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How do you use the select function in socket programming?
The select function returns the total number of socket handles that are ready and contained in the fd_set structures, zero if the time limit expired, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR, WSAGetLastError can be used to retrieve a specific error code.
How does select () work?
select() works by blocking until something happens on a file descriptor (aka a socket). Data coming in or being able to write to a file descriptor — you tell select() what you want to be woken up by.
What is the purpose of the select mechanism?
The mechanism therefore needs to serve two purposes: (1) directing one channel towards the camera, (2) shutting the second channel to not disturb the measurement of the observing channel.
What is select and poll function?
The select() and poll() methods are used for multiplexing network sockets. For instance, a programmer can use these calls to know when there is data to be read on a socket. By delegating responsibility to select() and poll(), you don’t have to constantly check whether there is data to be read.
What does select () do in C?
The select() function indicates which of the specified file descriptors is ready for reading, ready for writing, or has an error condition pending.
What is select select in Python?
Python’s select() function is a direct interface to the underlying operating system implementation. It monitors sockets, open files, and pipes (anything with a fileno() method that returns a valid file descriptor) until they become readable or writable, or a communication error occurs.
Why Epoll is faster than select?
The main difference between epoll and select is that in select() the list of file descriptors to wait on only exists for the duration of a single select() call, and the calling task only stays on the sockets’ wait queues for the duration of a single call.
How does poll and select functions work in socket programming?
The function fileno converts a standard I/O file pointer into its corresponding descriptor, since select (and poll ) work only with descriptors. select is called after calculating the maximum of the two descriptors. In the call, the write-set pointer and the exception-set pointer are both null pointers.
Should I use select or poll?
select() only uses (at maximum) three bits of data per file descriptor, while poll() typically uses 64 bits per file descriptor. In each syscall invoke poll() thus needs to copy a lot more over to kernel space. A small win for select().
What select function returns on success?
On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets (that is, the total number of bits that are set in readfds, writefds, exceptfds). The return value may be zero if the timeout expired before any file descriptors became ready.
What is selectselect used for in Linux?
Select allows you to monitor the activity of several different file descriptors without having to dedicate a thread of your program to each function call. In order for you to get a more specific answer, you will probably have to mention what language you are programming in.
What is the correct way to use the select() method?
The correct way to use select() (or poll(), which is usually a better option) is: Set all of the file descriptors involved to nonblocking mode. For most use cases you want this because you want to do all your blocking inside select() (or poll()), not while servicing individual file descriptors.
What does the Select system call do?
Fortunately, the select system call does just that. The select system call monitors three sets of independent file descriptors. The file descriptors to be monitored are specified in the three file descriptor sets pointed by the second, third and fourth parameters to the select call.
How does a socket work in a server?
The server has a socket listening for new connections from clients. Assuming the socket to be non-blocking, the server blocks for accepting a connection from a prospective client. However, the server has to receive messages from existing clients on different sockets and has to block while receiving messages on those sockets.