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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



gethostbyname, gethostbyname_r, gethostbyaddr, gethostbyaddr_r, gethostent, gethostent_r, sethostent, endhostent - get network host entry


#include <netdb.h>

struct hostent *gethostbyname(const char *name);
struct hostent *gethostbyname_r(const char *name,
     struct hostent *result, char *buffer, int buflen,
     int *h_errnop);
struct hostent *gethostbyaddr(const char *addr, int len,
     int type);
struct hostent *gethostbyaddr_r(const char *addr, int length,
     int type, struct hostent *result, char *buffer,
     int buflen, int *h_errnop);
struct hostent *gethostent(void);
struct hostent *gethostent_r(struct hostent *result,
     char *buffer, int buflen, int *h_errnop);
int sethostent(int stayopen);
int endhostent(void);


These functions are used to obtain entries describing hosts. An entry can come from any of the sources for the host database specified in the nsswitch.conf(5) configuration. These functions have been superseded by getaddrinfo(3C), which provides greater portability to applications when multithreading is performed or technologies such as IPv6 are used. For example, the functions described in the following cannot be used with applications targeted to work with IPv6.

The gethostbyname() function searches for information for a host with the hostname specified by the character-string parameter name.

The gethostbyaddr() function searches for information for a host with a given host address. The parameter type specifies the family of the address. This should be one of the address families defined in <sys/socket.h>. See the NOTES section for more information. Also see the EXAMPLES section for information on how to convert an Internet IP address notation that is separated by periods (.) into an addr parameter. The parameter len specifies the length of the buffer indicated by addr.

All addresses are returned in network order. In order to interpret the addresses, ntohl(3C) must be used for byte order conversion.

The sethostent(), gethostent(), and endhostent() functions are used to enumerate host entries from the database.

The sethostent() function sets or resets the enumeration to the beginning of the set of host entries. This function should be called before the first call to gethostent(). Calls to gethostbyname() and gethostbyaddr() leave the enumeration position in an indeterminate state. If the stayopen flag is non-zero, the system can keep allocated resources such as open file descriptors until a subsequent call to endhostent().

Successive calls to the gethostent() function return either successive entries or NULL, indicating the end of the enumeration.

The endhostent() function can be called to indicate that the caller expects to do no further host entry retrieval operations; the system can then deallocate resources it was using. It is still allowed, but possibly less efficient, for the process to call more host retrieval functions after calling endhostent().

Reentrant Interfaces

The gethostbyname(), gethostbyaddr(), and gethostent() functions use static storage that is reused in each call, making these functions unsafe for use in multithreaded applications.

The gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() functions provide reentrant interfaces for these operations.

Each reentrant interface performs the same operation as its non-reentrant counterpart, named by removing the _r suffix. The reentrant interfaces, however, use buffers supplied by the caller to store returned results and the interfaces are safe for use in both single-threaded and multithreaded applications.

Each reentrant interface takes the same parameters as its non-reentrant counterpart, as well as the following additional parameters. The parameter result must be a pointer to a struct hostent structure allocated by the caller. On successful completion, the function returns the host entry in this structure. The parameter buffer must be a pointer to a buffer supplied by the caller. This buffer is used as storage space for the host data. All of the pointers within the returned struct hostent result point to data stored within this buffer. See the RETURN VALUES section for more information. The buffer must be large enough to hold all of the data associated with the host entry. The parameter buflen should give the size in bytes of the buffer indicated by buffer. The parameter h_errnop should be a pointer to an integer. An integer error status value is stored there on certain error conditions. See the ERRORS section for more information.

For enumeration in multithreaded applications, the position within the enumeration is a process-wide property shared by all threads. The sethostent() function can be used in a multithreaded application but resets the enumeration position for all threads. If multiple threads interleave calls to gethostent_r(), the threads will enumerate disjoint subsets of the host database.

Like their non-reentrant counterparts, gethostbyname_r() and gethostbyaddr_r() leave the enumeration position in an indeterminate state.

Return Values

Host entries are represented by the struct hostent structure defined in <netdb.h>:

struct hostent {
    char    *h_name;         /* canonical name of host */
    char    **h_aliases;     /* alias list */
    int     h_addrtype;      /* host address type */
    int     h_length;        /* length of address */
    char    **h_addr_list;   /* list of addresses */

See the EXAMPLES section for information about how to retrieve a ‘.’ separated Internet IP address string from the h_addr_list field of struct hostent.

The gethostbyname(), gethostbyname_r(), gethostbyaddr(), and gethostbyaddr_r() functions each return a pointer to a struct hostent if they successfully locate the requested entry; otherwise they return NULL.

The gethostent() and gethostent_r() functions each return a pointer to a struct hostent if they successfully enumerate an entry; otherwise they return NULL, indicating the end of the enumeration.

The gethostbyname(), gethostbyaddr(), and gethostent() functions use static storage, so returned data must be copied before a subsequent call to any of these functions if the data is to be saved.

When the pointer returned by the reentrant functions gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() is not NULL, it is always equal to the result pointer that was supplied by the caller.

The sethostent() and endhostent() functions return 0 on success.


The reentrant functions gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() will return NULL and set errno to ERANGE if the length of the buffer supplied by caller is not large enough to store the result. See Intro(2) for the proper usage and interpretation of errno in multithreaded applications.

The reentrant functions gethostbyname_r() and gethostbyaddr_r() set the integer pointed to by h_errnop to one of these values in case of error.

On failures, the non-reentrant functions gethostbyname() and gethostbyaddr() set a global integer h_errno to indicate one of these error codes (defined in <netdb.h>): HOST_NOT_FOUND, TRY_AGAIN, NO_RECOVERY, NO_DATA, and NO_ADDRESS.

If a resolver is provided with a malformed address, or if any other error occurs before gethostbyname() is resolved, then gethostbyname() returns an internal error with a value of −1.

The gethostbyname() function will set h_errno to NETDB_INTERNAL when it returns a NULL value.


Example 1 Using gethostbyaddr()

Here is a sample program that gets the canonical name, aliases, and ‘.’ separated Internet IP addresses for a given ‘.’ separated IP address:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

int main(int argc, const char **argv)
    in_addr_t addr;
    struct hostent *hp;
    char **p;
    if (argc != 2) {
        (void) printf("usage: %s IP-address\n", argv[0]);
        exit (1);
    if ((int)(addr = inet_addr(argv[1])) == -1) {
        (void) printf("IP-address must be of the form a.b.c.d\n");
        exit (2);
    hp = gethostbyaddr((char *)&addr, 4, AF_INET);
    if (hp == NULL) {
        (void) printf("host information for %s not found\n", argv[1]);
        exit (3);
    for (p = hp->h_addr_list; *p != 0; p++) {
        struct in_addr in;
        char **q;
        (void) memcpy(&in.s_addr, *p, sizeof (in.s_addr));
        (void) printf("%s %s", inet_ntoa(in), hp−>h_name);
        for (q = hp->h_aliases; *q != 0; q++)
            (void) printf(" %s", *q);
        (void) putchar('\n');
    exit (0);

Note that the preceding sample program is unsafe for use in multithreaded applications.



hosts file that associates the names of hosts with their Internet Protocol (IP) addresses


network configuration database


See attributes(7) for descriptions of the following attributes:

Interface Stability
See Reentrant Interfaces in the DESCRIPTION section.

See Also

Intro(2), getaddrinfo(3C), inet(3C), netdir(3C), ntohl(3C), netdb.h(3HEAD), hosts(5), netconfig(5), nss(5), nsswitch.conf(5), attributes(7)


The reentrant interfaces gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() are included in this release on an uncommitted basis only and are subject to change or removal in future minor releases.


To ensure that they all return consistent results, gethostbyname(), gethostbyname_r(), and netdir_getbyname() are implemented in terms of the same internal library function. This function obtains the system-wide source lookup policy based on the inet family entries in netconfig(5) and the hosts: entry in nsswitch.conf(5). Similarly, gethostbyaddr(), gethostbyaddr_r(), and netdir_getbyaddr() are implemented in terms of the same internal library function. If the inet family entries in netconfig(5) have a ‘-’ in the last column for nametoaddr libraries, then the entry for hosts in nsswitch.conf will be used; nametoaddr libraries in that column will be used, and nsswitch.conf will not be consulted.

There is no analogue of gethostent() and gethostent_r() in the netdir functions, so these enumeration functions go straight to the hosts entry in nsswitch.conf. Thus enumeration can return results from a different source than that used by gethostbyname(), gethostbyname_r(), gethostbyaddr(), and gethostbyaddr_r().

All the functions that return a struct hostent must always return the canonical name in the h_name field. This name, by definition, is the well-known and official hostname shared between all aliases and all addresses. The underlying source that satisfies the request determines the mapping of the input name or address into the set of names and addresses in hostent. Different sources might do that in different ways. If there is more than one alias and more than one address in hostent, no pairing is implied between them.

By default, the system attempts to put those addresses that are on the same subnet as the caller before addresses that are on different subnets. Alternative options are available. See nss(5) for more information.

Use of the enumeration interfaces gethostent() and gethostent_r() is discouraged; enumeration might not be supported for all database sources. The semantics of enumeration are discussed further in nsswitch.conf(5).

The current implementations of these functions only return or accept addresses for the Internet address family (type AF_INET).

The form for an address of type AF_INET is a struct in_addr defined in <netinet/in.h>. The functions described in inet(3C), and illustrated in the EXAMPLES section, are helpful in constructing and manipulating addresses in this form.

When the caller provides the IP address (the addr argument of gethostbyaddr() and gethostbyaddr_r()), the addr argument should be aligned on a word boundary or the code must be changed to memcpy(3C) the argument to an aligned area; otherwise an error such as a SIGBUS may result.


The reentrant interfaces gethostbyname_r(), gethostbyaddr_r(), and gethostent_r() were added to Oracle Solaris in the Solaris 2.3 release.

The functions gethostbyname(), gethostbyaddr(), gethostent(), sethostent(), and addthostent() have been present since the initial release of Solaris.