14.10 Summary
There are three main ways to set a time limit on
a socket operation:
-
Use the alarm function and the
SIGALRM signal
-
Use the time limit that is provided by
select
-
Use the newer SO_RCVTIMEO and
SO_SNDTIMEO socket options
The first is easy to use, but involves signal
handling, and as we will see in Section 20.5, can
lead to race conditions. Using select means that we block
in this function with its provided time limit instead of blocking
in a call to read, write, or connect.
The third alternative, to use the new socket options, is also easy,
but is not provided by all implementations.
recvmsg and sendmsg are the
most general of the five groups of I/O functions provided. They
combine the ability to specify an MSG_xxx flag (from recv and
send), plus employ the ability to return or specify the
peer's protocol address (from recvfrom and
sendto), with the ability to use multiple buffers (from
readv and writev), along with two new features:
returning flags to the application and receiving or sending
ancillary data.
We describe ten different forms of ancillary
data in the text, six of which are new with IPv6. Ancillary data
consists of one or more ancillary data objects, each object
preceded by a cmsghdr structure specifying its length,
protocol level, and type of data. Five functions beginning with
CMSG_ are used to build and parse ancillary data.
Sockets can be used with the C standard I/O
library, but doing this adds another level of buffering to that
already being performed by TCP. Indeed, a lack of understanding of
the buffering performed by the standard I/O library is the most
common problem with the library. Since a socket is not a terminal
device, the common solution to this potential problem is to set the
standard I/O stream to unbuffered, or to simply avoid standard I/O
on sockets completely.
Many vendors provide advanced ways to poll for
many events without the overhead required by select and
poll. While writing nonportable code should be avoided
whenever possible, sometimes the benefits of performance
improvements outweigh the risk of nonportability.
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