• Advertisement
Sign in to follow this  

memcpy / sendto issue

This topic is 4758 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

I'm attemping to send some header information across the network using UDP sockets. The client generates the following message and sends it the server.
char *msg;
u_short length = htons(514);
u_long source = htonl(ip_address);
u_long dest = htonl(ip_address);
u_char id = 50;
u_short offset = htons(500);
u_char flag = 49;

msg = (char *)malloc(sizeof(char)*14);
memset(msg, 0, 14);  //zero out the memory
memcpy(msg, &length, 2);  //add length
memcpy(msg+2, &source, 4);  //add source
memcpy(msg+6, &destination, 4);  //add destination
memcpy(msg+10, &id, 1);  //add id
memcpy(msg+11, &offset, 2);  //add offset
memcpy(msg+13, &flag, 1); //add flag

sendto(sock, msg, strlen(msg), 0, (struct sockaddr *)&dest, (socklen_t)sizeof(dest));

The server receives the message and parses the string.
u_short size;
u_long source;
u_long dest;
u_char id;
u_short offset;
u_char flag;

recvfrom(sock, msg, sizeof(msg), 0, (struct sockaddr *)&from, (socklen_t *)&fsize);

memcpy(&size, msg, 2);
memcpy(&source, msg+2, 4);
memcpy(&dest, msg+6, 4);
memcpy(&id, msg+10, 1);
memcpy(&offset, msg+11, 2);
memcpy(&flag, msg+13, 1);

This works as expected unless I set one of the u_shorts(id, length) to a value less than 256. Is there anyway I can make this work with values between 0 and 255?

Share this post


Link to post
Share on other sites
Advertisement
Quote:


strlen(msg)



You formed "msg" using memcpy(), not strcpy(). It's a binary blob of data, not a C string. Thus, you should send "msg" using the length of the msg buffer, not using the strlen() of it. Just put a breakpoint at the send line, and examine the memory at "msg" and you'll understand why this is.

Share this post


Link to post
Share on other sites
Thanks for taking the time to point out my mistake hplus0603. Everything works fine now.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement