gethostbyname(3)                                      Library Functions Manual                                     gethostbyname(3)

       gethostbyname,  gethostbyaddr,  sethostent,  gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostby‐
       name2, gethostbyname2_r, gethostbyname_r, gethostent_r - get network host entry

       Standard C library (libc, -lc)

       #include <netdb.h>

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

       [[deprecated]] extern int h_errno;

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

       [[deprecated]] void herror(const char *s);
       [[deprecated]] 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 buf[restrict .buflen], size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);

       int gethostbyaddr_r(const void addr[restrict .len], socklen_t len,
                        int type,
                        struct hostent *restrict ret,
                        char buf[restrict .buflen], size_t buflen,
                        struct hostent **restrict result,
                        int *restrict h_errnop);
       int gethostbyname_r(const char *restrict name,
                        struct hostent *restrict ret,
                        char buf[restrict .buflen], 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 buf[restrict .buflen], 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(), gethostbyname2_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 glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE
           Before glibc 2.8:

           Since glibc 2.19
               _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
           glibc 2.12 to glibc 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  getad‐
       drinfo(3), getnameinfo(3), and gai_strerror(3) instead.

       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

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

       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 (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 (nss‐
       witch.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

       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 pointer.

              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 er‐
       ror,  the  h_errno  variable holds an error number.  When non-NULL, the return value may point at static data, see the notes

       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 do‐
              main may return an answer.  The constant NO_ADDRESS is a synonym for NO_DATA.

              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(), endhostent(), │ Thread safety │ MT-Unsafe race:hostent env locale                                            │
       │gethostent_r()              │               │                                                                              │
       │herror(), hstrerror()       │ Thread safety │ MT-Safe                                                                      │
       │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(),  gethos‐
       tent_r(), or endhostent() are used in parallel in different threads of a program, then data races could occur.

       POSIX.1-2001  specifies  gethostbyname(), gethostbyaddr(), sethostent(), endhostent(), gethostent(), and h_errno; gethostby‐
       name(), 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 required.

       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 argument.

   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 ver‐
       sion gethostent_r().

   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 h_errnop.

       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),

Linux man-pages 6.03                                         2023-02-05                                            gethostbyname(3)