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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
netdb.h — definitions for network database operations
SYNOPSIS
#include <netdb.h>
DESCRIPTION
The <netdb.h> header may define the in_port_t type and the in_addr_t type as described in <netinet/in.h>.
The <netdb.h> header shall define the hostent structure, which shall include at least the following
members:
char *h_name Official name of the host.
char **h_aliases A pointer to an array of pointers to
alternative host names, terminated by a
null pointer.
int h_addrtype Address type.
int h_length The length, in bytes, of the address.
char **h_addr_list A pointer to an array of pointers to network
addresses (in network byte order) for the host,
terminated by a null pointer.
The <netdb.h> header shall define the netent structure, which shall include at least the following
members:
char *n_name Official, fully-qualified (including the
domain) name of the host.
char **n_aliases A pointer to an array of pointers to
alternative network names, terminated by a
null pointer.
int n_addrtype The address type of the network.
uint32_t n_net The network number, in host byte order.
The <netdb.h> header shall define the uint32_t type as described in <inttypes.h>.
The <netdb.h> header shall define the protoent structure, which shall include at least the following
members:
char *p_name Official name of the protocol.
char **p_aliases A pointer to an array of pointers to
alternative protocol names, terminated by
a null pointer.
int p_proto The protocol number.
The <netdb.h> header shall define the servent structure, which shall include at least the following
members:
char *s_name Official name of the service.
char **s_aliases A pointer to an array of pointers to
alternative service names, terminated by
a null pointer.
int s_port A value which, when converted to uint16_t,
yields the port number in network byte order
at which the service resides.
char *s_proto The name of the protocol to use when
contacting the service.
The <netdb.h> header shall define the IPPORT_RESERVED symbolic constant with the value of the highest
reserved Internet port number.
Address Information Structure
The <netdb.h> header shall define the addrinfo structure, which shall include at least the following
members:
int ai_flags Input flags.
int ai_family Address family of socket.
int ai_socktype Socket type.
int ai_protocol Protocol of socket.
socklen_t ai_addrlen Length of socket address.
struct sockaddr *ai_addr Socket address of socket.
char *ai_canonname Canonical name of service location.
struct addrinfo *ai_next Pointer to next in list.
The <netdb.h> header shall define the following symbolic constants that evaluate to bitwise-distinct
integer constants for use in the flags field of the addrinfo structure:
AI_PASSIVE Socket address is intended for bind().
AI_CANONNAME Request for canonical name.
AI_NUMERICHOST
Return numeric host address as name.
AI_NUMERICSERV
Inhibit service name resolution.
AI_V4MAPPED If no IPv6 addresses are found, query for IPv4 addresses and return them to the caller as
IPv4-mapped IPv6 addresses.
AI_ALL Query for both IPv4 and IPv6 addresses.
AI_ADDRCONFIG Query for IPv4 addresses only when an IPv4 address is configured; query for IPv6 addresses
only when an IPv6 address is configured.
The <netdb.h> header shall define the following symbolic constants that evaluate to bitwise-distinct
integer constants for use in the flags argument to getnameinfo():
NI_NOFQDN Only the nodename portion of the FQDN is returned for local hosts.
NI_NUMERICHOST
The numeric form of the node's address is returned instead of its name.
NI_NAMEREQD Return an error if the node's name cannot be located in the database.
NI_NUMERICSERV
The numeric form of the service address is returned instead of its name.
NI_NUMERICSCOPE
For IPv6 addresses, the numeric form of the scope identifier is returned instead of its
name.
NI_DGRAM Indicates that the service is a datagram service (SOCK_DGRAM).
Address Information Errors
The <netdb.h> header shall define the following symbolic constants for use as error values for
getaddrinfo() and getnameinfo(). The values shall be suitable for use in #if preprocessing directives.
EAI_AGAIN The name could not be resolved at this time. Future attempts may succeed.
EAI_BADFLAGS The flags had an invalid value.
EAI_FAIL A non-recoverable error occurred.
EAI_FAMILY The address family was not recognized or the address length was invalid for the specified
family.
EAI_MEMORY There was a memory allocation failure.
EAI_NONAME The name does not resolve for the supplied parameters.
NI_NAMEREQD is set and the host's name cannot be located, or both nodename and servname
were null.
EAI_SERVICE The service passed was not recognized for the specified socket type.
EAI_SOCKTYPE The intended socket type was not recognized.
EAI_SYSTEM A system error occurred. The error code can be found in errno.
EAI_OVERFLOW An argument buffer overflowed.
The following shall be declared as functions and may also be defined as macros. Function prototypes shall
be provided.
void endhostent(void);
void endnetent(void);
void endprotoent(void);
void endservent(void);
void freeaddrinfo(struct addrinfo *);
const char *gai_strerror(int);
int getaddrinfo(const char *restrict, const char *restrict,
const struct addrinfo *restrict,
struct addrinfo **restrict);
struct hostent *gethostent(void);
int getnameinfo(const struct sockaddr *restrict, socklen_t,
char *restrict, socklen_t, char *restrict,
socklen_t, int);
struct netent *getnetbyaddr(uint32_t, int);
struct netent *getnetbyname(const char *);
struct netent *getnetent(void);
struct protoent *getprotobyname(const char *);
struct protoent *getprotobynumber(int);
struct protoent *getprotoent(void);
struct servent *getservbyname(const char *, const char *);
struct servent *getservbyport(int, const char *);
struct servent *getservent(void);
void sethostent(int);
void setnetent(int);
void setprotoent(int);
void setservent(int);
The <netdb.h> header shall define the socklen_t type through typedef, as described in <sys/socket.h>.
Inclusion of the <netdb.h> header may also make visible all symbols from <netinet/in.h>, <sys/socket.h>,
and <inttypes.h>.
The following sections are informative.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
<inttypes.h>, <netinet_in.h>, <sys_socket.h>
The System Interfaces volume of POSIX.1‐2017, bind(), endhostent(), endnetent(), endprotoent(),
endservent(), freeaddrinfo(), gai_strerror(), getnameinfo()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .