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The GNU C Library

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16.6.2.2 Host Address Data Type

IPv4 Internet host addresses are represented in some contexts as integers (type uint32_t). In other contexts, the integer is packaged inside a structure of type struct in_addr. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.

You will find older code that uses unsigned long int for IPv4 Internet host addresses instead of uint32_t or struct in_addr. Historically unsigned long int was a 32-bit number but with 64-bit machines this has changed. Using unsigned long int might break the code if it is used on machines where this type doesn't have 32 bits. uint32_t is specified by Unix98 and guaranteed to have 32 bits.

IPv6 Internet host addresses have 128 bits and are packaged inside a structure of type struct in6_addr.

The following basic definitions for Internet addresses are declared in the header file `netinet/in.h':

Data Type: struct in_addr
This data type is used in certain contexts to contain an IPv4 Internet host address. It has just one field, named s_addr, which records the host address number as an uint32_t.

Macro: uint32_t INADDR_LOOPBACK
You can use this constant to stand for "the address of this machine," instead of finding its actual address. It is the IPv4 Internet address `127.0.0.1', which is usually called `localhost'. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements INADDR_LOOPBACK specially, avoiding any network traffic for the case of one machine talking to itself.

Macro: uint32_t INADDR_ANY
You can use this constant to stand for "any incoming address" when binding to an address. See section 16.3.2 Setting the Address of a Socket. This is the usual address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.

Macro: uint32_t INADDR_BROADCAST
This constant is the address you use to send a broadcast message.

Macro: uint32_t INADDR_NONE
This constant is returned by some functions to indicate an error.

Data Type: struct in6_addr
This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways.

Constant: struct in6_addr in6addr_loopback
This constant is the IPv6 address `::1', the loopback address. See above for a description of what this means. The macro IN6ADDR_LOOPBACK_INIT is provided to allow you to initialize your own variables to this value.

Constant: struct in6_addr in6addr_any
This constant is the IPv6 address `::', the unspecified address. See above for a description of what this means. The macro IN6ADDR_ANY_INIT is provided to allow you to initialize your own variables to this value.


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