I am trying to use unix sockets to test sending some udp packets to localhost.
It is my understanding that when setting ip address and port in order to send packets, I would fill my sockaddr_in
with values converted to network-byte order.
I am on OSX and I'm astonished that this
printf("ntohl: %d\n", ntohl(4711));
printf("htonl: %d\n", htonl(4711));
printf("plain: %d\n", 4711);
Prints
ntohl: 1729232896
htonl: 1729232896
plain: 4711
So neither function actually returns the plain value. I would have expected to see either the results differ, as x86 is little-endian (afaik), or be identical and the same as the actual number 4711. Clearly I do not understand what htonl
and ntohl
and their variants do. What am I missing?
The relevant code is this:
int main(int argc, char *argv[])
{
if (argc != 4)
{
fprintf(stderr, "%s\n", HELP);
exit(-1);
}
in_addr_t rec_addr = inet_addr(argv[1]); // first arg is '127.0.0.1'
in_port_t rec_port = atoi(argv[2]); // second arg is port number
printf("Address is %s\nPort is %d\n", argv[1], rec_port);
char* inpath = argv[3];
char* file_buf;
unsigned long file_size = readFile(inpath, &file_buf); // I am trying to send a file
if (file_size > 0)
{
struct sockaddr_in dest;
dest.sin_family = AF_INET;
dest.sin_addr.s_addr = rec_addr; // here I would use htons
dest.sin_port = rec_port;
printf("ntohs: %d\n", ntohl(4711));
printf("htons: %d\n", htonl(4711));
printf("plain: %d\n", 4711);
int socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
if (socket_fd != -1)
{
int error;
error = sendto(socket_fd, file_buf, file_size + 1, 0, (struct sockaddr*)&dest, sizeof(dest));
if (error == -1)
fprintf(stderr, "%s\n", strerror(errno));
else printf("Sent %d bytes.\n", error);
}
}
free(file_buf);
return 0;
}
As others have mentioned, both htons
and ntohs
reverse the byte order on a little-endian machine, and are no-ops on big-endian machines.
What wasn't mentioned is that these functions take a 16-bit value and return a 16-bit value. If you want to convert 32-bit values, you want to use htonl
and ntohl
instead.
The names of these functions come from the traditional sizes of certain datatypes. The s
stands for short
while the l
stands for long
. A short
is typically 16-bit while on older systems long
was 32-bit.
In your code, you don't need to call htonl
on rec_addr
, because that value was returned by inet_addr
, and that function returns the address in network byte order.
You do however need to call htons
on rec_port
.