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Name

select - synchronous I/O multiplexing

Synopsis


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

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

void FD_SET(int fd, fd_set &fdset);

void FD_CLR(int fd, fd_set &fdset);

int FD_ISSET(int fd, fd_set &fdset);

void FD_ZERO(fd_set &fdset);

MT-Level

MT-Safe

Description

select() examines the I /O file descriptor sets whose addresses are passed in readfds, writefds, and exceptfds to see if any of their file descriptors are ready for reading, are ready for writing, or have an exceptional condition pending, respectively. Out-of-band data is the only exceptional condition. nfds is the number of bits to be checked in each bit mask that represents a file descriptor; the file descriptors from 0 to nfds -1 in the file descriptor sets are examined. On return, select() replaces the given file descriptor sets with subsets consisting of those file descriptors that are ready for the requested operation. The return value from the call to select() is the number of ready file descriptors.

The file descriptor sets are stored as bit fields in arrays of integers. The following macros are provided for manipulating such file descriptor sets: FD_ZERO () initializes a file descriptor set fdset to the null set. FD_SET () includes a particular file descriptor fd in fdset. FD_CLR () removes fd from fdset. FD_ISSET () is nonzero if fd is a member of fdset, zero otherwise. The behavior of these macros is undefined if a file descriptor value is less than zero or greater than or equal to FD_SETSIZE . FD_SETSIZE is a constant defined in <sys/select.h>.

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 structure.

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

Return Values

select() returns the number of ready file descriptors contained in the file descriptor sets or -1 if an error occurred. If the time limit expires, then select() returns 0.

Errors

The call fails if:
EBADF
One of the I /O file descriptor sets specified an invalid I /O file descriptor.
EINTR
A signal was delivered before any of the selected events occurred, or 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

poll(2) , read(2) , write(2)

Notes

The default value for FD_SETSIZE (currently 1024) is larger than the default limit on the number of open files. In order to accommodate programs that may 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>.

The file descriptor sets are always modified on return, even if the call returns as the result of a timeout.

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