--- name: "gethostbyname" section: "3" platform: "macos" package: "Libinfo" version: "600" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/gethostbyname" --- # gethostbyname(3) get network host entry ## NAME **gethostbyname**, **gethostbyname2**, **gethostbyaddr**, **gethostent**, **sethostent**, **endhostent**, **herror**, **hstrerror** – get network host entry ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *int* *h\_errno*; *struct* *hostent* *\**\ **gethostbyname**(*const char \*name*); *struct* *hostent* *\**\ **gethostbyname2**(*const char \*name*, *int af*); *struct* *hostent* *\**\ **gethostbyaddr**(*const void \*addr*, *socklen\_t len*, *int type*); *struct* *hostent* *\**\ **gethostent**(*void*); *void*\ **sethostent**(*int stayopen*); *void*\ **endhostent**(*void*); *void*\ **herror**(*const char \*string*); *const* *char* *\**\ **hstrerror**(*int err*); ## DESCRIPTION The [getaddrinfo(3)](/getaddrinfo/3.md) and [getnameinfo(3)](/getnameinfo/3.md) functions are preferred over the **gethostbyname**(), **gethostbyname2**(), and **gethostbyaddr**() functions. The **gethostbyname**(), **gethostbyname2**() and **gethostbyaddr**() functions each return a pointer to an object with the following structure describing an internet host referenced by name or by address, respectively. The *name* argument passed to **gethostbyname**() or **gethostbyname2**() should point to a NUL-terminated hostname. The *addr* argument passed to **gethostbyaddr**() should point to an address which is *len* bytes long, in binary form (i.e., not an IP address in human readable ASCII form). The *type* argument specifies the address family (e.g. AF\_INET, AF\_INET6, etc.) of this address. The structure returned contains information obtained from [mDNSResponder(8)](/mDNSResponder/8.md), including records in */etc/hosts*. ``` 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 from name server */ }; #define h_addr h_addr_list[0] /* address, for backward compatibility */ ``` The members of this structure are: - *h\_name* Official name of the host. - *h\_aliases* A NULL-terminated array of alternate names for the host. - *h\_addrtype* The type of address being returned; usually AF\_INET. - *h\_length* The length, in bytes, of the address. - *h\_addr\_list* A NULL-terminated array of network addresses for the host. Host addresses are returned in network byte order. - *h\_addr* The first address in *h\_addr\_list*; this is for backward compatibility. When using the nameserver, **gethostbyname**() and **gethostbyname2**() will search for the named host in the current domain and its parents unless the name ends in a dot. The **gethostbyname2**() function is an evolution of **gethostbyname**() which is intended to allow lookups in address families other than AF\_INET, for example AF\_INET6. The **herror**() function writes a message to the diagnostic output consisting of the string argument *string*, the constant string ": ", and a message corresponding to the value of *h\_errno*. The **hstrerror**() function returns a string which is the message text corresponding to the value of the *err* argument. Historically, passing a host's own hostname to **gethostbyname**() or **gethostbyname2**() has been a popular technique for determining that host's IP address(es), but this is fragile, and doesn't work reliably in all cases. The appropriate way for software to discover the IP address(es) of the host it is running on is to use [getifaddrs(3)](/getifaddrs/3.md). ## FILES - */etc/hosts* - */etc/resolv.conf* ## EXAMPLES Print out the hostname associated with a specific IP address: ``` const char *ipstr = "127.0.0.1"; struct in_addr ip; struct hostent *hp; if (!inet_aton(ipstr, &ip)) errx(1, "can't parse IP address %s", ipstr); if ((hp = gethostbyaddr((const void *)&ip, sizeof ip, AF_INET)) == NULL) errx(1, "no name associated with %s", ipstr); printf("name associated with %s is %s\n", ipstr, hp->h_name); ``` ## DIAGNOSTICS Error return status from **gethostbyname**(), **gethostbyname2**() and **gethostbyaddr**() is indicated by return of a NULL pointer. The integer *h\_errno* may then be checked to see whether this is a temporary failure or an invalid or unknown host. The routine **herror**() can be used to print an error message describing the failure. If its argument *string* is non-NULL, it is printed, followed by a colon and a space. The error message is printed with a trailing newline. The variable *h\_errno* can have the following values: - HOST\_NOT\_FOUND No such host is known. - TRY\_AGAIN This is usually a temporary error and means that the local server did not receive a response from an authoritative server. A retry at some later time may succeed. - NO\_RECOVERY Some unexpected server failure was encountered. This is a non-recoverable error. - NO\_DATA The requested name is valid but does not have an IP address; this is not a temporary error. This means that the name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name will result in an answer; for example, a mail-forwarder may be registered for this domain. ## SEE ALSO [getaddrinfo(3)](/getaddrinfo/3.md), [getnameinfo(3)](/getnameinfo/3.md), [inet\_aton(3)](/inet_aton/3.md), [resolver(3)](/resolver/3.md), [hosts(5)](/hosts/5.md), [hostname(7)](/hostname/7.md), [mDNSResponder(8)](/mDNSResponder/8.md) ## CAVEAT The **gethostent**() function is defined, and **sethostent**() and **endhostent**() are redefined, when Standard C Library (libc, -lc) is built to use only the routines to lookup in */etc/hosts* and not the name server. The **gethostent**() function reads the next line of */etc/hosts*, opening the file if necessary. The **sethostent**() function opens and/or rewinds the file */etc/hosts*. If the *stayopen* argument is non-zero, the file will not be closed after each call to **gethostbyname**(), **gethostbyname2**() or **gethostbyaddr**(). The **endhostent**() function closes the file. ## HISTORY The **herror**() function appeared in 4.3BSD. The **endhostent**(), **gethostbyaddr**(), **gethostbyname**(), **gethostent**(), and **sethostent**() functions appeared in 4.2BSD. The **gethostbyname2**() function first appeared in BIND version 4.9.4. ## BUGS These functions use a thread-specific data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it. Though these functions are thread-safe, still it is recommended to use the [getaddrinfo(3)](/getaddrinfo/3.md) family of functions, instead. Only the Internet address format is currently understood.