Here is an example of a program that uses gethostbyaddr_r().
All other routines with names that end in "_r" have similar semantics and are also threadsafe. This example program takes an IP address in the dotted-decimal notation and prints the host name.
/********************************************************/
/* Header files */
/********************************************************/
#include </netdb.h>
#include <sys/param.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#define HEX00 '\x00'
#define NUMPARMS 2
/********************************************************/
/* Pass one parameter that is the IP address in */
/* dotted decimal notation. The host name will be */
/* displayed if found; otherwise, a message states */
/* host not found. */
/********************************************************/
int main(int argc, char *argv[])
{
int rc;
struct in_addr internet_address;
struct hostent hst_ent;
struct hostent_data hst_ent_data;
char dotted_decimal_address [16];
char host_name[MAXHOSTNAMELEN];
/**********************************************************/
/* Verify correct number of arguments have been passed */
/**********************************************************/
if (argc != NUMPARMS)
{
printf("Wrong number of parms passed\n");
exit(-1);
}
/**********************************************************/
/* Obtain addressability to parameters passed */
/**********************************************************/
strcpy(dotted_decimal_address, argv[1]);
/**********************************************************/
/* Initialize the structure-field */
/* hostent_data.host_control_blk with hexadecimal zeros */
/* before its initial use. If you require compatibility */
/* with other platforms, then you must initialize the */
/* entire hostent_data structure with hexadecimal zeros. */
/**********************************************************/
/* Initialize to hex 00 hostent_data structure */
/**********************************************************/
memset(&hst_ent_data,HEX00,sizeof(struct hostent_data));
/**********************************************************/
/* Translate an IP address from dotted decimal */
/* notation to 32-bit IP address format. */
/**********************************************************/
internet_address.s_addr=inet_addr(dotted_decimal_address);
/**********************************************************/
/* Obtain host name */
/**********************************************************/
/**********************************************************/
/* NOTE: The gethostbyaddr_r() returns an integer. */
/* The following are possible values: */
/* -1 (unsuccessful call) */
/* 0 (successful call) */
/**********************************************************/
rc=gethostbyaddr_r((char *) &internet_address,
sizeof(struct in_addr), AF_INET,
&hst_ent, &hst_ent_data);
if (rc== -1)
{
printf("Host name not found\n");
exit(-1);
}
else
{
/*****************************************************/
/* Copy the host name to an output buffer */
/*****************************************************/
(void) memcpy((void *) host_name,
/****************************************************/
/* You must address all the results through the */
/* hostent structure hst_ent. */
/* NOTE: Hostent_data structure hst_ent_data is just */
/* a data repository that is used to support the */
/* hostent structure. Applications should consider */
/* hostent_data a storage area to put host level data */
/* that the application does not need to access. */
/****************************************************/
(void *) hst_ent.h_name,
MAXHOSTNAMELEN);
/*****************************************************/
/* Print the host name */
/*****************************************************/
printf("The host name is %s\n", host_name);
}
exit(0);
}