socket dont get/set values (c problem)

Hi i have used in the past many times sockets

My problem that i have for first time is that the recv seems to not set the correct values to the parameters

extern union qdisc qdisc_stats[10];
............
if ((n=recvfrom(sock,qdisc_stats,sizeof(qdisc_stats[10]),0,(struct sockaddr *) &client, &client_len)) <0){

After this call the struct has the same values... like not ever getting the values


I send the values from another pc



extern union qdisc reconfigure_qdisc[10];

 printf("I send for example %s \n",reconfigure_qdisc[0].htbstats.rate);
 if (sendto(sock,reconfigure_qdisc,sizeof(reconfigure_qdisc), 0,(struct sockaddr *) &server, server_len) <0){

omg i am searching for this thing all the night... i want so sleep

Try creating your own dummy "recvmsg()" and see what happens:

cut/pasted from your example code (in a separate thread):

#include <stdio.h>
#include <string.h>

/*
 * Here's the struct we're trying to read into
 */
typedef struct htb
{
  char *parameter;
} 
  htb;

union qdisc 
{
  int i;
  struct htb htb;
}
  qdisc_stats[10];

/*
 * Here's a substitute for the actual "read" procedure
 */
int
recvfrom2 (int  s, void *buf, size_t len, int flags, void *from, size_t *fromlen)
{
  strcpy ((char *)buf, "ABC");
  return 4;  /* 'A', 'B', 'C' and '\0' */
}

/*
 * And here's our test driver
 */
int
main(int argc, char *argv[])
{
  int n = -1, sock = 0, fromlen = 0;

  /* Initialize the data */
  memset (&qdisc_stats, sizeof qdisc_stats, 0xaa);
  qdisc_stats[0].htb.parameter="foo";
  printf ("Before: %s %d\n", 
    qdisc_stats[0].htb.parameter, qdisc_stats[0].i);

  /* Attempt read */
  n= recvfrom2 (sock, qdisc_stats, sizeof(qdisc_stats[10]), 0, 0, (size_t *)&fromlen);
  printf ("After: n= %d; %s %d\n", 
    n, qdisc_stats[0].htb.parameter, qdisc_stats[0].i);

  return 0;
}

I've cut and pasted the "qdisc_stats" data definition from another thread you posted.

If you run this under a debugger, you'll see that it should probably *crash*.

Please consider at least *borrowing* a copy of K&R from a library until I can get you a copy. Please?

PS:
"sizeof (qdisk_stats[10])" for a 10-element array is syntactically legal, it won't crash ... but it's
extremely poor form. Stylistically, it would be *much* better to write "sizeof (qdisk_stats[0])".
And to declare "main()" either "int" (the standard definition), or "void" (if you really don't want
to return anything. But I think you should always return at least a "0").

IMHO.

PPS:
The goal here is to compare how you *meant* to pass your struct to recvfrom() (presumably, you mean to pass the address of some "parameter" string) with the buffer that you *actually* passed into recvfrom().

K&R would be of help. The tutorial on arrays and pointers would be of help. a debugger like gdb (Linux) or Visual Studio (Windows) would be of *enormous* help.

PPS:
One final question: how big can your parameter strings be? And where are you allocating the space for them? Again: K&R discusses all of these issues.

Thx paul basically i am very tired
i have changed the line to the send to
if (sendto(sock,reconfigure_qdisc,sizeof(reconfigure_qdisc[0]), 0,(struct sockaddr *) &server, server_len) <0){

and now it works


The line before was
if (sendto(sock,reconfigure_qdisc,sizeof(reconfigure_qdisc), 0,(struct sockaddr *) &server, server_len) <0){


Can u explain me whats the difference?

Hi -

int
main(int argc, char *argv[])
{
  int n = -1, sock = 0, fromlen = 0;

  /* SAMPLE OUTPUT:
   * sizeof qdisc_stats: 40, sizeof qdisc_stats[0]: 4
   */
  printf ("sizeof qdisc_stats: %d, sizeof qdisc_stats[0]: %d\n",
     sizeof qdisc_stats, sizeof qdisc_stats[0]);

"sizeof qdisc_stats[0]" gives the size of one "qdisk" element (which happens to be a union of "struct htb" and "int", which happens to be 4 bytes). "sizeof qdisc_stats", on the other hand, gives the size of the whole thing (which happens to be 10 "qdisc" elements, with no padding and no extra space). One is "4", the other is "40".

Now: what's an "htb"? It's a struct that contains a pointer to a string. *Not* a string, but a *pointer* to a string. I would humbly suggest this might be "better":

#define MAX_STRING 4
typedef struct htb
{
  char parameter[MAX_STRING];
} 
  htb;

IMHO ...

Originally posted by alaios Thx paul basically i am very tired i have changed the line to the send to if (sendto(sock,reconfigure_qdisc,sizeof(reconfigure_qdisc[0]), 0,(struct sockaddr *) &server, server_len) <0){ and now it works The line before was if (sendto(sock,reconfigure_qdisc,sizeof(reconfigure_qdisc), 0,(struct sockaddr *) &server, server_len) <0){ Can u explain me whats the difference?

The difference is in the use of the name of an array. The sizeof() an array[0] will be the size of one element, the sizeof() the name of the array will the size of its pointer. Confusing, but logical, in a K&R way.

thx a lot but if i receive a pointer how i can calculate the correct size to feed the function correctly?