CS558 Sylabus

Selecting (clearing/setting) the (socket) descriptors in general

The data type of the descriptor sets recv_fds

What is a Descriptor set:

The variable recv_fds in the select( ) call:
is the descriptor set (that is being checked for readiness)

The data type of the recv_fds variable is:

fd_set recv_fds ;

The fd_set variable is:

a bit array of size FD_SETSIZE (=1024) number of bits
(Use: "man select" to find out more)

In my previous example, I used a short bit array:
This will work as long as you use ≤ 32 descriptors (i.e., use less than around 30 sockets !!!)

Using select( ) with a fd_set variable

How the select( ) function with a fd_set variable:

fd_set recv_fds; // bit array variable int s = socket(PF_INET, SOCK_DGRAM, 0); // Socket that we want to // test for receive readiness (1) clear all bits in the bit array variable recv_fds (2) set the bit position s in the bit array variable recv_fds (3) call: select ( FD_SETSIZE, &recv_fds, NULL, NULL, timeout )

Compare to my simplified usage (with an int variable):

int recv_fds; // Shorter bit array variable int s = socket(PF_INET, SOCK_DGRAM, 0); // Socket that we want to // test for receive readiness (1) clear all bits in recv_fds recv_fds = 0 (2) set the bit position s in recv_fds recv_fds = recv_fds | ( 1 << s ) (3) Call: select ( 32, &recv_fds, NULL, NULL, timeout ); (I need to use 32 to tell select( ) that it only need to scan the first 32 entries in the descriptor table)

Clearing, Setting and Testing bits in a bit array

Bit array manipulation macros (defined inside select.h):

void FD_ZERO(fd_set *fdset)

Clear all bits in the "fdset" bit array variable

Example:

fd_set recv_fds; FD_ZERO ( & recv_fds );
Result: recv_fds = 0000....000

void FD_SET(int s, fd_set *fdset)

Set bit "s" in the "fdset" bit array variable to 1 (i.e., select "s" for checking)

Example:

fd_set recv_fs; FD_ZERO ( & recv_fs ); FD_SET ( 2, & recv_fs );
Result: recv_fds = 0000....0100 // Bit position 2 is set

void FD_CLR(int fd, fd_set *fdset)
int FD_ISSET(int fd, fd_set *fdset)
Returns:

The general usage of select( ) to check receive readiness of a socket

Here is the same demo program re-written using a fd_set typed variable:

I have highlighted the changes

// 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 select ================================== */ fd_set recv_fds; /* I'm using a fd_set var now */ struct timeval timeout; /* Time value for time out */ /* =============================== Select the bit for socket s =============================== */ FD_ZERO( &recv_fds ); // Clear all bits in recv_fds FD_SET( s, &recv_fds); // Set bit for socket s /* =================================== Set timeout value to 5 sec =================================== */ timeout.tv_sec = 5; // Timeout = 5 sec + 0 micro sec timeout.tv_usec = 0; // number of micro seconds int result ; printf("Now send a UDP message to port %s before %d sec !!!....\n", argv[1], timeout.tv_sec ); result = select(FD_SETSIZE, &recv_fds, NULL, NULL, &timeout); printf("select( ) returns: %d\n", result);

Example Program: (Demo above code)

The modified select( ) 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 select2 test prog: gcc -o select2 select2.c
To compile the udp4-s3 test prog: gcc -o udp4-s3 udp4-s3.c -lnsl
To run: select2 8000 on machine X
To send a msg to select prog: udp4-s3 X 8000