GETHOSTBYNAME(3)            Linux Programmer's Manual           GETHOSTBYNAME(3)

       gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
       h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2,
       gethostbyname2_r, gethostbyname_r, gethostent_r - get network host entry

       #include <netdb.h>

       extern int h_errno;

       struct hostent *gethostbyname(const char *name);
       struct hostent *gethostbyaddr(const void *addr,
                                     socklen_t len, int type);

       void sethostent(int stayopen);
       void endhostent(void);

       void herror(const char *s);
       const char *hstrerror(int err);

       /* System V/POSIX extension */
       struct hostent *gethostent(void);

       /* GNU extensions */
       struct hostent *gethostbyname2(const char *name, int af);

       int gethostent_r(struct hostent *restrict ret,
                        char *restrict buf, size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);

       int gethostbyaddr_r(const void *restrict addr, socklen_t len, int type,
                        struct hostent *restrict ret,
                        char *restrict buf, size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);
       int gethostbyname_r(const char *restrict name,
                        struct hostent *restrict ret,
                        char *restrict buf, size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);
       int gethostbyname2_r(const char *restrict name, int af,
                        struct hostent *restrict ret,
                        char *restrict buf, size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(),
           Since glibc 2.19:
           Glibc up to and including 2.19:
               _BSD_SOURCE || _SVID_SOURCE

       herror(), hstrerror():
           Since glibc 2.19:
           Glibc 2.8 to 2.19:
               _BSD_SOURCE || _SVID_SOURCE
           Before glibc 2.8:

           Since glibc 2.19
               _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
           Glibc 2.12 to 2.19:
               _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
           Before glibc 2.12:

       The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror()
       functions are obsolete.  Applications should use getaddrinfo(3),
       getnameinfo(3), and gai_strerror(3) instead.

       The gethostbyname() function returns a structure of type hostent for the
       given host name.  Here name is either a hostname or an IPv4 address in
       standard dot notation (as for inet_addr(3)).  If name is an IPv4 address,
       no lookup is performed and gethostbyname() simply copies name into the
       h_name field and its struct in_addr equivalent into the h_addr_list[0]
       field of the returned hostent structure.  If name doesn't end in a dot
       and the environment variable HOSTALIASES is set, the alias file pointed
       to by HOSTALIASES will first be searched for name (see hostname(7) for
       the file format).  The current domain and its parents are searched unless
       name ends in a dot.

       The gethostbyaddr() function returns a structure of type hostent for the
       given host address addr of length len and address type type.  Valid
       address types are AF_INET and AF_INET6 (defined in <sys/socket.h>).  The
       host address argument is a pointer to a struct of a type depending on the
       address type, for example a struct in_addr * (probably obtained via a
       call to inet_addr(3)) for address type AF_INET.

       The sethostent() function specifies, if stayopen is true (1), that a
       connected TCP socket should be used for the name server queries and that
       the connection should remain open during successive queries.  Otherwise,
       name server queries will use UDP datagrams.

       The endhostent() function ends the use of a TCP connection for name
       server queries.

       The (obsolete) herror() function prints the error message associated with
       the current value of h_errno on stderr.

       The (obsolete) hstrerror() function takes an error number (typically
       h_errno) and returns the corresponding message string.

       The domain name queries carried out by gethostbyname() and
       gethostbyaddr() rely on the Name Service Switch (nsswitch.conf(5))
       configured sources or a local name server (named(8)).  The default action
       is to query the Name Service Switch (nsswitch.conf(5)) configured
       sources, failing that, a local name server (named(8)).

       The nsswitch.conf(5) file is the modern way of controlling the order of
       host lookups.

       In glibc 2.4 and earlier, the order keyword was used to control the order
       of host lookups as defined in /etc/host.conf (host.conf(5)).

       The hostent structure is defined in <netdb.h> as follows:

           struct hostent {
               char  *h_name;            /* official 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 */
           #define h_addr h_addr_list[0] /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name of the host.

              An array of alternative names for the host, terminated by a null

              The type of address; always AF_INET or AF_INET6 at present.

              The length of the address in bytes.

              An array of pointers to network addresses for the host (in network
              byte order), terminated by a null pointer.

       h_addr The first address in h_addr_list for backward compatibility.

       The gethostbyname() and gethostbyaddr() functions return the hostent
       structure or a null pointer if an error occurs.  On error, the h_errno
       variable holds an error number.  When non-NULL, the return value may
       point at static data, see the notes below.

       The variable h_errno can have the following values:

              The specified host is unknown.

              The requested name is valid but does not have an IP address.
              Another type of request to the name server for this domain may
              return an answer.  The constant NO_ADDRESS is a synonym for

              A nonrecoverable name server error occurred.

              A temporary error occurred on an authoritative name server.  Try
              again later.

              resolver configuration file

              host database file

              name service switch configuration

       For an explanation of the terms used in this section, see attributes(7).

       │Interface          Attribute     Value                              │
       │gethostbyname()    │ Thread safety │ MT-Unsafe race:hostbyname env      │
       │                   │               │ locale                             │
       │gethostbyaddr()    │ Thread safety │ MT-Unsafe race:hostbyaddr env      │
       │                   │               │ locale                             │
       │sethostent(),      │ Thread safety │ MT-Unsafe race:hostent env locale  │
       │endhostent(),      │               │                                    │
       │gethostent_r()     │               │                                    │
       │herror(),          │ Thread safety │ MT-Safe                            │
       │hstrerror()        │               │                                    │
       │gethostent()       │ Thread safety │ MT-Unsafe race:hostent             │
       │                   │               │ race:hostentbuf env locale         │
       │gethostbyname2()   │ Thread safety │ MT-Unsafe race:hostbyname2 env     │
       │                   │               │ locale                             │
       │gethostbyaddr_r(), │ Thread safety │ MT-Safe env locale                 │
       │gethostbyname_r(), │               │                                    │
       │gethostbyname2_r() │               │                                    │
       In the above table, hostent in race:hostent signifies that if any of the
       functions sethostent(), gethostent(), gethostent_r(), or endhostent() are
       used in parallel in different threads of a program, then data races could

       POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(), sethostent(),
       endhostent(), gethostent(), and h_errno; gethostbyname(),
       gethostbyaddr(), and h_errno are marked obsolescent in that standard.
       POSIX.1-2008 removes the specifications of gethostbyname(),
       gethostbyaddr(), and h_errno, recommending the use of getaddrinfo(3) and
       getnameinfo(3) instead.

       The functions gethostbyname() and gethostbyaddr() may return pointers to
       static data, which may be overwritten by later calls.  Copying the struct
       hostent does not suffice, since it contains pointers; a deep copy is

       In the original BSD implementation the len argument of gethostbyname()
       was an int.  The SUSv2 standard is buggy and declares the len argument of
       gethostbyaddr() to be of type size_t.  (That is wrong, because it has to
       be int, and size_t is not.  POSIX.1-2001 makes it socklen_t, which is
       OK.)  See also accept(2).

       The BSD prototype for gethostbyaddr() uses const char * for the first

   System V/POSIX extension
       POSIX requires the gethostent() call, which should return the next entry
       in the host data base.  When using DNS/BIND this does not make much
       sense, but it may be reasonable if the host data base is a file that can
       be read line by line.  On many systems, a routine of this name reads from
       the file /etc/hosts.  It may be available only when the library was built
       without DNS support.  The glibc version will ignore ipv6 entries.  This
       function is not reentrant, and glibc adds a reentrant version

   GNU extensions
       Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
       permits to specify the address family to which the address must belong.

       Glibc2 also has reentrant versions gethostent_r(), gethostbyaddr_r(),
       gethostbyname_r(), and gethostbyname2_r().  The caller supplies a hostent
       structure ret which will be filled in on success, and a temporary work
       buffer buf of size buflen.  After the call, result will point to the
       result on success.  In case of an error or if no entry is found result
       will be NULL.  The functions return 0 on success and a nonzero error
       number on failure.  In addition to the errors returned by the
       nonreentrant versions of these functions, if buf is too small, the
       functions will return ERANGE, and the call should be retried with a
       larger buffer.  The global variable h_errno is not modified, but the
       address of a variable in which to store error numbers is passed in

       gethostbyname() does not recognize components of a dotted IPv4 address
       string that are expressed in hexadecimal.

       getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3),
       resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

                                   2021-03-22                   GETHOSTBYNAME(3)