SELECT(2)

NAME
pselect, select - synchronous I/O multiplexing

SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <sys/select.h>
#include <signal.h>

nfound = pselect(nfds, readfds, writefds, exceptfds, timeout, sigmask);
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timespec *timeout;
sigset_t *sigmask;

nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
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
Pselect and select examine 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 two functions are identical except for the type and format of the timeout value, and the additonal sigmask parameter supplied to the pselect() call.

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 total number of ready descriptors in all the sets is returned in nfound.

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 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 normally at least equal to the maximum number of descriptors supported by the system.

If timeout is a non-zero pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a zero pointer, select blocks indefinitely. To affect a poll, the timeout argument should be non-zero, pointing to a zero-valued timeval structure.

If the sigmask parameter to pselect() is not NULL, it points to a signal mask that replaces the previous signal mask for the process for the duration of the call, and the previous mask is restored upon return; see sigprocmask(3). This is normally used so that signals can be blocked while preparing for a call to pselect() and then atomically unblocking the signals while selecting.

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

RETURN VALUE
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 then select returns 0. If select returns with an error, including one due to an interrupted call, the descriptor sets will be unmodified.

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), read(2), write(2), recv(2), send(2), getdtablesize(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 256) is somewhat larger than the current kernel limit to the number of open files. However, in order to accommodate programs which might potentially 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 return the time remaining from the original timeout, if any, by modifying the time value in place. This may be implemented in future versions of the system. Thus, it is unwise to assume that the timeout value will be unmodified by the select call.

In 2BSD the timeout is implemented in the kernel using the callout table. Since a callout structure only has a signed short to store the number of ticks till expiration the maximum value of a kernel timeout is 32767 ticks. In the US (60hz power) this gives a maximum timeout of approximately 9 minutes. In countries using 50hz power the maximum timeout is about 13 minutes.

struct timespec on a PDP-11 is silly since the hardware has nowhere near microsecond much less nanosecond clock resolution.