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357 lines
11 KiB
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<title>gethostbyname()--Get Host Information for Host Name</title>
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<h2>gethostbyname()--Get Host Information for Host Name</h2>
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<div class="box" style="width: 70%;">
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<br> BSD 4.3 Syntax<br>
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<pre>
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#include <netdb.h>
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struct hostent *gethostbyname(char *<em>host_name</em>)
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</pre>
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<br>
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Service Program Name: QSOSRV2<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: No; see <a href="#USAGE_NOTES">Usage Notes</a>.<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<br>
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<div class="box" style="width: 70%;">
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<br>
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<a href="_xopen_source.htm">UNIX 98 Compatible Syntax</a><br>
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<pre>
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#define _XOPEN_SOURCE 520
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#include <netdb.h>
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struct hostent *gethostbyname(const char *<em>host_name</em>)
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</pre>
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<br>
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Service Program Name: QSOSRV2<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: No; see <a href="#USAGE_NOTES">Usage Notes</a>.<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<br>
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<p>The <em>gethostbyname()</em> function is used to retrieve information about
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a host.</p>
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<p>There are two versions of the API, as shown above. The base i5/OS API uses BSD 4.3
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structures and syntax. The other uses syntax and structures compatible with the UNIX 98
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programming interface specifications. You can select the UNIX 98 compatible
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interface with the <a href="_xopen_source.htm">_XOPEN_SOURCE</a> macro.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>host_name</strong></dt>
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<dd>(Input) The pointer to the character string that contains the name of the
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host for which information is to be retrieved.</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p>Authorization of *R (allow access to the object) to the host aliases file
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specified by the <em>HOSTALIASES</em> environment variable.</p>
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<p>You also need *X authority to each directory in the path of the host aliases
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file.</p>
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<br>
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<h3>Return Value</h3>
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<p><em>gethostbyname()</em> returns a pointer. Possible values are:</p>
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<ul>
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<li>NULL (unsuccessful)<br>
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<br>
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</li>
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<li>p (successful), where p is a pointer to <strong>struct
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hostent</strong>.</li>
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</ul>
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<p>The structure <strong>struct hostent</strong> is defined in
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<strong><netdb.h></strong>.</p>
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<pre>
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struct hostent {
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char *h_name;
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char **h_aliases;
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int h_addrtype;
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int h_length;
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char **h_addr_list;
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};
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#define h_addr h_addr_list[0]
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</pre>
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<p><em>h_name</em> points to the character string that contains the name of the
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host. <em>h_aliases</em> is a pointer to a NULL-terminated list of pointers,
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each of which points to a character string that represents an alternative name
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for the host. <em>h_addrtype</em> contains the address type of the host (for
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example, <samp>AF_INET</samp>). <em>h_length</em> contains the address length.
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<em>h_addr_list</em> is a pointer to a NULL-terminated list of pointers, each
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of which points to a network address for the host, in network byte order. Note
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that the array of address pointers points to structures of type
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<strong>in_addr</strong> defined in <strong><netinet/in.h></strong>.</p>
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<br>
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<h3>Error Conditions</h3>
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<p>When <em>gethostbyname()</em> fails, <em>h_errno</em> (defined in
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<strong><netdb.h></strong>) can be set to one of the following:</p>
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<dl>
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<dd></dd>
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<dt><em>[HOST_NOT_FOUND]</em></dt>
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<dd><p>The host name specified by the <em>host_name</em>
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parameter was not found.</p></dd>
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<dt><em>[NO_DATA]</em></dt>
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<dd><p>The host name is a valid name, but there is no
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corresponding IP address.</p></dd>
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<dt><em>[NO_RECOVERY]</em></dt>
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<dd><p>An unrecoverable error has occurred.</p></dd>
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<dt><em>[TRY_AGAIN]</em></dt>
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<dd><p>The local server did not receive a response from
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an authoritative server. An attempt at a later time may succeed.</p></dd>
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</dl>
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<p>When the <em>gethostbyname()</em> function fails, <em>errno</em> can be set
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to:</p>
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<dl>
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<dt><em>[EACCES]</em></dt>
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<dd><p>Permission denied. The process does not have the
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appropriate privileges to the host aliases file specified by the
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<em>HOSTALIASES</em> environment variable.</p></dd>
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</dl>
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<br>
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<h3><a name="USAGE_NOTES">Usage Notes</a></h3>
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<ol>
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<li>The iSeries Navigator or the following CL commands can be
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used to access the host database file:<br>
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<br>
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<ul>
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<li>ADDTCPHTE (Add TCP/IP Host Table Entry)<br>
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<br>
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</li>
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<li>RMVTCPHTE (Remove TCP/IP Host Table Entry)<br>
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<br>
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</li>
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<li>CHGTCPHTE (Change TCP/IP Host Table Entry)<br>
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<br>
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</li>
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<li>RNMTCPHTE (Rename TCP/IP Host Table Entry)<br>
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<br>
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</li>
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<li>MRGTCPHT (Merge TCP/IP Host Tables)</li>
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</ul>
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<br>
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<br>
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</li>
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<li>The pointer returned by <em>gethostbyname()</em> points to static storage
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that is overwritten on subsequent calls to the <em>gethostbyname()</em>,
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<em>gethostbyaddr()</em>, or <em>gethostent()</em> functions.<br>
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<br>
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</li>
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<li>There are two sources from which host information can be obtained: the
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domain name server, and the host database file. The path taken depends on
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whether an IP address is configured for a name server using the iSeries Navigator or option 12,
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Change TCP/IP domain information, on the Configure TCP/IP (CFGTCP) menu.
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<p><strong>Note:</strong> A person with a UNIX background would expect this
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information to exist in a file known as <strong>/etc/resolv.conf</strong>.</p>
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<p>If the IP address is found (indicating that the local network is a domain
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network), the <em>gethostbyaddr()</em> function attempts to query the domain
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name server for information about a host. If the query fails, the information
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is obtained from the host database file. If the name server IP address is not
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found (indicating that local network is a flat network), the host database file
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is used to obtain the address.</p>
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</li>
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<li>If the <em>host_name</em> parameter does specify a domain qualified name,
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the <em>gethostbyaddr()</em> function appends a domain name to the specified
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host name, if possible. The domain name that is appended is configured using the
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iSeries Navigator or CFGTCP menu option 12, Change TCP/IP domain information.<br>
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<br>
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</li>
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<li>When the host information is obtained from the host database file, the file
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is opened and the host information is retrieved (if it exists) from the file.
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The file is then closed only if a <em>sethostent()</em> with a nonzero
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parameter value was not previously done.<br>
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<br>
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</li>
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<li>If a <em>sethostent()</em> with a nonzero parameter value was previously
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done, the <em>gethostbyname()</em> routine, when obtaining host information
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from the domain name server, communicates with the domain name server over a
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connection-oriented transport service (for example, TCP). Otherwise,
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<em>gethostbyname()</em> uses a connectionless transport service (for example,
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UDP).<br>
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<br>
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</li>
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<li>A job has a coded character set identifier (CCSID) and a default CCSID. The
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default CCSID is the same as the job CCSID unless the job CCSID specifies
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65535, which requests that no database translation be performed. In this case,
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the default CCSID is set by the system based on the language ID in effect for
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the job.
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<p>If the host information is retrieved from the domain name server, sockets
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converts the host name specified by the <em>host_name</em> parameter from the
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default (CCSID) to ASCII before communicating with the domain name server. If
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the host information is retrieved from the host database file, no conversion is
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done on the host name specified by the <em>host_name</em> parameter unless the
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CCSID of the job is something other than 65535. In addition, the host names
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returned in the hostent structure will be returned in the default CCSID of the
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job if they are obtained from the domain name server. For translation to occur
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for the host names returned in the hostent structure when they are obtained
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from the host database file, you must use a job CCSID of something other than
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65535.</p>
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</li>
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<li>Address families are defined in <strong><sys/socket.h></strong>, and
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the <strong>in_addr</strong> structure is defined in
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<strong><netinet/in.h></strong>.<br>
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<br>
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</li>
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<li>Do not use the <em>gethostbyname()</em> function in a multithreaded
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environment. See the multithread alternative <em>gethostbyname_r()</em>
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function.<br>
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<br>
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</li>
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<li><em>gethostbyname()</em> will resolve local host aliases to a domain name
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which are then resolved with a query using DNS. See <a href=
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"reshostalias.htm">res_hostalias()</a> for more information on aliases. <br>
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<br>
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</li>
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<li>When you develop in C-based languages and an application is compiled with the _XOPEN_SOURCE
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macro defined to the value 520 or greater, the <em>gethostbyname()</em> API is mapped to
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<em>qso_gethostbyname98()</em>.</li>
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</ol>
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<br>
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<h3>Related Information</h3>
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<ul>
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<li><a href="_xopen_source.htm">_XOPEN_SOURCE</a>--Using _XOPEN_SOURCE for the UNIX 98
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compatible interface<br>
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<br>
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</li>
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<li><a href="hstrerror.htm">hstrerror()</a>--Retrieve Resolver Error
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Message<br>
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<br>
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</li>
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<li><a href="reshostalias.htm">res_hostalias()</a>--Retrieve the host alias<br>
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<br>
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</li>
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<li><a href="ghosta.htm">gethostbyaddr()</a>--Get Host Information for IP
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Address<br>
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<br>
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</li>
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<li><a href="ghostnt.htm">gethostent()</a>--Get Next Entry from Host
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Database<br>
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<br>
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</li>
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<li><a href="shostnt.htm">sethostent()</a>--Open Host Database<br>
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<br>
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</li>
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<li><a href="ehostnt.htm">endhostent()</a>--Close Host Database</li>
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</ul>
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<br>
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<hr>
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API introduced: V3R1
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a> </td>
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</center>
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