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Typically, one of the ends of a socket-based data communication is a <emphasis>server</emphasis>, the other is a <emphasis>client</emphasis>.
The Common Elements
The one function used by both, clients and servers, is <citerefentry><refentrytitle>socket</refentrytitle><manvolnum>2</manvolnum></citerefentry>. It is declared this way:
int socket(int domain, int type, int protocol);
The return value is of the same type as that of <function>open</function>, an integer. FreeBSD allocates its value from the same pool as that of file handles. That is what allows sockets to be treated the same way as files.
The <varname>domain</varname> argument tells the system what <emphasis>protocol family</emphasis> you want it to use. Many of them exist, some are vendor specific, others are very common. They are declared in <filename>sys/socket.h</filename>.
Use <constant>PF_INET</constant> for <acronym>UDP</acronym>, <acronym>TCP</acronym> and other Internet protocols (<acronym>IP</acronym>v4).
Five values are defined for the <varname>type</varname> argument, again, in <filename>sys/socket.h</filename>. All of them start with <quote><constant>SOCK_</constant></quote>. The most common one is <constant>SOCK_STREAM</constant>, which tells the system you are asking for a <emphasis>reliable stream delivery service</emphasis> (which is <acronym>TCP</acronym> when used with <constant>PF_INET</constant>).
If you asked for <constant>SOCK_DGRAM</constant>, you would be requesting a <emphasis>connectionless datagram delivery service</emphasis> (in our case, <acronym>UDP</acronym>).
If you wanted to be in charge of the low-level protocols (such as <acronym>IP</acronym>), or even network interfaces (e.g., the Ethernet), you would need to specify <constant>SOCK_RAW</constant>.
Finally, the <varname>protocol</varname> argument depends on the previous two arguments, and is not always meaningful. In that case, use <constant>0</constant> for its value.
The Unconnected Socket
Nowhere, in the <function>socket</function> function have we specified to what other system we should be connected. Our newly created socket remains <emphasis>unconnected</emphasis>.
This is on purpose: To use a telephone analogy, we have just attached a modem to the phone line. We have neither told the modem to make a call, nor to answer if the phone rings.
Various functions of the sockets family expect the address of (or pointer to, to use C terminology) a small area of the memory. The various C declarations in the <filename>sys/socket.h</filename> refer to it as <varname>struct sockaddr</varname>. This structure is declared in the same file:
* Structure used by kernel to store most
* addresses.
struct sockaddr {
unsigned char sa_len; /* total length */
sa_family_t sa_family; /* address family */
char sa_data[14]; /* actually longer; address value */
#define SOCK_MAXADDRLEN 255 /* longest possible addresses */
Please note the <emphasis>vagueness</emphasis> with which the <varname>sa_data</varname> field is declared, just as an array of <constant>14</constant> bytes, with the comment hinting there can be more than <constant>14</constant> of them.
This vagueness is quite deliberate. Sockets is a very powerful interface. While most people perhaps think of it as nothing more than the Internet interface—and most applications probably use it for that nowadays—sockets can be used for just about <emphasis>any</emphasis> kind of interprocess communications, of which the Internet (or, more precisely, <acronym>IP</acronym>) is only one.
The <filename>sys/socket.h</filename> refers to the various types of protocols sockets will handle as <emphasis>address families</emphasis>, and lists them right before the definition of <varname>sockaddr</varname>:
* Address families.
#define AF_UNSPEC 0 /* unspecified */
#define AF_LOCAL 1 /* local to host (pipes, portals) */
#define AF_UNIX AF_LOCAL /* backward compatibility */
#define AF_INET 2 /* internetwork: UDP, TCP, etc. */
#define AF_IMPLINK 3 /* arpanet imp addresses */
#define AF_PUP 4 /* pup protocols: e.g. BSP */
#define AF_CHAOS 5 /* mit CHAOS protocols */
#define AF_NS 6 /* XEROX NS protocols */
#define AF_ISO 7 /* ISO protocols */
#define AF_OSI AF_ISO
#define AF_ECMA 8 /* European computer manufacturers */
#define AF_DATAKIT 9 /* datakit protocols */
#define AF_CCITT 10 /* CCITT protocols, X.25 etc */
#define AF_SNA 11 /* IBM SNA */
#define AF_DECnet 12 /* DECnet */
#define AF_DLI 13 /* DEC Direct data link interface */
#define AF_LAT 14 /* LAT */
#define AF_HYLINK 15 /* NSC Hyperchannel */
#define AF_APPLETALK 16 /* Apple Talk */
#define AF_ROUTE 17 /* Internal Routing Protocol */
#define AF_LINK 18 /* Link layer interface */
#define pseudo_AF_XTP 19 /* eXpress Transfer Protocol (no AF) */
#define AF_COIP 20 /* connection-oriented IP, aka ST II */
#define AF_CNT 21 /* Computer Network Technology */
#define pseudo_AF_RTIP 22 /* Help Identify RTIP packets */
#define AF_IPX 23 /* Novell Internet Protocol */
#define AF_SIP 24 /* Simple Internet Protocol */
#define pseudo_AF_PIP 25 /* Help Identify PIP packets */
#define AF_ISDN 26 /* Integrated Services Digital Network*/
#define AF_E164 AF_ISDN /* CCITT E.164 recommendation */
#define pseudo_AF_KEY 27 /* Internal key-management function */
#define AF_INET6 28 /* IPv6 */
#define AF_NATM 29 /* native ATM access */
#define AF_ATM 30 /* ATM */
#define pseudo_AF_HDRCMPLT 31 /* Used by BPF to not rewrite headers
* in interface output routine
#define AF_NETGRAPH 32 /* Netgraph sockets */
#define AF_SLOW 33 /* 802.3ad slow protocol */
#define AF_SCLUSTER 34 /* Sitara cluster protocol */
#define AF_ARP 35
#define AF_BLUETOOTH 36 /* Bluetooth sockets */
#define AF_MAX 37
The one used for <acronym>IP</acronym> is <symbol>AF_INET</symbol>. It is a symbol for the constant <constant>2</constant>.
It is the <emphasis>address family</emphasis> listed in the <varname>sa_family</varname> field of <varname>sockaddr</varname> that decides how exactly the vaguely named bytes of <varname>sa_data</varname> will be used.
Specifically, whenever the <emphasis>address family</emphasis> is <symbol>AF_INET</symbol>, we can use <varname>struct sockaddr_in</varname> found in <filename>netinet/in.h</filename>, wherever <varname>sockaddr</varname> is expected:
* Socket address, internet style.
struct sockaddr_in {
uint8_t sin_len;
sa_family_t sin_family;
in_port_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
We can visualize its organization this way:
_ external ref='sockets/sain' md5='__failed__'
0 1 2 3
0 | 0 | Family | Port |
4 | IP Address |
8 | 0 |
12 | 0 |
<imageobject> <imagedata fileref="sockets/sain"/> </imageobject> <textobject> <_:literallayout-1/> </textobject> <textobject> <phrase>sockaddr_in</phrase> </textobject>
The three important fields are <varname>sin_family</varname>, which is byte 1 of the structure, <varname>sin_port</varname>, a 16-bit value found in bytes 2 and 3, and <varname>sin_addr</varname>, a 32-bit integer representation of the <acronym>IP</acronym> address, stored in bytes 4-7.


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Trailing space: Portuguese (Brazil)
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(itstool) path: sect4/title
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String age
a year ago
Source string age
a year ago
Translation file
books/developers-handbook.pot, string 895