gethostbyname_r()--Get Host Information for Host Name

  #include <netdb.h>

  int gethostbyname_r(char *host_name,
                      struct hostent *hostent_struct_addr,
                      struct hostent_data *hostent_data_struct_addr)

  #define _XOPEN_SOURCE 520
  #include <netdb.h>

  int gethostbyname_r(const char *host_name,
                       struct hostent *hostent_struct_addr,
                       struct hostent_data *hostent_data_struct_addr)

The gethostbyname_r() function is used to retrieve information about a host.

There are two versions of the API, as shown above. The base i5/OS API uses BSD 4.3 structures and syntax. The other uses syntax and structures compatible with the UNIX 98 programming interface specifications. You can select the UNIX 98 compatible interface with the _XOPEN_SOURCE macro.

Parameters

host_name  (input) 

Specifies the pointer to the character string that contains the name of the host for which information is to be retrieved.

hostent_struct_addr  (input/output) 

Specifies the pointer to a hostent structure where the results will be placed. All results must be referenced through this structure.

hostent_data_struct_addr  (input/output) 

Specifies the pointer to the hostent_data structure, which is used to pass and preserve results between function calls. The field host_control_blk in the hostent_data structure must be initialized with hexadecimal zeros before its initial use. If compatibility with other platforms is required, then the entire hostent_data structure must be initialized with hexadecimal zeros before initial use.

Authorities:

Authorization of *R (allow access to the object) to the host aliases file specified by the hostaliases environment variable.

You also need *X authority to each directory in the path of the host aliases file.

Return Value

The gethostbyname_r() function returns an integer. Possible values are:

• -1 (unsuccessful call)
• 0 (successful call)

The struct hostent denoted by hostent_struct_addr and struct hostent_datadenoted by hostent_data_struct_addr are both defined in <netdb.h>. The structure struct hostentis defined as:

      struct hostent [
        char   *h_name;
        char   **h_aliases;
        int    h_addrtype;
        int    h_length;
        char   **h_addr_list;
      ];

      #define h_addr  h_addr_list[0]

h_name points to the character string that contains the name of the host. h_aliases is a pointer to a NULL-terminated list of pointers, each of which points to a character string that represents an alternative name for the host. h_addrtype contains the address type of the host (for example, af_inet). h_length contains the size of an address in octets (for example, the size of an Internet address is 4 octets). h_addr_list is a pointer to a NULL-terminated list of pointers, each of which points to a network address (in network byte order) for the host.

Error Conditions

When the gethostbyname_r() function fails, h_errno (defined in <netdb.h>) can be set to:

[HOST_NOT_FOUND]

The host name specified by the host_name parameter was not found.

[NO_DATA]

The host name is a valid name, but there is no corresponding IP address.

[NO_RECOVERY]

An unrecoverable error has occurred.

[TRY_AGAIN]

The local server did not receive a response from an authoritative server. An attempt at a later time may succeed.

When the gethostbyname_r() function fails, errno can be set to:

[EACCES]

Permission denied. The process does not have the appropriate privileges to the host aliases file specified by the HOSTALIASES environment variable.

[EINVAL]

The hostent_data structure was not initialized with hexadecimal zeros before initial use. For corrective action, see the description for structure hostent_data.

Usage Notes

1. The iSeries Navigator or the following CL commands can be used to access the host database file:
   
   
   • ADDTCPHTE (Add TCP/IP Host Table Entry)
   • RMVTCPHTE (Remove TCP/IP Host Table Entry)
   • CHGTCPHTE (Change TCP/IP Host Table Entry)
   • RNMTCPHTE (Rename TCP/IP Host Table Entry)
   • MRGTCPHT (Merge TCP/IP Host Tables)
   
   
2. There are two sources from which host information can be obtained: the domain name server and the host database file. The path taken depends on whether an IP address is configured for a name server using the iSeries Navigator or option 12, Change TCP/IP domain information, on the CFGTCP menu.

   Note: A person with a UNIX background would expect this information to exist in a file known as /etc/resolv.conf. If the IP address is found (indicating that the local network is a domain network), the gethostbyaddr_r() function will attempt to query the domain name server for information about a host. If the query fails, the information will be obtained from the host database file. If the name server IP address is not found (indicating that local network is a flat network), the host database file is used to obtain the address.

3. If the host_name parameter does specify a domain qualified name, the gethostbyaddr_r() function will append a domain name to the specified host name, if possible. The domain name that will be appended is configured using the iSeries Navigator or CFGTCP menu option 12, Change TCP/IP domain information.
   
   
4. When the host information is obtained from the host database file, the file is opened and the host information is retrieved (if it exists) from the file. The file is then closed only if a sethostent_r() call with a non-zero parameter value was not previously done.
   
   
5. If a sethostent_r() call with a non-zero parameter value was previously done, the gethostbyname_r() routine, when obtaining host information from the domain name server, will communicate with the domain name server over a connection-oriented transport service (for example, TCP). Otherwise, gethostbyname_r() will use a connectionless transport service (for example, UDP).
   
   
6. A job has a coded character set identifier (CCSID) and a default CCSID. The default CCSID is the same as the job CCSID unless the job CCSID specifies 65535, which requests that no database translation be performed. In this case, the default CCSID is set by the system based on the language ID in effect for the job.

   If the host information is retrieved from the domain name server, sockets converts the host name specified by the host_name parameter to ASCII before communicating with the domain name server. If the host information is retrieved from the host database file, no conversion is done on the host name specified by the host_name parameter unless the CCSID of the job is something other than 65535. In addition, host names returned in the hostent will be returned in the default CCSID of the job if they are obtained from the domain name server. For translation to occur for the host names returned in the hostent structure when they are obtained from the host database file, you must use a job CCSID of something other than 65535.

7. Address families are defined in <sys/socket.h>, and the in_addr structure is defined in <netinet/in.h>.
   
   
8. gethostbyname_r() will resolve local host aliases to a domain name which are then resolved with a query using DNS. See res_hostalias() for more information on aliases.
   
   
9. When you develop in C-based languages and an application is compiled with the _XOPEN_SOURCE macro defined to the value 520 or greater, the gethostbyname_r() API is mapped to qso_gethostbyname_r98().

Related Information

• _XOPEN_SOURCE--Using _XOPEN_SOURCE for the UNIX 98 compatible interface
  
  
• hstrerror()--Retrieve Resolver Error Message
  
  
• res_hostalias()--Retrieve the host alias
  
  
• endhostent_r()--Close Host Database
  
  
• gethostbyaddr_r()--Get Host Information for IP Address
  
  
• gethostent_r()--Get Next Entry from Host Database
  
  
• sethostent_r()--Open Host Database