The first function that you will learn about is an older function, which should probably no longer be used in new code. However, you will find it in a lot of existing network code, and so you should become familiar with it and know its limitations. The synopsis for inet_addr(3) is as follows:
unsigned long inet_addr(const char *string);
This function accepts an input C string argument string and parses the dotted-quad notation into a 32-bit Internet address value. The 32-bit value returned is in network byte order. If the input argument string does not represent a valid dotted-quad address, the value INADDR_NONE is returned. Any other returned value represents the converted value.
The 32-bit value returned by inet_addr(3) is in network byte order. Do not use htonl(3) on the returned value, because it is already in network byte order.
The inet_addr(3) does not establish a reason code in errno when
INADDR_NONE is returned. So, do not test errno or report it when an error indication is returned by this function. The program shown below is an example of how you would use this function. To compile and run the program, perform the following:
gcc -c -D_GNU_SOURCE -Wall inetaddr.c
gcc inetaddr.o -o inetaddr
The program below when it is run, converts a C string constant containing an IP number into a network sequenced 32-bit IP address. This value is then placed into an AF_INET socket address
and bound to the socket.