gethostbyname_r()

Get a network host entry by name

Synopsis:

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

struct hostent *gethostbyname_r(
                   const char * name,
                   struct hostent * result,
                   char * buffer,
                   int bufflen,
                   int * h_errnop );

Arguments:

name: The name of the Internet host whose entry you want to find.
result: A pointer to a struct hostent where the function can store the host entry.
buffer: A pointer to a buffer that the function can use during the operation to store host database entries; buffer should be large enough to hold all of the data associated with the host entry. A 2 KB buffer is usually more than enough; a 256-byte buffer is safe in most cases.
buflen: The length of the area pointed to by buffer.
h_errnop: A pointer to a location where the function can store an herrno value if an error occurs.

Library:

libsocket

Use the -l socket option to qcc to link against this library.

Description:

The gethostbyname_r() function is a thread-safe version of gethostbyname(). This function gets the network host entry for the host specified by name, and stores the entry in the struct hostent pointed to by result.

Returns:

A pointer to result, or NULL if an error occurs.

Errors:

If an error occurs, the int pointed to by h_errnop is set to:

ERANGE: The supplied buffer isn't large enough to store the result.
HOST_NOT_FOUND: Authoritative answer: Unknown host.
NO_ADDRESS: No address associated with name; look for an MX record.
NO_DATA: Valid name, but no data record of the requested type. The name is known to the name server, but has no IP address associated with it—this isn't a temporary error. Another type of request to the name server using this domain name will result in an answer (e.g., a mail-forwarder may be registered for this domain).
NO_RECOVERY: Unknown server error. An unexpected server failure was encountered. This is a nonrecoverable network error.
TRY_AGAIN: Nonauthoritative answer: Host name lookup failure. This is usually a temporary error and means that the local server didn't receive a response from an authoritative server. A retry at some later time may succeed.

Files:

/etc/hosts: Local host database file.
/etc/nsswitch.conf: Name-service switch configuration file.

For information about these files, see the Utilities Reference.

Classification:

Unix


Safety:
Cancellation point	Yes
Interrupt handler	No
Signal handler	No
Thread	Yes

Page updated: March 12, 2026