Byte Order (The GNU C Library)

#### 16.6.5 Byte Order Conversion ¶

Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called “big-endian” order), and others put it last (“little-endian” order).

So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as network byte order.

When establishing an Internet socket connection, you must make sure that the data in the `sin_port` and `sin_addr` members of the `sockaddr_in` structure are represented in network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don’t do this, your program may fail when running on or talking to other kinds of machines.

If you use `getservbyname` and `gethostbyname` or `inet_addr` to get the port number and host address, the values are already in network byte order, and you can copy them directly into the `sockaddr_in` structure.

Otherwise, you have to convert the values explicitly. Use `htons` and `ntohs` to convert values for the `sin_port` member. Use `htonl` and `ntohl` to convert IPv4 addresses for the `sin_addr` member. (Remember, `struct in_addr` is equivalent to `uint32_t`.) These functions are declared in `netinet/in.h`.

Function: `uint16_t` htons `(uint16_t hostshort)` ¶

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

  This function converts the `uint16_t` integer hostshort from host byte order to network byte order.

Function: `uint16_t` ntohs `(uint16_t netshort)` ¶

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

  This function converts the `uint16_t` integer netshort from network byte order to host byte order.

Function: `uint32_t` htonl `(uint32_t hostlong)` ¶

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

  This function converts the `uint32_t` integer hostlong from host byte order to network byte order.

  This is used for IPv4 Internet addresses.

Function: `uint32_t` ntohl `(uint32_t netlong)` ¶

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

  This function converts the `uint32_t` integer netlong from network byte order to host byte order.

  This is used for IPv4 Internet addresses.