netdb.h(7POSIX)

📦 version 2017a, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu resolute, ubuntu stonking

🦋 netdb.h — definitions for network database operations 🦋

okay bestie, so this one is from the POSIX Programmer's Manual — the manual page is literally netdb.h(7POSIX). It's the header that holds all the network database ✨lore✨. Sit down, hydrate, we're doing this.

🌸 PROLOG 🌸

This manual page is part of the POSIX Programmer's Manual. Heads up though: the Linux implementation of this interface may differ (so go check the corresponding Linux manual page for details on how Linux actually behaves), or honestly the interface may not even be implemented on Linux at all. 💀

🎀 NAME 🎀

netdb.h — definitions for network database operations

✨ SYNOPSIS ✨

#include <netdb.h>

that's it. one include. iconic. 💅

💖 DESCRIPTION 💖

The <netdb.h> header may define the in_port_t type and the in_addr_t type, exactly as described in <netinet/in.h>.

The <netdb.h> header shall define the hostent structure, and it shall include at least these 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, at least these 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, at least these 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, at least these 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, and its value is the highest reserved Internet port number. 🔒

🌈 Address Information Structure 🌈

The <netdb.h> header shall define the addrinfo structure, at least these 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 these symbolic constants, which 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.

And the <netdb.h> header shall define these symbolic constants too, also bitwise-distinct integer constants, but this batch is for 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 these 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>.

Also, ngl, including the <netdb.h> header may also make visible all symbols from <netinet/in.h>, <sys/socket.h>, and <inttypes.h>. surprise package deal. 🎁

The following sections are informative. (translation: the real spec part is done, this is the bonus commentary track 🎧)

💜 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()

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 .

and that's the whole page, bestie 🫶 this one's brought to you by IEEE/The Open Group, 2017 — stan them for real, they've been holding the network standards down since forever. mwah. 💋 stay resolving ✨

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