Graphics Programs Reference
In-Depth Information
htons(
short value
)
Host-to-Network Short
Converts a 16-bit integer from the host's byte order to network byte order
ntohl(
long value
)
Network-to-Host Long
Converts a 32-bit integer from network byte order to the host's byte order
ntohs(
long value
)
Network-to-Host Short
Converts a 16-bit integer from network byte order to the host's byte order
For compatibility with all architectures, these conversion functions should
still be used even if the host is using a processor with big-endian byte ordering.
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Internet Address Conversion
When you see 12.110.110.204, you probably recognize this as an Internet
address (IP version 4). This familiar dotted-number notation is a common
way to specify Internet addresses, and there are functions to convert this
notation to and from a 32-bit integer in network byte order. These functions
are defined in the arpa/inet.h include file, and the two most useful con-
version functions are:
inet_aton(char *ascii_addr, struct in_addr *network_addr)
ASCII to Network
This function converts an ASCII string containing an IP address in dotted-
number format into an
in_addr
structure, which, as you remember, only
contains a 32-bit integer representing the IP address in network byte
order.
inet_ntoa(struct in_addr *network_addr)
Network to ASCII
This function converts the other way. It is passed a pointer to an
in_addr
structure containing an IP address, and the function returns a character
pointer to an ASCII string containing the IP address in dotted-number
format. This string is held in a statically allocated memory buffer in the
function, so it can be accessed until the next call to
inet_ntoa()
, when the
string will be overwritten.
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A Simple Server Example
The best way to show how these functions are used is by example. The following
server code listens for TCP connections on port 7890. When a client connects,
it sends the message
Hello, world!
and then receives data until the connection
is closed. This is done using socket functions and structures from the include
files mentioned earlier, so these files are included at the beginning of the
program. A useful memory dump function has been added to hacking.h,
which is shown on the following page.
