Unix Environment Programming Chapter 14 14.4 I/O Multiplexing

14.4.1 Function select and pselect

#include int select(int maxfdpl, fd_set *restrict readset, fd_set *restrict writeset, fd_set* restrict exceptset, struct timeval *restrict tvptr);

Parameter: struct timeval *restrict tvptr.

It indicates the length of time it is willing to wait, in seconds and microseconds.There are three situations:

• tvptr==NULL.wait forever.This indefinite wait is interrupted if a signal is caught.Returns when one of the specified descriptors is ready or catches a signal.If a signal is caught, select returns -1 and errno is set to EINTR.
• tvptr->tv_sec==0 && tvptr->tv_usec==0.Do not wait at all, test all specified descriptors and return immediately.
• tvptr->tv_sec!=0 || tvptr->tv_usec!=0.Wait the specified number of seconds and microseconds.Returns immediately when one of the specified descriptors is ready, or when the specified time value has passed.If no descriptor is ready by the time the timeout expires, the return value is 0.

The middle three parameters: readfs, writefds, exceptfds are pointers to descriptor sets.

These 3 descriptor sets describe the set of descriptors we care about that are readable, writable, or in exceptional conditions.Each descriptor set is stored in a fd_set data type.

After declaring a descriptor set, we must use FD_ZERO to set the descriptor set to 0, and then set the bits of each descriptor we care about in it.

Any of the three parameters in the middle of the selection can be a null pointer, which means that it does not care about the corresponding conditions.

Select the first parameter maxfdp1 means "the maximum file descriptor number value plus 1".

select has three possible return values

• A return value of -1 indicates an error, in which case none of the descriptor sets are modified.
• A return value of 0 means that no descriptors are ready, and all descriptor sets will have bit 0 set.
• A positive return value indicates the number of descriptors that have been prepared.The value is the sum of the number of descriptors that are ready for the 3 descriptor sets, so if the same descriptor is ready for reading and writing, it will be counted twice in the return value.

Some clarification on what "ready" means:

A descriptor in the read set (readfds) is considered ready if a read operation does not block.

A descriptor in the write set (writefds) is considered ready if the write operation does not block.

If there is a pending exception condition for a descriptor in the exception condition set, the descriptor set is considered to be active.

For read, write and exception conditions, file descriptors for ordinary files always return ready.

14.4.2 Function poll

The poll function is similar to the select function, but the programmer interface is different.The poll function can be used with any type of file descriptor.

#include int poll(struct pollfd fd[], nfds_t nfds, int timeout);//Return value: the number of descriptors that are ready, if it times out, return 0, if there is an error, return -1

Unlike select, poll does not construct a descriptor set for each condition, but an array of pollfd structures, each array element specifying a descriptor number and the condition we are interested in changing the descriptor.

struct pollfd{int fd;short events;short revents;};

The elements in the fdarray array are specified by nfds.

poll's events and revents flags:

The last parameter to poll specifies how long we are willing to wait.

timeout==-1

Wait forever, returning when one of the specified descriptors is ready, or catches a signal.If a signal is caught, poll returns -1 and errno is set to EINTR.

timeout==0

Don't wait.Test all descriptors and return immediately.

timeout>0

Wait timeout milliseconds.Returns immediately when one of the specified descriptors is ready, or when the timeout expires.If no descriptor is ready when the timeout expires, the return value is 0.