select()

The socket call gets read, write, and exception status on a group of sockets.

The BSD version monitors the activity on sockets by specifying an array (fd_set) of socket numbers for which the caller wants to read the data, write the data, and check exception-pending conditions. The BSD version provides FD_SET, FD_CLR, FD_ISSET, and FD_ZERO macros to add or delete socket numbers from the array.

Syntax

#include <types.h>
#include <unistd.h>
#include <sys\time.h>
int select(nfds, readfds, writefds, exceptfds, timeout)
int nfds;
fd_set *readfds;
fd_set *writefds;
fd_set *exceptfds;
struct timeval *timeout;

Parameters

nfds

This parameter is unused; it is maintained for compatibility with BSD. readfds

Pointer to a list of descriptors to be checked for reading. writefds

Pointer to a list of descriptors to be checked for writing. exceptfds

Pointer to a list of descriptors to be checked for exception-pending conditions. For networking services sockets, the only exception-pending condition is out-of-band data in the receive buffer. timeout

Pointer to the time to wait for the select() call to complete.

Description

This call monitors activity on a set of different sockets until a timeout expires, to see if any sockets are ready for reading or writing, or if any exception-pending conditions are pending.

Reinitializing readfds, writefds, and exceptfds every time select() is called is required.

If timeout is a NULL pointer, the call blocks indefinitely until one of the requested conditions is satisfied. If timeout is non-NULL, it specifies the maximum time to wait for the call to complete. To poll a set of sockets, the timeout pointer should point to a zeroed timeval structure. The timeval structure is defined in the <SYS\TIME.H> header file and contains the following fields:

struct timeval {       long tv_sec;  /* Number of seconds */
       long tv_usec; /* Number of microseconds */
}

An fd_set is made up of an array of integers. Macros are provided to manipulate the array.

Macro

Description

FD _ SET ( socket ,array _ address )

array _ address

FD _ CLR ( socket ,array _ address )

Removesthesocketfromthelist . FD _ ISSET ( socket ,array _ address )

Returnstrueifthedescriptorispartofthearray ;otherwise ,returnsfalse . FD _ ZERO ( socket ,array _ address )

Clearstheentirearrayforallsocketdescriptors .

Note: For macros FD_SET, FD_CLR, FD_ISSET, and FD_ZERO, define the parameters socket and array_address in the following manner:

int socket;struct fd_set *array_address;

Setting any of the descriptor pointers to zero indicates that no checks are to be made for the conditions. For example, setting exceptfds to be a NULL pointer causes the select call to check for only read and write conditions.

Return Values

The total number of ready sockets (in all arrays) is returned. The value -1 indicates an error. The value 0 indicates an expired time limit. If the return value is greater than 0, the socket descriptors in each array that are not ready are removed from the array and fd_array is rearranged so that the ready sockets are at the top. The fd_count parameter is adjusted accordingly and returned. You can get the specific error code by calling sock_errno() or psock_errno().

sock_errno() Value

Description

SOCENOTSOCK

SOCEFAULT

Theaddressisnotvalid . SOCEINVAL

Invalidargument .

Examples

Following is an example of the select() call.

...fd_set readsocks;
fd_set writesocks;
fd_set exceptsocks;
struct timeval timeout;
int number_found;
...
/* add socket to read/write/except arrays.  To add descriptor s use
 *   FD_SET (s, &readsocks);
 *
 */
...
number_found = select(0,&readsocks, &writesocks,
                      &exceptsocks, &timeout);

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