gethostbyname: Get IP address information by host name

The gethostbyname socket function returns information about a host specified by a host name.

Last updated

Added for PUT00.

Format

#include  <netdb.h>
struct hostent *gethostbyname(char *name);

name: The name of the host that is being queried. If the specified host name does not contain a period, the defined default domain name is appended. For example, if you specify tpf for this parameter and the defined default domain name is ibm.com®, the host name resolves to tpf.ibm.com.
See the ZDTCP command in z/TPF Operations for information about defining, displaying, and deleting the default domain name.

Normal return

This function returns a pointer to a hostent structure for the host name specified on the call. The netdb.h header file defines the hostent structure, which contains the following elements:

h_name: Official name of the host.
h_aliases: Zero-terminated array of alternative names for the host.
h_addrtype: Type of address being returned, always set to AF_INET.
h_length: Length of the address in bytes.
h_addr: Pointer to the network address of the host in network byte order.

Note: Subsequent gethostbyname calls overwrite the data in the hostent structure.

Error return

A NULL pointer indicates an error. The value of h_errno indicates the specific error.

HOST_NOT_FOUND: The host name specified by the name parameter was not found.
TRY_AGAIN: The local server did not receive a response from an authorized server. Try again later.
NO_RECOVERY: An irrecoverable error has occurred.
NO_DATA: The host name is a valid name, but there is no corresponding Internet Protocol (IP) address.

Programming considerations

The gethostbyname function tries to resolve the host name through a name server if one is present.

Examples

The following example obtains the IP address associated with a given host name.

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
⋮
struct sockaddr whereto;
struct hostent *hp;
struct sockaddr_in *to;
char *target;
char *hostname;

memset(&whereto, 0, sizeof(struct sockaddr));
to = (struct sockaddr_in *)&whereto;
to->sin_family = AF_INET;
to->sin_addr.s_addr = inet_addr(target);
if (to->sin_addr.s_addr != -1)
   hostname = target;
else
{
   hp = gethostbyname(target);
   if (!hp)
      printf("unknown host %s\n", target);
   else
   {
      to->sin_family = hp->h_addrtype;
      memcpy(&(to->sin_addr.s_addr), hp->h_addr, hp->h_length);
      hostname = hp->h_name;
      printf("gethostbyname was successful\n");
   }
}

Related information

gethostbyaddr: Get host information for IP address.

See z/TPF Transmission Control Protocol/Internet Protocol for information about z/TPF TCP/IP support.