select Subroutine

Purpose

Checks the I/O status of multiple file descriptors and message queues.

Library

Standard C Library (libc.a)

Syntax

#include <sys/time.h>
#include <sys/select.h>
#include <sys/types.h>

Description

The select subroutine checks the specified file descriptors and message queues to see if they are ready for reading (receiving) or writing (sending), or if they have an exceptional condition pending.

When selecting on an unconnected stream socket, select returns when the connection is made. If selecting on a connected stream socket, then the ready message indicates that data can be sent or received. Files descriptors of regular files always select true for read, write, and exception conditions. For more information on sockets, refer to "Understanding Socket Connections" and the related "Checking for Pending Connections Example Program" dealing with pending connections in AIX® Version 6.1 Communications Programming Concepts.

The select subroutine is also supported for compatibility with previous releases of this operating system and with BSD systems.

On shared memory descriptors, the select subroutine returns true.

Parameters

struct timeval {
   int tv_sec;        /* seconds       */
   int tv_usec;       /* microseconds  */
    };

struct sellist
{
ulong fdsmask[F];        /* file descriptor bit mask  */
int msgids[M];         /* message queue identifiers */
};

SELLIST(f, m) declarator . . . ;

Return Values

Upon successful completion, the select subroutine returns a value that indicates the total number of file descriptors and message queues that satisfy the selection criteria. The fdsmask bit masks are modified so that bits set to 1 indicate file descriptors that meet the criteria. The msgids arrays are altered so that message queue identifiers that do not meet the criteria are replaced with a value of -1.

The return value is similar to the Nfdsmsgs parameter in that the low-order 16 bits give the number of file descriptors, and the high-order 16 bits give the number of message queue identifiers. These values indicate the sum total that meet each of the read, write, and exception criteria. Therefore, the same file descriptor or message queue can be counted up to three times. You can use the NFDS and NMSGS macros found in the sys/select.h file to separate out these two values from the return value. For example, if rc contains the value returned from the select subroutine, NFDS(rc) is the number of files selected, and NMSGS(rc) is the number of message queues selected.

If the time limit specified by the TimeOut parameter expires, the select subroutine returns a value of 0.

If a connection-based socket is specified in the Readlist parameter and the connection disconnects, the select subroutine returns successfully, but the recv subroutine on the socket will return a value of 0 to indicate the socket connection has been closed.

For nonbloking connection-based sockets, both successful and unsuccessful connections will cause the select subroutine to return successfully without any error.

When the connection completes successfully the socket becomes writable, and if the connection encounters an error the socket becomes both readable and writable.

When using the select subroutine, you can not check any pending errors on the socket. You need to call the getsockopt subroutine with SOL_SOCKET and SOL_ERROR to check for a pending error.

If the select subroutine is unsuccessful, it returns a value of -1 and sets the global variable errno to indicate the error. In this case, the contents of the structures pointed to by the ReadList, WriteList, and ExceptList parameters are unpredictable.

Error Codes

The select subroutine is unsuccessful if one of the following are true:

Examples

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <fcntl.h>
#include <sys/time.h>
#include <errno.h>
#include <stdio.h>

int main()
{
    int sockfd, cnt, i = 1;
    struct sockaddr_in  serv_addr;

    bzero((char *)&serv_addr, sizeof (serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr("172.16.55.25");
    serv_addr.sin_port = htons(102);

    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
        exit(1);
    if (fcntl(sockfd, F_SETFL, FNONBLOCK) < 0)
        exit(1);
    if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof
            (serv_addr)) < 0 && errno != EINPROGRESS)
        exit(1);
    for (cnt=0; cnt<2; cnt++) {
        fd_set  readfds, writefds;

        FD_ZERO(&readfds);
        FD_SET(sockfd, &readfds);
        FD_ZERO(&writefds);
        FD_SET(sockfd, &writefds);

        if (select(sockfd + 1, &readfds, &writefds, NULL,
                NULL) < 0)
            exit(1);
        printf("Iteration %d ==============\n", i);
        printf("FD_ISSET(sockfd, &readfds) == %d\n",
            FD_ISSET(sockfd, &readfds));
        printf("FD_ISSET(sockfd, &writefds) == %d\n",
            FD_ISSET(sockfd, &writefds));
        i++;
    }
    return 0;
}

Iteration 1 ==============
FD_ISSET(sockfd, &readfds) == 0
FD_ISSET(sockfd, &writefds) == 1
Iteration 2 ==============
FD_ISSET(sockfd, &readfds) == 1
FD_ISSET(sockfd, &writefds) == 1

In the first iteration, select notifies the write event only. In the second iteration, select notifies both the read and write events.

Notes

FD_SETSIZE is the #define variable that defines how many file descriptors the various FD macros will use. The default value for FD_SETSIZE will vary, depending on the version of AIX. As the number of open files supported has increased, the default value of FD_SETSIZE has increased.

In AIX 4.3.1, the size increased to 32767 open file descriptors (from 2000 in prior releases). In AIX 5.2, the size increased to 65534 open file descriptors. This value can not be set greater than OPEN_MAX, which also varies from one AIX Version to another.

For more information, refer to the /usr/include/sys/time.h file.

The user may override FD_SETSIZE to select a smaller value before including the system header files. This is desirable for performance reasons, because of the overhead in FD_ZERO to zero 65534 bits.

Performance Issues and Recommended Coding Practices

select(FD_SETSIZE, ....)

Performance will be poor if the program uses FD_ZERO and the default FD_SETSIZE. FD_ZERO should not be used in any loops or before each select call. However, using it one time to zero the bit string will not cause problems. If you plan to use this simple programming method, you should override FD_SETSIZE to define a smaller number of FDs. For example, if your process will only open two FDs that you will be selecting on, and there will never be more than a few hundred other FDs open in the process, you should lower FD_SETSIZE to approximately 1024.

Do not pass FD_SETSIZE as the first parameter to select. This specifies the maximum number of file descriptors the system should check for. The program should keep track of the highest FD that has been assigned or use the getdtablesize subroutine to determine this value. This saves passing excessively long bit maps in and out of the kernel and reduces the number of FDs that select must check.

Use the poll system call instead of select. The poll system call has the same functionality as select, but it uses a list of FDs instead of a bit map. Thus, if you are only selecting on a single FD, you would only pass one FD to poll. With select, you have to pass a bit map that is as long as the FD number assigned for that FD. If AIX assigned FD 4000, for example, you would have to pass a bit map 4001 bits long.