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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.