CS558 Sylabus

Timeout Programming using the `poll( )` system call

Timeout programming using the poll() system call

The poll( ) system call:

The poll( ) performs a similar task to select( ):

Differences:

Header file for using poll( )
- Header file for poll( ):

The poll( ) function

The poll( ) function syntax:

Syntax:

#include <poll.h> /* Header file */ int poll( struct pollfd fds[], nfds_t nfds, int n_msec );

Meaning of the parameters:

fds = an array of "struct pollfd" This array contains the file descriptors (sockets) that you want to check. (The pollfd structure will be explained later) n_fds = the length of the "struct pollfd" array (= number of descriptors to poll). n_msec = the timeout value in milli-seconds Special timeout values: n_msec = 0 ==> poll() returns immediately with the poll results n_msec = -1 ==> poll() blocks until a requested event occurs

Effect:

The poll() function examines the array of file/socket descriptors fds[ ] for n_msec milli-seconds.

If some descriptor (e.g., a socket) becomes ready before the before the time out value of n_msec time:

poll( ) will return immediately, and:

The return value is equal to the number of descriptors (sockets) that are ready
The fds[ ] array will contain information on which of the descriptors/sockets are ready

If after waiting for n_msec, there are not any descriptor (= socket) ready:

The pollfd (poll file descriptor) data structure

The struct pollfd data structure is as follows:

struct pollfd { int fd; // A descriptor (e.g., socket) short events; // This is a bit array // Each bit represents a certain operation // Set that bit to check for readiness // E.g, one or the bit represent receive short revents; // ready events // This is also a bit array // Each bit represents whether some operation is ready }

Meaning of the parameters:

fd = the (one) file descriptor/socket to be checked events = input bit array of events that will be checked revents = output bit array of events that are ready

The types of events are defined by the following bit masks:

POLLIN (= 00001) = ready for input (receive) POLLOUT (= 00010) = ready for output (send)
Other less used functions: POLLRDBAND (= 00100) = high priority read data ready POLLWRBAND (= 01000) = high priority output (send) ready POLLERR (= 10000) = error detected

Examples on how the data field is used:

Suppose you want to check if:
then you set the values as follows:
Result:

Checking for ready events in poll( )
- Checking for readiness:
- Example:

Setting the descriptor sets in the pollfd structure with sockets

Example 1:

We have 1 socket:

int s = socket( PF_INET, SOCK_DGRAM, 0 );

in our program

We want to check the socket s for receive readiness

Solution:

Define an pollfd arary of 1 element:

struct pollfd fds[1]; // Array of 1 element (The array element has index 0: fds[0])

Initialize the poll structure fd[0] to test s for read readiness:

fds[0].fd = s; // set descriptor fd to s fds[0].events = 0 // Clear the bit array fds[0].events = fds[0].events | POLLIN ; // Set the POLLIN bit

Call poll( ) with timeout:

timeout = 100; // 100 msec = 0.1 sec if ( poll ( fds, 1, timeout ) == 0 ) { // Time out } else { // Ready }

Example 2:

We have 2 sockets:

int s1 = socket( PF_INET, SOCK_DGRAM, 0 ); int s2 = socket( PF_INET, SOCK_DGRAM, 0 );

in our program

We want to check when either socket s1 or s2 has some message that can be received

Solution:

Define a pollfd arary of 2 elements (because we have 2 sockets):

struct pollfd fds[2]; // Array of 2 element (The array elements are: fds[0] and fds[1])

Initialize the poll structure fd[0] to test s1 for read readiness:

fds[0].fd = s1; // set descriptor fd to s1 fds[0].events = 0 // Clear the bit array fds[0].events = fds[0].events | POLLIN ; // Set the POLLIN bit

Initialize the poll structure fd[1] to test s2 for read readiness:

fds[1].fd = s2; // set descriptor fd to s2 fds[1].events = 0 // Clear the bit array fds[1].events = fds[0].events | POLLIN ; // Set the POLLIN bit

Call poll( ):

timeout = 100; // 100 msec = 0.1 sec if ( poll ( fds, 2, timeout ) == 0 ) { // Time out } else { for ( i = 0; i < nfds; i++ ) { if ( (fds[i].revents | POLLIN) == POLLIN ) { // fds[i].fd is ready } } }

Example: Timeout programming with poll() system call

Here is the select example (discussed here: click here) re-written using poll():

// I need to create a scoket and bind it first.... /* --- Create a socket --- */ s = socket(PF_INET, SOCK_DGRAM, 0); /* --- Set up socket end-point info for binding --- */ in_addr.sin_family = AF_INET; /* Protocol family */ in_addr.sin_addr.s_addr = htonl(INADDR_ANY); /* Wildcard IP address */ in_addr.sin_port = htons(atoi(argv[1])); /* Use any UDP port */ /* --- Bind socket s --- */ bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) /* ============================================== How to use poll( ) ============================================== */ struct pollfd fds[1]; // 1 socket int timeout = 5000; // 5000 msec = 5 sec /* ====================================== Set up fds[0] to monitor socket s ====================================== */ fds[0].fd = s; // Test socket s fds[0].events = 0; // Clear events fds[0].events = fds[0].events | POLLIN; ; // Set INPUT bit int result ; printf("Now use udp4-s3 and send a UDP message to port %s before %d sec\n", argv[1], timeout/1000 ); /* ================================ Here we go.... ================================ */ result = poll( fds, 1, timeout ); printf("poll( ) returns: %d\n", result);

Example Program: (Demo above code)

The poll( ) test Prog file: click here
Use this sender to send a message to the select program: click here

How to run the program:

Right click on link(s) and save in a scratch directory
To compile the poll1 test prog: gcc -o poll1 poll1.c
To compile the udp4-s3 test prog: gcc -o udp4-s3 udp4-s3.c -lnsl
To run: poll1 8000 on machine X
To send a msg to select prog: udp4-s3 X 8000

Timeout Programming using the poll( ) system call

Timeout Programming using the `poll( )` system call