gethostbyname

gethostbyname is a function from the BSD sockets API used to translate a hostname into one or more IPv4 addresses. It is declared in netdb.h and is part of the legacy resolver interface. The function signature is struct hostent *gethostbyname(const char *name);

On success it returns a pointer to a statically allocated hostent structure containing information about the host, and on failure it returns NULL. When NULL is returned, the global h_errno variable is set to indicate the error, such as HOST_NOT_FOUND, NO_ADDRESS, or NO_RECOVERY. The returned data is stored in a static buffer and is overwritten by subsequent calls, which makes the function not thread-safe without external synchronization.

The hostent structure includes several fields: h_name (official name of the host), h_aliases (null-terminated array of alternative names), h_addrtype (address type, typically AF_INET for IPv4), h_length (length of the address in bytes, usually 4), and h_addr_list (NULL-terminated array of pointers to network-order IPv4 addresses). The addresses returned are in network byte order, and the list may contain multiple addresses for a single hostname.

Limitations and modern usage: gethostbyname is largely considered deprecated for new code due to thread safety concerns and limited IPv6 support. It can consult local sources such as /etc/hosts or a DNS resolver, but its behavior varies across systems. Modern programs are encouraged to use getaddrinfo and related functions, which are thread-safe, support both IPv4 and IPv6, and provide more robust error reporting. Some platforms offer gethostbyname2 to specify address family, but getaddrinfo remains the portable, recommended API.