fcntl and ioctl System Calls in Python

To control files and io, we should usefcntlmodule. It is basicallyfcntl()andioctl()An interface for Unix routines.

All methods in this module take an integer or io.IOBase file descriptor as their first parameter.

To use this module, we should use the import statement.

import fcntl

There are some modules in the fcntl module, which are-

The method fcntl.fcntl(fd, op[, arg])

This method is used to perform operations on a file using a file descriptor. The operation is determined byopDefinition. The third parameter is optional. It can be an integer type value or a string type value. When the parameter is of integer type, the return value will be Cfcntl()The value called. If it is a string, it represents a binary structure. When this function fails, it will raise an IOError.

The method fcntl.ioctl(fd, op[, arg[, mutate_flag]])

This method is the same as this method.fcntl(), but in this case, the parameter handling is more complex. In the parameters, if a variable buffer is passed, its behavior will depend on mutate_flag. If it is true, the buffer can be variable, otherwise it will act as a read-only buffer.

Method fcntl.flock(fd, op)

This method is used to perform locking operations on a file using file_descriptor. On some systems, it can be usedfcntl()This method simulates this method.

Method fcntl.lockf(fd, operation [, length [, start [, whence]]])

This method is used as a wrapper for locking calls. Pass the operation parameter to lock or unlock the file. There are different operation values.

• LOCK_UN-Unlock File

• LOCK_SH-Shared Lock

• LOCK_EX-Exclusive Lock

Example Code

import fcntl, os, time
counter_file = 'my_counter.txt'
if not os.path.exists(counter_file):
　　　counter_file = open('my_counter.txt', 'w')
　　　counter_file.write('0') # Store 0 as starting number
　　　counter_file.close()
for i in range(15)
　　　counter_file = open('my_counter.txt', 'r+)
　　　fcntl.flock(counter_file.fileno(), fcntl.LOCK_EX)
　　　count = int(counter_file.readline())　+　1
　　　counter_file.seek(0)
　　　counter_file.write(str(count))
　　　counter_file.close()
　　　print('Process ID: '　+　str(os.getpid())　+　', Count: '　+　str(count))
　　　time.sleep(0.2)

Output Result

$ python3　example.py
Process ID:　12698, Count:　1
Process ID:　12698, Count:　2
Process ID:　12698, Count:　3
Process ID:　12698, Count:　4
Process ID:　12698, Count:　5
Process ID:　12698, Count:　6
Process ID:　12698, Count:　7
Process ID:　12698, Count:　8
Process ID:　12698, Count:　9
Process ID:　12698, Count:　10
Process ID:　12698, Count:　11
Process ID:　12698, Count:　12
Process ID:　12698, Count:　13
Process ID:　12698, Count:　14
Process ID:　12698, Count:　15
$
$
$ cat my_counter.txt
15
$