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| <h3 class="section">16.13 Networks Database</h3> |
| |
| <p><a name="index-networks-database-1834"></a><a name="index-converting-network-number-to-network-name-1835"></a><a name="index-converting-network-name-to-network-number-1836"></a> |
| <a name="index-g_t_002fetc_002fnetworks-1837"></a><a name="index-netdb_002eh-1838"></a>Many systems come with a database that records a list of networks known |
| to the system developer. This is usually kept either in the file |
| <samp><span class="file">/etc/networks</span></samp> or in an equivalent from a name server. This data |
| base is useful for routing programs such as <code>route</code>, but it is not |
| useful for programs that simply communicate over the network. We |
| provide functions to access this database, which are declared in |
| <samp><span class="file">netdb.h</span></samp>. |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Data Type: <b>struct netent</b><var><a name="index-struct-netent-1839"></a></var><br> |
| <blockquote><p>This data type is used to represent information about entries in the |
| networks database. It has the following members: |
| |
| <dl> |
| <dt><code>char *n_name</code><dd>This is the “official” name of the network. |
| |
| <br><dt><code>char **n_aliases</code><dd>These are alternative names for the network, represented as a vector |
| of strings. A null pointer terminates the array. |
| |
| <br><dt><code>int n_addrtype</code><dd>This is the type of the network number; this is always equal to |
| <code>AF_INET</code> for Internet networks. |
| |
| <br><dt><code>unsigned long int n_net</code><dd>This is the network number. Network numbers are returned in host |
| byte order; see <a href="Byte-Order.html#Byte-Order">Byte Order</a>. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>Use the <code>getnetbyname</code> or <code>getnetbyaddr</code> functions to search |
| the networks database for information about a specific network. The |
| information is returned in a statically-allocated structure; you must |
| copy the information if you need to save it. |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: struct netent * <b>getnetbyname</b> (<var>const char *name</var>)<var><a name="index-getnetbyname-1840"></a></var><br> |
| <blockquote><p>The <code>getnetbyname</code> function returns information about the network |
| named <var>name</var>. It returns a null pointer if there is no such |
| network. |
| </p></blockquote></div> |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: struct netent * <b>getnetbyaddr</b> (<var>unsigned long int net, int type</var>)<var><a name="index-getnetbyaddr-1841"></a></var><br> |
| <blockquote><p>The <code>getnetbyaddr</code> function returns information about the network |
| of type <var>type</var> with number <var>net</var>. You should specify a value of |
| <code>AF_INET</code> for the <var>type</var> argument for Internet networks. |
| |
| <p><code>getnetbyaddr</code> returns a null pointer if there is no such |
| network. |
| </p></blockquote></div> |
| |
| <p>You can also scan the networks database using <code>setnetent</code>, |
| <code>getnetent</code> and <code>endnetent</code>. Be careful when using these |
| functions because they are not reentrant. |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>setnetent</b> (<var>int stayopen</var>)<var><a name="index-setnetent-1842"></a></var><br> |
| <blockquote><p>This function opens and rewinds the networks database. |
| |
| <p>If the <var>stayopen</var> argument is nonzero, this sets a flag so that |
| subsequent calls to <code>getnetbyname</code> or <code>getnetbyaddr</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 netent * <b>getnetent</b> (<var>void</var>)<var><a name="index-getnetent-1843"></a></var><br> |
| <blockquote><p>This function returns the next entry in the networks database. It |
| returns a null pointer if there are no more entries. |
| </p></blockquote></div> |
| |
| <!-- netdb.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>endnetent</b> (<var>void</var>)<var><a name="index-endnetent-1844"></a></var><br> |
| <blockquote><p>This function closes the networks database. |
| </p></blockquote></div> |
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