Host Address Functions (The GNU C Library)

#### 16.6.2.3 Host Address Functions ¶

These additional functions for manipulating Internet addresses are declared in the header file `arpa/inet.h`. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. See Byte Order Conversion, for an explanation of network and host byte order.

Function: `int` inet_aton `(const char *name, struct in_addr *addr)` ¶

  Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data and stores it in the `struct in_addr` that addr points to. `inet_aton` returns nonzero if the address is valid, zero if not.

Function: `uint32_t` inet_addr `(const char *name)` ¶

  Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data. If the input is not valid, `inet_addr` returns `INADDR_NONE`. This is an obsolete interface to `inet_aton`, described immediately above. It is obsolete because `INADDR_NONE` is a valid address (255.255.255.255), and `inet_aton` provides a cleaner way to indicate error return.

Function: `uint32_t` inet_network `(const char *name)` ¶

  Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function extracts the network number from the address name, given in the standard numbers-and-dots notation. The returned address is in host order. If the input is not valid, `inet_network` returns `-1`.

  The function works only with traditional IPv4 class A, B and C network types. It doesn’t work with classless addresses and shouldn’t be used anymore.

Function: `char *` inet_ntoa `(struct in_addr addr)` ¶

  Preliminary: | MT-Safe locale | AS-Unsafe race | AC-Safe | See POSIX Safety Concepts.

  This function converts the IPv4 Internet host address addr to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.

  In multi-threaded programs each thread has its own statically-allocated buffer. But still subsequent calls of `inet_ntoa` in the same thread will overwrite the result of the last call.

  Instead of `inet_ntoa` the newer function `inet_ntop` which is described below should be used since it handles both IPv4 and IPv6 addresses.

Function: `struct in_addr` inet_makeaddr `(uint32_t net, uint32_t local)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function makes an IPv4 Internet host address by combining the network number net with the local-address-within-network number local.

Function: `uint32_t` inet_lnaof `(struct in_addr addr)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function returns the local-address-within-network part of the Internet host address addr.

  The function works only with traditional IPv4 class A, B and C network types. It doesn’t work with classless addresses and shouldn’t be used anymore.

Function: `uint32_t` inet_netof `(struct in_addr addr)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function returns the network number part of the Internet host address addr.

  The function works only with traditional IPv4 class A, B and C network types. It doesn’t work with classless addresses and shouldn’t be used anymore.

Function: `int` inet_pton `(int af, const char *cp, void *buf)` ¶

  Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. af should be either `AF_INET` or `AF_INET6`, as appropriate for the type of address being converted. cp is a pointer to the input string, and buf is a pointer to a buffer for the result. It is the caller’s responsibility to make sure the buffer is large enough.

  The return value is `1` on success and `0` if cp does not point to a valid address string for the address family af requested. On failure, the function’s return value is `-1` and `errno` is set accordingly. The following `errno` values are specific to this function:

  `EAFNOSUPPORT`

    The address family requested is neither `AF_INET` nor `AF_INET6`.

Function: `const char *` inet_ntop `(int af, const void *cp, char *buf, socklen_t len)` ¶

  Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. af should be either `AF_INET` or `AF_INET6`, as appropriate. cp is a pointer to the address to be converted. buf should be a pointer to a buffer to hold the result, and len is the length of this buffer.

  The return value is buf on success. On failure, the function’s return value is a null pointer and `errno` is set accordingly. The following `errno` values are specific to this function:

  `EAFNOSUPPORT`

    The address family requested is neither `AF_INET` nor `AF_INET6`.

  `ENOSPC`

    Insufficient space available for the result in the buffer provided.