#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(), gethostbyname2_r():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc up to and including 2.19:
_BSD_SOURCE || _SVID_SOURCE
herror(), hstrerror():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.8 to 2.19:
_BSD_SOURCE || _SVID_SOURCE
Before glibc 2.8:
none
h_errno:
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:
none
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)).
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:
| Interface | Attribute | Value |
| gethostbyname() | Thread safety |
MT-Unsafe race:hostbyname env
locale
|
| gethostbyaddr() | Thread safety |
MT-Unsafe race:hostbyaddr env
locale
|
| sethostent(), endhostent(), gethostent_r() | Thread safety |
MT-Unsafe race:hostent env
locale
|
| 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(), gethostbyname_r(), gethostbyname2_r() | Thread safety | MT-Safe env locale |
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.
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.
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.