Send int over socket in C/C++

Question

I have troubles with sending an array of ints over a socket. the code looks like this

Program 1: (running on windows)

int bmp_info_buff[3];

/* connecting and others */

/* Send informations about bitmap */
send(my_socket, (char*)bmp_info_buff, 3, 0);

Program 2: (running on neutrino)

/*buff to store bitmap information size, with, length */
int bmp_info_buff[3];

/* stuff */

/* Read informations about bitmap */
recv(my_connection, bmp_info_buff, 3, NULL);
printf("Size of bitmap: %d\nwidth: %d\nheight: %d\n", bmp_info_buff[0], bmp_info_buff[1], bmp_info_buff[2]);

It should print Size of bitmap: 64
width: 8
height: 8

Size of bitmap: 64
width: 6
height: 4096
What do I do wrong?

Noontime

Answer 1

When you send the bmp_info_buff array as char array, the size of bmp_info_buff is not 3 but is 3 * sizeof(int)

The same for recv

Replace

send(my_socket, (char*)bmp_info_buff, 3, 0);
recv(my_connection, bmp_info_buff, 3, NULL);

by

send(my_socket, (char*)bmp_info_buff, 3*sizeof(int), 0);
recv(my_connection, bmp_info_buff, 3*sizeof(int), NULL);

Anton

But what about byte ordering issues? Writing ints from a big-endian machine to a little-endian machine requires usage of htonl, ntohl, etc. – Brannon

In addition, make sure you check the return values from send and recv. They're the only way to be sure what really happened. – Waxy

Answer 2

The size argument to send() and recv() is in bytes, not ints. You're sending/receiving too little data.

You need:

send(my_socket, bmp_info_buff, sizeof bmp_info_buff, 0);

and

recv(my_connection, bmp_info_buff, sizeof bmp_info_buff, 0);

Also note:

• This makes your code sensitive to byte endianness issues.
• The size of int is not the same on all platforms, you need to consider this, too.
• No need to cast the pointer argument, it's void *.
• You should also add code to check the return values, I/O can fail!
• The last argument to recv() shouldn't be NULL as in your code, it's a flags integer just as in send().

Vulcan

+1. But don't people know about the sizeof keyword or what? – Hellbox

Also, there is an issue about possible 32/64bit mismatch to add. – Hadji