I'm writing a function to read from socket and return exact size of data. And I thought of codes like:
whlie len(buf) < required:
buf += socket.recv(required - len(buf))
return buf
But I think this might waste some CPU resources(not sure). I'm looking for a system call or something to return exact amount of data from socket, and it could be blocked until the required size is received.
This will not waste CPU resources.
sock.recv will block until at least one byte is available (but may buffer and return multiple bytes if they are available), so your application will not enter a busy loop.
In general, the buffer length of the sock.recv call should not have anything to do with the length of the message you'd like to retrieve.
String concatenation is not efficient for your buffer though.
You may want to consider using:
''.join() (b''.join() if using Python 3)StringIO and cStringIO modules (or io.BytesIO if using Python 3).sock.recv_into and StringIO / bytesIO . 
bytearray type, dabeaz.blogspot.com/2010/01/few-useful-bytearray-tricks.html
If you are using the default socket library you can use the following.
import socket
host, port = "localhost", 8000
num_bytes = 5000
sock = socket.socket()
sock.connect((host, port))
while True:
data = sock.recv(num_bytes)
handle(data)
You can take a look at the possible parameters here socket.recv documentation
num_bytes is the maximum number of bytes you will read, but may return less bytes than you intended to.See my example for Python 2.x socket module. Use memoryview() and recv_into(),
def readReliably(s,n):
buf = bytearray(n)
view = memoryview(buf)
sz = s.recv_into(view,n)
return sz,buf
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
...
sz,buf = io.readReliably(s,4)
a = struct.unpack("4B",buf)
print repr(a)
Size of the buffer returned by readReliably() can be greater than n, don't forget about it.
