select

Name

select - synchronous I/O multiplexing

Library

libc.lib

Synopsis

#include <sys/select.h>

int select (int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
FD_SET (fd, &fdset);
FD_CLR (fd, &fdset);
FD_ISSET (fd, &fdset);
FD_ZERO (&fdset);

Return values

The select system call returns the number of ready descriptors that are contained in the descriptor sets, or -1 if an error occurred. If the time limit expires, select returns 0. If select returns with an error, the descriptor sets will be unmodified.

Detailed description

The select system call examines the I/O descriptor sets whose addresses are passed in readfds, writefds, and exceptfds to see if some of their descriptors are ready for reading, are ready for writing, or have an exceptional condition pending, respectively. The only exceptional condition detectable is out-of-band data received on a socket. The first nfds descriptors are checked in each set; i.e., the descriptors from 0 through nfds -1 in the descriptor sets are examined. On return, select replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation. The select system call returns the total number of ready descriptors in all the sets.

The descriptor sets are stored as bit fields in arrays of integers. The following macros are provided for manipulating such descriptor sets: FD_ZERO (&fdset);
initializes a descriptor set fdset to the null set. FD_SET (fd, &fdset);
includes a particular descriptor fd in fdset. FD_CLR (fd, &fdset);
removes fd from fdset. FD_ISSET (fd, &fdset);
is non-zero if fd is a member of fdset, zero otherwise. The behavior of these macros is undefined if a descriptor value is less than zero or greater than or equal to FD_SETSIZE, which is normally at least equal to the maximum number of descriptors supported by the system.

If timeout is not a null pointer, it specifies the maximum interval to wait for the selection to complete. System activity can lengthen the interval by an indeterminate amount.

To effect a poll, the timeout argument should not be a null pointer, but it should point to a zero-valued timeval structure.

Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.

Examples

#include <sys/select.h>
#include <unistd.h>
/*
* A simple example of testing a single FD for readability
* This example returns 1 when the fd is ready for reading.
*/
#include <sys/select.h>
#include <unistd.h>
/*
* A simple example of testing a single FD for readability
* This example returns 1 when the fd is ready for reading.
*/
int isready(int fd)
{
    int rc;
    fd_set fds;
    struct timeval tv;
   
    /*
     * FD_ZERO() clears out the fd_set called fds, so that
     * it doesn’t contain any file descriptors.
     */
    FD_ZERO(&fds);
    /*
     * FD_SET() adds the file descriptor "fd" to the fd_set,
     * so that select() will return if fd is readable
     */
    FD_SET(fd,&fds);
    tv.tv_sec = tv.tv_usec = 0;
    /*
     * The first argument to select is the highest file
     * descriptor value plus 1.
     */
                        
     /* The second argument to select() is the address of
      * the fd_set that contains fd’s we’re waiting
      * to be readable.
      */
                        
     /* The third parameter is an fd_set that you want to
      * know if you can write on -- this example doesn’t
      * use it, so it passes 0, or NULL.
      */
     /* The fourth parameter is sockets you’re waiting for
      * out-of-band data for.
      */
                
     /* The last parameter to select() is a time-out of how
      * long select() should block.
      */
     rc = select(fd+1, &fds, NULL, NULL, &tv);
     /* select() returns the number of fd’s that are ready       
      * Once select() returns, the original fd_set has been
      * modified so it now reflects the state of why select()
      * woke up. i.e. If file descriptor 4 was originally in
      * the fd_set, and then it became readable, the fd_set
      * contains file descriptor 4 in it.
      */
     if (rc < 0)
       return -1;
     return FD_ISSET(fd,&fds) ? 1 : 0;
}

Errors

An error return from select indicates:

[`EBADF`]
	One of the descriptor sets specified an invalid descriptor.
[`EFAULT`]
	One of the arguments readfds, writefds, exceptfds, or timeout points to an invalid address.
[`EINVAL`]
	The specified time limit is invalid. One of its components is negative or too large.
[`EINVAL`]
	The nfds argument was invalid.

Notes

The default size of FD_SETSIZE is currently 1024. In order to accommodate programs which might potentially use a larger number of open files with select, it is possible to increase this size by having the program define FD_SETSIZE before the inclusion of any header which includes

#include <sys/types.h>

If nfds is greater than the number of open files, select is not guaranteed to examine the unused file descriptors. For historical reasons, select will always examine the first 256 descriptors.

The select system call appeared in BSD 4.2.

Bugs

-susv2 allows systems to modify the original timeout in place. Thus, it is unwise to assume that the timeout value will be unmodified by the select system call.

Limitations

select is not interrupted by the signals, as signals are not supported by Symbian. If timeout is set to null, select does not block forever, but waits for a maximum of 10 seconds and returns. select operation cannot be called for the second time on the same socket descriptor before the first select operation on the same socket descriptor completes. (i.e) Only one select operation can be outstanding on a Socket. This is because of the limitation of the underlying Ioctl operation. Only one Ioctl operation may be outstanding for each socket.

Feedback

For additional information or queries on this page send feedback