c socket send and receive an int instead of a char buffer

Question

1: The server copy the file size into the buffer and send it:

  snprintf(t_buf, 255, "%" PRIu32, fsize);
  if(send(f_sockd, t_buf, sizeof(t_buf), 0) < 0){
    perror("error on sending file size\n");
    onexit(f_sockd, m_sockd, 0, 2);
  }

2: The client receives the file size and put it into fsize:

  if(recv(f_sockd, t_buf, sizeof(t_buf), 0) < 0){
    perror("error on receiving file size");
    onexit(f_sockd, 0 ,0 ,1);
  }
  fsize = atoi(t_buf);

----------------- The code above makes my program working perfectly!

The problem happens if i write this code instead of the previous one:
1: The server send fsize:

  if(send(f_sockd, &fsize, sizeof(fsize), 0) < 0){
    perror("error on sending file size\n");
    onexit(f_sockd, m_sockd, 0, 2);
  }

2: The client receives fsize:

  if(recv(f_sockd, &fsize, sizeof(fsize), 0) < 0){
    perror("error on receiving file size");
    onexit(f_sockd, 0, 0, 1);
  }

Where uint32_t fsize; and char t_buf[256];.
The problem is that with the first method all work but with the second method the client doesn't receive all file but only a piece of it. What is wrong with this code?
Thanks!

possible duplicate of [Handling Partial return from recv() TCP in C](http://stackoverflow.com/questions/1386142/handling-partial-return-from-recv-tcp-in-c) — James M, Aug 29 '12 at 10:30
Are the client and server on the same machine? What does _only a piece of it_ mean? — hmjd, Aug 29 '12 at 10:32
Are you saying that the value of fsize is different or that subsequent reads behave differently? — Klas Lindbäck, Aug 29 '12 at 10:37
with the first method i receive the file, with the second nothing is received. Also the "printf fsize" is a strange number with the 2nd method! — polslinux, Aug 29 '12 at 10:39

James M · Answer 1 · 2012-08-29T10:46:56.297

recv(2) doesn't necessarily fill the complete output buffer - it might return fewer bytes depending on how much data is available:

The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.

The return value (when > 0) will be the number of bytes received, so you can call it in a loop if you want to be sure to receive everything.

Alternatively, you could pass the MSG_WAITALL flag:

This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned.

So in your case, you could do something like:

ssize_t bytes = recv(f_sockd, &fsize, sizeof(fsize), MSG_WAITALL);

if(bytes == sizeof(fsize))
{
    /* received everything */
}
else
{
    /* something went wrong */
}

the server sent 20 bytes and the client receives 1801745249 O.o why?? :( — polslinux, Aug 29 '12 at 12:51

score 0 · Answer 2 · answered Aug 29 '12 at 10:34

0

Hard to tell without more data.

Could be endianness. You should print out the value of fsize.

Could also be partial return from recv(), as was mentioned in a comment.

answered Aug 29 '12 at 10:34

unwind

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little endian, i'm running server and client on the same machine! – polslinux Aug 29 '12 at 10:35

c socket send and receive an int instead of a char buffer

2 Answers2