select.h File Reference

The select() function indicates which of the specified file descriptors is ready for reading, ready for writing, or has an error condition pending. If the specified condition is false for all of the specified file descriptors, select() blocks, up to the specified timeout interval, until the specified condition is true for at least one of the specified file descriptors.

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 maxfd descriptors are checked in each set; i.e., the descriptors from 0 through maxfd -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 tvptr 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 tvptr 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.

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 tvptr points to an invalid address. [EINVAL] The specified time limit is invalid. One of its components is negative or too large. [EINVAL] The maxfd argument was invalid.

Limitations:

#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 doesnt 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 fds were waiting
 to be readable.
      */
                        
     /* The third parameter is an fd_set that you want to
 know if you can write on -- this example doesnt
 use it, so it passes 0, or NULL.
      */
     /* The fourth parameter is sockets youre 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 fds 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;
}

See also: accept() connect() getdtablesize() gettimeofday() read() recv() send() write()

Notes:

The default size of FD_SETSIZE is currently set to 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 maxfd 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 tvptr in place. Thus, it is unwise to assume that the tvptr value will be unmodified by the select system call.