Name

Library

Synopsis

Return values

Detailed description

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

See also

Notes

The select system call appeared in BSD 4.2.

Bugs

Limitations

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