As I saw in the Python documentation,
https://docs.python.org/3/library/mmap.html.
Python in Linux can fully support the memory-mapped file. However, while I am trying to apply this idea to my application. I cannot run the sample.
My application is that sending the frames from Python file (client) to the another Python file (server).
Client Code
import mmap
import time
import os
import cv2 as cv
print("Opening camera...")
cap = cv.VideoCapture('/home/hunglv/Downloads/IMG_8442.MOV')
mm = None
try:
while True:
ret, img = cap.read()
if not ret:
break
if mm is None:
mm = mmap.mmap(-1,img.size,mmap.MAP_SHARED, mmap.PROT_WRITE)
# write image
start = time.time()
buf = img.tobytes()
mm.seek(0)
mm.write(buf)
mm.flush()
stop = time.time()
print("Writing Duration:", (stop - start) * 1000, "ms")
except KeyboardInterrupt:
pass
print("Closing resources")
cap.release()
mm.close()
Server Code
import mmap
import time
import os
import cv2 as cv
import numpy as np
shape = (1080, 1920, 3)
n = np.prod(shape)
mm = mmap.mmap(-1, n)
while True:
# read image
print (mm)
start = time.perf_counter()
mm.seek(0)
buf = mm.read(12)
img = np.frombuffer(buf, dtype=np.uint8).reshape(shape)
stop = time.perf_counter()
print("Reading Duration:", (stop - start) * 1000, "ms")
cv.imshow("img", img)
key = cv.waitKey(1) & 0xFF
key = chr(key)
if key.lower() == "q":
break
cv.destroyAllWindows()
mm.close()
On the server-side, I set the memory index at 0, and try to read the bytes from memory. However, it seems to be that the server cannot read correctly the data from client.
[Updated] I've tried to read out the first 12 bytes at the server-side. The value is constant, not changing anymore.
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
Besides, The first 12 bytes of a random frame is
b'\xf5\xff\xff\xf0\xfa\xfe\xdf\xe9\xed\xd2\xdc\xe0'
First I found example which probably works but it uses tagName (the same for client and server) and it means it is only for Window:
Next I found code which works on Linux:
Sharing Python data between processes using mmap.
It creates real file on disk, resizes it to size of image and then it uses its fd in mmap()
I use web camera for test.
Server
import mmap
import time
import os
import cv2
print("Opening camera...")
cap = cv2.VideoCapture(0)
#print(cap.get(cv.CAP_PROP_FRAME_WIDTH)) # 640
#print(cap.get(cv.CAP_PROP_FRAME_HEIGHT)) # 480
shape = (480, 640, 3)
n = (480*640*3)
fd = os.open('/tmp/mmaptest', os.O_CREAT | os.O_TRUNC | os.O_RDWR)
#os.write(fd, b'\x00' * n) # resize file
os.truncate(fd, n) # resize file
mm = None
try:
while True:
ret, img = cap.read()
if not ret:
break
if mm is None:
mm = mmap.mmap(fd, n, mmap.MAP_SHARED, mmap.PROT_WRITE) # it has to be only for writing
# write image
start = time.perf_counter()
buf = img.tobytes()
mm.seek(0)
mm.write(buf)
mm.flush()
stop = time.perf_counter()
print("Writing Duration:", (stop - start) * 1000, "ms")
except KeyboardInterrupt:
pass
print("Closing resources")
cap.release()
mm.close()
Client
import mmap
import time
import os
import cv2
import numpy as np
shape = (480, 640, 3)
n = (480*640*3)
fd = os.open('/tmp/mmaptest', os.O_RDONLY)
mm = mmap.mmap(fd, n, mmap.MAP_SHARED, mmap.PROT_READ) # it has to be only for reading
while True:
# read image
start = time.perf_counter()
mm.seek(0)
buf = mm.read(n)
img = np.frombuffer(buf, dtype=np.uint8).reshape(shape)
stop = time.perf_counter()
print("Reading Duration:", (stop - start) * 1000, "ms")
cv2.imshow("img", img)
key = cv2.waitKey(1) & 0xFF
key = chr(key)
if key.lower() == "q":
break
cv2.destroyAllWindows()
mm.close()
BTW: probably mmap() with -1 (without creating file on disk) could work with threads (or forks) because they share the same memory.