve it (which I think is new from the approaches mentioned above). With lsof we can basically check for the processes that are using this particular file. Here is how I did it:

from subprocess import check_output,Popen, PIPE
try:
   lsout=Popen(['lsof',filename],stdout=PIPE, shell=False)
   check_output(["grep",filename], stdin=lsout.stdout, shell=False)
except:
   #check_output will throw an exception here if it won't find any process using that file

just write your log processing code in the except part and you are good to go.

Answer 5

A slightly more polished version of one of the answers from above.

from pathlib import Path


def is_file_in_use(file_path):
    path = Path(file_path)
    
    if not path.exists():
        raise FileNotFoundError
    
    try:
        path.rename(path)
    except PermissionError:
        return True
    else:
        return False

Answer 6

Instead on using os.remove() you may use the following workaround on Windows:

import os

file = "D:\\temp\\test.pdf"
if os.path.exists(file):
    try:
        os.rename(file,file+"_")
        print "Access on file \"" + str(file) +"\" is available!"
        os.rename(file+"_",file)
    except OSError as e:
        message = "Access-error on file \"" + str(file) + "\"!!! \n" + str(e)
        print message

Answer 7

You can use inotify to watch for activity in file system. You can watch for file close events, indicating that a roll-over has happened. You should also add additional condition on file-size. Make sure you filter out file close events from the second thread.

Answer 8

On Windows, you can also directly retrieve the information by leveraging on the NTDLL/KERNEL32 Windows API. The following code returns a list of PIDs, in case the file is still opened/used by a process (including your own, if you have an open handle on the file):

import ctypes
from ctypes import wintypes

path = r"C:\temp\test.txt"

# -----------------------------------------------------------------------------
# generic strings and constants
# -----------------------------------------------------------------------------

ntdll = ctypes.WinDLL('ntdll')
kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)

NTSTATUS = wintypes.LONG

INVALID_HANDLE_VALUE = wintypes.HANDLE(-1).value
FILE_READ_ATTRIBUTES = 0x80
FILE_SHARE_READ = 1
OPEN_EXISTING = 3
FILE_FLAG_BACKUP_SEMANTICS = 0x02000000

FILE_INFORMATION_CLASS = wintypes.ULONG
FileProcessIdsUsingFileInformation = 47

LPSECURITY_ATTRIBUTES = wintypes.LPVOID
ULONG_PTR = wintypes.WPARAM


# -----------------------------------------------------------------------------
# create handle on concerned file with dwDesiredAccess == FILE_READ_ATTRIBUTES
# -----------------------------------------------------------------------------

kernel32.CreateFileW.restype = wintypes.HANDLE
kernel32.CreateFileW.argtypes = (
    wintypes.LPCWSTR,      # In     lpFileName
    wintypes.DWORD,        # In     dwDesiredAccess
    wintypes.DWORD,        # In     dwShareMode
    LPSECURITY_ATTRIBUTES,  # In_opt lpSecurityAttributes
    wintypes.DWORD,        # In     dwCreationDisposition
    wintypes.DWORD,        # In     dwFlagsAndAttributes
    wintypes.HANDLE)       # In_opt hTemplateFile
hFile = kernel32.CreateFileW(
    path, FILE_READ_ATTRIBUTES, FILE_SHARE_READ, None, OPEN_EXISTING,
    FILE_FLAG_BACKUP_SEMANTICS, None)
if hFile == INVALID_HANDLE_VALUE:
    raise ctypes.WinError(ctypes.get_last_error())


# -----------------------------------------------------------------------------
# prepare data types for system call
# -----------------------------------------------------------------------------

class IO_STATUS_BLOCK(ctypes.Structure):
    class _STATUS(ctypes.Union):
        _fields_ = (('Status', NTSTATUS),
                    ('Pointer', wintypes.LPVOID))
    _anonymous_ = '_Status',
    _fields_ = (('_Status', _STATUS),
                ('Information', ULONG_PTR))


iosb = IO_STATUS_BLOCK()


class FILE_PROCESS_IDS_USING_FILE_INFORMATION(ctypes.Structure):
    _fields_ = (('NumberOfProcessIdsInList', wintypes.LARGE_INTEGER),
                ('ProcessIdList', wintypes.LARGE_INTEGER * 64))


info = FILE_PROCESS_IDS_USING_FILE_INFORMATION()

PIO_STATUS_BLOCK = ctypes.POINTER(IO_STATUS_BLOCK)
ntdll.NtQueryInformationFile.restype = NTSTATUS
ntdll.NtQueryInformationFile.argtypes = (
    wintypes.HANDLE,        # In  FileHandle
    PIO_STATUS_BLOCK,       # Out IoStatusBlock
    wintypes.LPVOID,        # Out FileInformation
    wintypes.ULONG,         # In  Length
    FILE_INFORMATION_CLASS)  # In  FileInformationClass

# -----------------------------------------------------------------------------
# system call to retrieve list of PIDs currently using the file
# -----------------------------------------------------------------------------
status = ntdll.NtQueryInformationFile(hFile, ctypes.byref(iosb),
                                      ctypes.byref(info),
                                      ctypes.sizeof(info),
                                      FileProcessIdsUsingFileInformation)
pidList = info.ProcessIdList[0:info.NumberOfProcessIdsInList]
print(pidList)

Answer 9

I provided one solution. please see the following code.

def isFileinUsed(ifile):
    widlcard = "/proc/*/fd/*"
    lfds = glob.glob(widlcard)
    for fds in lfds:
        try:
            file = os.readlink(fds)
            if file == ifile:
                return True            
        except OSError as err:
            if err.errno == 2:     
                file = None
            else:
                raise(err)
    return False

You can use this function to check if a file is in used.

Note: This solution only can be used for Linux system.