select
Prototype: 
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int hi_fd, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
General Description: Wait for any I/O status changes from the file descriptor sets. When any of the specified sets changes, the call returns. You have four macros to help construct and manage the file descriptor sets: FD_CLR - Remove a descriptor from the set. FD_SET - Add a descriptor to a set. FD_ISSET - Test if specified descriptor is ready for I/O. FD_ZERO - Initialize the set to empty.
Return Value: The number of descriptors that have changed states. If an error occurred, the return value is negative. If the timeout expired, the return value is zero.
Parameters
hi_fd This is the highest file descriptor number +1. So if you have four files open, plus the stdio, your descriptors could be [0,1,2,3,5,6,8]. The highest is 8. So, if you include fd(8) in your select statement, hi_fd would equal 9. However, if the highest fd was 5, this parameter would be 6.
readfds The set of descriptors to test for readability.
writefds The set of descriptors to test for writing.
exceptfds The set of descriptors to test for out-of-band data.
timeout The maximum time to wait for data to arrive in microseconds. This is a pointer to a number. If the number is a zero (not the pointer), the call returns immediately after checking all the descriptors. If the pointer is NULL (zero), the select's timeout feature is disabled.
fd The file descriptor to add, remove or test.
set The file descriptor set.
Possible Errors
EBADF An invalid file descriptor was given in one of the sets.
EINTR A non blocked signal was caught.
EINVAL n is negative.
ENOMEM select was unable to allocate memory for internal tables.
Examples 
int i, ports[]={9001, 9002, 9004, -1};
int sockfd, max=0;
fd_set set;
struct sockaddr_in addr;
struct timeval timeout={2,500000}; /* 2.5 sec. */

FD_ZERO(&set);
bzero(&addr, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
for ( i = 0; ports[i] > 0; i++ )
{
    sockfd = socket(PF_INET, SOCK_STREAM, 0);
    addr.sin_port = htons(ports[i]);
    if ( bind(sockfd, &addr, sizeof(addr)) != 0)
        perror("bind() failed");
    else
    {
        FD_SET(sockfd, &set);
        if ( max < sockfd )
            max = sockfd;
    }
}
if ( select(max+1, &set, 0, &set, &timeout) > 0 )
{
    for ( i = 0; i <= max; i++ )
        if ( FD_ISSET(i, &set) )
        {   int client = accept(i, 0, 0);
            /**** process the client's requests ****/
        }
}