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 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 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 timeout 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.
If timeout is a null pointer, the select blocks indefinitely.
To effect a poll, the timeout 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.
The select system call 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 system call, the descriptor sets will be unmodified.
An error return from select indicates: