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select(2) [ultrix man page]

select(2)							System Calls Manual							 select(2)

Name
       select - synchronous I/O multiplexing

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

       int select (nfds, readfds, writefds, exceptfds, timeout)
       int nfsd;
       fd_set *readfds, *writefds, *exceptfds;
       struct timeval *timeout;

       FD_SET (fd, fdset)
       FD_CLR (fd, fdset)
       FD_ISSET (fd, fdset)
       FD_ZERO (fdset)
       int fd;
       fd_set *fdset;

Description
       The  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, ready for writing, or have an exceptional condition pending. The nfds parameter is the number of bits to
       be  checked  in	each bit mask that represent a file descriptor; the descriptors from 0 through nfds-1 in the descriptor sets are examined.
       Typically nfds has the value returned by for the maximum number of file descriptors. On return, replaces the  given  descriptor	sets  with
       subsets	consisting  of those descriptors that are ready for the requested operation. The total number of ready descriptors in all the sets
       is returned.

       The descriptor sets are stored as bit fields in arrays of integers. The following macros are  provided  for  manipulating  such	descriptor
       sets:  FD_ZERO(fds  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 nonzero 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 equal to the maximum number of descriptors that
       can be supported by the system.

       If timeout is not a NULL pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a NULL pointer,  the
       select  blocks  indefinitely. To effect a poll, the timeout argument should be a non-NULL pointer, pointing to a zero-valued timeval struc-
       ture.

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

       Selecting true for reading on a socket descriptor upon which a call has been performed indicates that a subsequent call on that	descriptor
       will not block.

Restrictions
       The  call  may  indicate  that  a  descriptor  is ready for writing when in fact an attempt to write would block. This can happen if system
       resources necessary for a write are exhausted or otherwise unavailable.	If an application deems it critical that writes to a file descrip-
       tor not block, it should set the descriptor for non-blocking I/O using the F_SETFL request to the call.

Return Values
       The  call  returns a non-negative value on success.  A positive value indicates the number of ready descriptors in the descriptor sets. A 0
       indicates that the time limit referred to by timeout expired.  On failure, returns -1, sets errno to indicate the error, and the descriptor
       sets are not changed.

Diagnostics
       [EBADF]	   One of the descriptor sets specified an invalid descriptor.

       [EFAULT]    One of the pointers given in the call referred to a non-existent portion of the process' address space.

       [EINTR]	   A signal was delivered before any of the selected events occurred, or before the time limit expired.

       [EINVAL]    A  component  of the pointed-to time limit is outside the acceptable range; t_sec must be between 0 and 10^8, inclusive. t_usec
		   must be greater than or equal to 0, and less than 10^6.

See Also
       accept(2), connect(2), fcntl(2), gettimeofday(2), listen(2), read(2), recv(2), send(2), write(2), getdtablesize(2)

																	 select(2)

Check Out this Related Man Page

SELECT(2)						      BSD System Calls Manual							 SELECT(2)

NAME
select -- synchronous I/O multiplexing SYNOPSIS
#include <sys/select.h> - or - #include <sys/types.h> #include <sys/time.h> #include <unistd.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); DESCRIPTION
Select() 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 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. Select() 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 a non-nil pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a nil pointer, the select blocks indefinitely. To effect a poll, the timeout argument should be non-nil, pointing to a zero-valued timeval structure. Timeout is not changed by select(), and may be reused on subsequent calls, however it is good style to re-initialize it before each invocation of select(). Any of readfds, writefds, and exceptfds may be given as nil pointers if no descriptors are of interest. RETURN VALUES
Select() 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, including one due to an interrupted call, the descriptor sets will be unmod- ified. ERRORS
An error return from select() indicates: [EBADF] One of the descriptor sets specified an invalid descriptor. [EINTR] A signal was delivered before the time limit expired and before any of the selected events occurred. [EINVAL] The specified time limit is invalid. One of its components is negative or too large. SEE ALSO
accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2) BUGS
Although the provision of getdtablesize(2) was intended to allow user programs to be written independent of the kernel limit on the number of open files, the dimension of a sufficiently large bit field for select remains a problem. The default size FD_SETSIZE (currently 1024) is somewhat smaller than the current kernel limit to the number of open files. However, in order to accommodate programs which might poten- tially use a larger number of open files with select, it is possible to increase this size within a program by providing a larger definition of FD_SETSIZE before the inclusion of <sys/types.h>. Select() should probably have been designed to return the time remaining from the original timeout, if any, by modifying the time value in place. However, it is unlikely this semantic will ever be implemented, as the change would cause source code compatibility problems. In general it is unwise to assume that the timeout value will be unmodified by the select() call, and the caller should reinitialize it on each invocation. HISTORY
The select() function call appeared in 4.2BSD. 4.2 Berkeley Distribution March 25, 1994 4.2 Berkeley Distribution
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