Server-client in C: Weird behaviour when sending bytes - c

Question

I'm trying to write a server-client program in C wherein the client will send a bunch of messages in the form of 5 bytes: the first byte will contain a command, and the next four will contain a key. It looks something like this:
rc = write(sockfd, &op, 1);
if (rc != 1)
{
printf("error! write() failed: %s\n", strerror(errno));
break;
}
uint32_t net_num = htonl(num);
int nsent = 0;
while (nsent < 4)
{
rc = write(sockfd, &net_num + nsent, 4 - nsent);
if (rc <= 0)
{
printf("error! write() failed: %s\n", strerror(errno));
break;
}
nsent += rc;
}
if (rc <= 0)
break;
}
On the receiving end, I have:
while((bytes = recv(socket,buffer,5,0)) > 0)
{
//printf("%d\t%d\t%d\t%d\t%d\n",(int)buffer[0],(int)buffer[1], (int)buffer[2],(int)buffer[3],(int)buffer[4]);
key = ((buffer[4] << 24) | (buffer[3] << 16) | (buffer[2] << 8) | (buffer[1]));
if((int)buffer[0] == 0)
{
do command 0, etc...
The problem I'm having is that I cant get the key. I've tried switching the order of the shifts, but all I'm getting are numbers that don't match the keys that the client is sending. I'm at a loss.
Even stranger, is that if I compile the server without the print under the while, I get seg-faulted. If I uncomment the printf, it works fine. This seems super strange.
Does anyone know what might be causing this?
Thanks in advance.

Answer 1

rc = write(sockfd, &net_num + nsent, 4 - nsent); is wrong, &net_num is a pointer to a 32 bits object, so &net_num+1 would point to the next 32bits object. That is beyond the object. You could cast to a char pointer, or copy to a small char buffer before sending, like below:
uint32_t net_num = htonl(num);
char buff[sizeof net_num];
memcpy (buff, &net_num, sizeof buff);
int nsent;
for nsent=0; nsent < sizeof buff; nsent += rc;)
{
rc = write(sockfd, buff + nsent, 4 - nsent);
if (rc == -1 && errno == EAGAIN) { rc=0; continue; }
if (rc <= 0)
{
printf("error! write() failed: %s\n", strerror(errno));
break;
}
}