h_errno(3) — Linux manual page


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

NAME         top

       gethostbyname,  gethostbyaddr,  sethostent,  gethostent,  endhostent,
       h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethost‐
       byname2_r, gethostbyname_r, gethostent_r - get network host entry

SYNOPSIS         top

       #include <netdb.h>
       extern int h_errno;

       struct hostent *gethostbyname(const char *name);

       #include <sys/socket.h>       /* for AF_INET */
       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 *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

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

       int gethostbyname_r(const char *name,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

       int gethostbyname2_r(const char *name, int af,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *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 versions 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:

DESCRIPTION         top

       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.  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 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.

RETURN VALUE         top

       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.

ERRORS         top

       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.

FILES         top

              resolver configuration file

              host database file

              name service switch configuration

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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    │
       │endhostent(),      │               │ locale                        │
       │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 occur.

CONFORMING TO         top

       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.

NOTES         top

       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

   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 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

       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.

BUGS         top

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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

                                 2020-06-09                 GETHOSTBYNAME(3)