The communications protocol used with a socket controls low-level
details of how data are exchanged. For example, the protocol implements
things like checksums to detect errors in transmissions, and routing
instructions for messages. Normal user programs have little reason to
mess with these details directly.
The default communications protocol for the Internet namespace depends on
the communication style. For stream communication, the default is TCP
(“transmission control protocol”). For datagram communication, the
default is UDP (“user datagram protocol”). For reliable datagram
communication, the default is RDP (“reliable datagram protocol”).
You should nearly always use the default.
Internet protocols are generally specified by a name instead of a
number. The network protocols that a host knows about are stored in a
database. This is usually either derived from the file
/etc/protocols, or it may be an equivalent provided by a name
server. You look up the protocol number associated with a named
protocol in the database using the getprotobyname function.
Here are detailed descriptions of the utilities for accessing the
protocols database. These are declared in netdb.h.
— Data Type: struct protoent
This data type is used to represent entries in the network protocols
database. It has the following members:
char *p_name
This is the official name of the protocol.
char **p_aliases
These are alternate names for the protocol, specified as an array of
strings. The last element of the array is a null pointer.
int p_proto
This is the protocol number (in host byte order); use this member as the
protocol argument to socket.
You can use getprotobyname and getprotobynumber to search
the protocols database for a specific protocol. The information is
returned in a statically-allocated structure; you must copy the
information if you need to save it across calls.
The getprotobynumber function returns information about the
network protocol with number protocol. If there is no such
protocol, it returns a null pointer.
You can also scan the whole protocols database one protocol at a time by
using setprotoent, getprotoent and endprotoent.
Be careful when using these functions because they are not reentrant.
— Function: void setprotoent (int stayopen)
This function opens the protocols database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that
subsequent calls to getprotobyname or getprotobynumber will
not close the database (as they usually would). This makes for more
efficiency if you call those functions several times, by avoiding
reopening the database for each call.
— Function: struct protoent * getprotoent (void)
This function returns the next entry in the protocols database. It
returns a null pointer if there are no more entries.
— Function: void endprotoent (void)
This function closes the protocols database.
Published under the terms of the GNU General Public License
