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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |FILES |ATTRIBUTES |STANDARDS |HISTORY |NOTES |BUGS |SEE ALSO |COLOPHON | |
gethostbyname(3) Library Functions Manualgethostbyname(3)gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostbyname2_r, gethostbyname_r, gethostent_r - get network host entry
Standard C library (libc,-lc)
#include <netdb.h>void sethostent(intstayopen);void endhostent(void);[[deprecated]] extern int h_errno;[[deprecated]] struct hostent *gethostbyname(const char *name);[[deprecated]] struct hostent *gethostbyaddr(socklen_t size;const voidaddr[size],socklen_tsize, inttype);[[deprecated]] void herror(const char *s);[[deprecated]] const char *hstrerror(interr); /* System V/POSIX extension */struct hostent *gethostent(void); /* GNU extensions */[[deprecated]]struct hostent *gethostbyname2(const char *name, intaf);int gethostent_r(size_t bufsize;struct hostent *restrictret,charbuf[restrictbufsize], size_tbufsize,struct hostent **restrictresult,int *restricth_errnop);[[deprecated]]int gethostbyaddr_r(socklen_t size, size_t bufsize;const voidaddr[restrictsize], socklen_tsize,inttype,struct hostent *restrictret,charbuf[restrictbufsize], size_tbufsize,struct hostent **restrictresult,int *restricth_errnop);[[deprecated]]int gethostbyname_r(size_t bufsize;const char *restrictname,struct hostent *restrictret,charbuf[restrictbufsize], size_tbufsize,struct hostent **restrictresult,int *restricth_errnop);[[deprecated]]int gethostbyname2_r(size_t bufsize;const char *restrictname, intaf,struct hostent *restrictret,charbuf[restrictbufsize], size_tbufsize,struct hostent **restrictresult,int *restricth_errnop); Feature Test Macro Requirements for glibc (seefeature_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_SOURCEherror(),hstrerror(): Since glibc 2.19: _DEFAULT_SOURCE glibc 2.8 to glibc 2.19: _BSD_SOURCE || _SVID_SOURCE Before glibc 2.8: noneh_errno: 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: none
Thegethostbyname*(),gethostbyaddr*(),herror(), andhstrerror() functions are obsolete. Applications should usegetaddrinfo(3),getnameinfo(3), andgai_strerror(3) instead. Thesethostent() function specifies, ifstayopen 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. Theendhostent() function ends the use of a TCP connection for name server queries. Thegethostbyname() function returns a structure of typehostent for the given hostname. Herename is either a hostname or an IPv4 address in standard dot notation (as forinet_addr(3)). Ifname is an IPv4 address, no lookup is performed andgethostbyname() simply copiesname into theh_name field and itsstruct in_addr equivalent into theh_addr_list[0] field of the returnedhostent structure. Ifname doesn't end in a dot and the environment variableHOSTALIASESis set, the alias file pointed to byHOSTALIASESwill first be searched forname (seehostname(7) for the file format). The current domain and its parents are searched unlessname ends in a dot. Thegethostbyaddr() function returns a structure of typehostent for the given host addressaddr of sizelen and address typetype. Valid address types areAF_INETandAF_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 astruct in_addr * (probably obtained via a call toinet_addr(3)) for address typeAF_INET. The (obsolete)herror() function prints the error message associated with the current value ofh_errno onstderr. The (obsolete)hstrerror() function takes an error number (typicallyh_errno) and returns the corresponding message string. The domain name queries carried out bygethostbyname() andgethostbyaddr() 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)).Historical Thensswitch.conf(5) file is the modern way of controlling the order of host lookups. In glibc 2.4 and earlier, theorder keyword was used to control the order of host lookups as defined in/etc/host.conf (host.conf(5)). Thehostent 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; /* size of address */ char **h_addr_list; /* list of addresses */ } #define h_addr h_addr_list[0] /* for backward compatibility */ The members of thehostent structure are:h_name The official name of the host.h_aliases An array of alternative names for the host, terminated by a null pointer.h_addrtype The type of address; alwaysAF_INETorAF_INET6at present.h_length The size of the address in bytes.h_addr_list An array of pointers to network addresses for the host (in network byte order), terminated by a null pointer.h_addr The first address inh_addr_list for backward compatibility.Thegethostbyname() andgethostbyaddr() functions return thehostent structure or a null pointer if an error occurs. On error, theh_errno variable holds an error number. When non-NULL, the return value may point at static data, see the notes below.
The variableh_errno can have the following values:HOST_NOT_FOUND The specified host is unknown.NO_DATA 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 constantNO_ADDRESS is a synonym forNO_DATA.NO_RECOVERY A nonrecoverable name server error occurred.TRY_AGAIN A temporary error occurred on an authoritative name server. Try again later.
/etc/host.conf resolver configuration file/etc/hosts host database file/etc/nsswitch.conf name service switch configuration
For an explanation of the terms used in this section, seeattributes(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 │ │endhostent(), │ │ env 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 inrace:hostent signifies that if any of the functionssethostent(),gethostent(),gethostent_r(), orendhostent() are used in parallel in different threads of a program, then data races could occur.
sethostent()endhostent()gethostent() POSIX.1-2008.gethostent_r() GNU. Others: None.
sethostent()endhostent()gethostent() POSIX.1-2001.gethostbyname()gethostbyaddr()h_errno Marked obsolescent in POSIX.1-2001. Removed in POSIX.1-2008, recommending the use ofgetaddrinfo(3) andgetnameinfo(3) instead.
The functionsgethostbyname() andgethostbyaddr() may return pointers to static data, which may be overwritten by later calls. Copying thestruct hostent does not suffice, since it contains pointers; a deep copy is required. In the original BSD implementation thesize argument ofgethostbyname() was anint. The SUSv2 standard is buggy and declares thesize argument ofgethostbyaddr() to be of typesize_t. (That is wrong, because it has to beint, andsize_t is not. POSIX.1-2001 makes itsocklen_t, which is OK.) See alsoaccept(2). The BSD prototype forgethostbyaddr() usesconst char * for the first argument.System V/POSIX extension POSIX requires thegethostent() 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 versiongethostent_r().GNU extensions glibc2 also has agethostbyname2() that works likegethostbyname(), but permits to specify the address family to which the address must belong. glibc2 also has reentrant versionsgethostent_r(),gethostbyaddr_r(),gethostbyname_r(), andgethostbyname2_r(). The caller supplies ahostent structureret which will be filled in on success, and a temporary work bufferbuf of sizebufsize. After the call,result will point to the result on success. In case of an error or if no entry is foundresult 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, ifbuf is too small, the functions will returnERANGE, and the call should be retried with a larger buffer. The global variableh_errno is not modified, but the address of a variable in which to store error numbers is passed inh_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),named(8)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28gethostbyname(3)Pages that refer to this page:getent(1), pmhostname(1), byteorder(3), getaddrinfo(3), gethostid(3), getipnodebyname(3), getnameinfo(3), inet(3), pmdatrace(3), pmgetarchivelabel(3), pmnewcontext(3), pmreconnectcontext(3), rcmd(3), resolver(3), rexec(3), setnetgrent(3), sockaddr(3type), host.conf(5), nsswitch.conf(5), resolv.conf(5), environ(7), hostname(7), ip(7), nscd(8), rpcctl(8), systemd-machined.service(8), systemd-resolved.service(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |