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| <h4 class="subsection">16.6.6 Protocols Database</h4> |
| |
| <p><a name="index-protocols-database-1757"></a> |
| 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. |
| |
| <p><a name="index-TCP-_0028Internet-protocol_0029-1758"></a>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. |
| |
| <p><a name="index-g_t_002fetc_002fprotocols-1759"></a>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 |
| <samp><span class="file">/etc/protocols</span></samp>, 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 <code>getprotobyname</code> function. |
| |
| <p>Here are detailed descriptions of the utilities for accessing the |
| protocols database. These are declared in <samp><span class="file">netdb.h</span></samp>. |
| <a name="index-netdb_002eh-1760"></a> |
| <!-- netdb.h --> |
| <!-- BSD --> |
| |
| <div class="defun"> |
| — Data Type: <b>struct protoent</b><var><a name="index-struct-protoent-1761"></a></var><br> |
| <blockquote><p>This data type is used to represent entries in the network protocols |
| database. It has the following members: |
| |
| <dl> |
| <dt><code>char *p_name</code><dd>This is the official name of the protocol. |
| |
| <br><dt><code>char **p_aliases</code><dd>These are alternate names for the protocol, specified as an array of |
| strings. The last element of the array is a null pointer. |
| |
| <br><dt><code>int p_proto</code><dd>This is the protocol number (in host byte order); use this member as the |
| <var>protocol</var> argument to <code>socket</code>. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>You can use <code>getprotobyname</code> and <code>getprotobynumber</code> 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. |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: struct protoent * <b>getprotobyname</b> (<var>const char *name</var>)<var><a name="index-getprotobyname-1762"></a></var><br> |
| <blockquote><p>The <code>getprotobyname</code> function returns information about the |
| network protocol named <var>name</var>. If there is no such protocol, it |
| returns a null pointer. |
| </p></blockquote></div> |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: struct protoent * <b>getprotobynumber</b> (<var>int protocol</var>)<var><a name="index-getprotobynumber-1763"></a></var><br> |
| <blockquote><p>The <code>getprotobynumber</code> function returns information about the |
| network protocol with number <var>protocol</var>. If there is no such |
| protocol, it returns a null pointer. |
| </p></blockquote></div> |
| |
| <p>You can also scan the whole protocols database one protocol at a time by |
| using <code>setprotoent</code>, <code>getprotoent</code> and <code>endprotoent</code>. |
| Be careful when using these functions because they are not reentrant. |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>setprotoent</b> (<var>int stayopen</var>)<var><a name="index-setprotoent-1764"></a></var><br> |
| <blockquote><p>This function opens the protocols database to begin scanning it. |
| |
| <p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that |
| subsequent calls to <code>getprotobyname</code> or <code>getprotobynumber</code> 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. |
| </p></blockquote></div> |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: struct protoent * <b>getprotoent</b> (<var>void</var>)<var><a name="index-getprotoent-1765"></a></var><br> |
| <blockquote><p>This function returns the next entry in the protocols database. It |
| returns a null pointer if there are no more entries. |
| </p></blockquote></div> |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>endprotoent</b> (<var>void</var>)<var><a name="index-endprotoent-1766"></a></var><br> |
| <blockquote><p>This function closes the protocols database. |
| </p></blockquote></div> |
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