send and recv are not consistent

Question

Ok so guys i've been working like 2 days straight, searching for bugs and looking for last years solutions and other software but found

My problem is that i need to program a fairly easy server client application in which both take responses and reply to them. I wrote smaller programs to just demonstrate my problem.

And before you complain about the lack of completeness with send and recv to be checked not be < 0. I know that i gotta check that.

common.h is just a header for all the inputs and static variables.

// guard block:
#ifndef COMMON_H
#define COMMON_H

// default hostname and port:
#define DEFAULT_HOST    "localhost"
#define DEFAULT_PORT    "1280"

#include 
#include  

// IO, C standard library, POSIX API, data types:
#include 
#include 
#include 
#include 
#include 
#include 
#include 

// Sockets, TCP, ... :
#include 
#include 
#include 
#include 

// Assertions, errors, signals:
#include 
#include 
#include 

static struct addrinfo *ai = NULL;      // stores address information
static int sockfd = -1;                 // socket file descriptor
static int connfd = -1;                 // connection file descriptor

void terminate(int sig);
void init_signal_handler(void);
void free_ressources(void);
void error(char* format, ...);

void terminate(int sig) {
  error("Caught signal %d. Terminating...
",sig);
}

void init_signal_handler(void) {
  struct sigaction sa;
  sa.sa_handler = terminate;
  if (-1 == sigemptyset(&(sa.sa_mask))) {
    error("sigemptyset()");
  }
  if (-1 == sigaction(SIGINT, &sa, NULL)) {
    error("sigaction()");
  }
  if (-1 == sigaction(SIGTERM, &sa, NULL)) {
    error("sigaction()");
  }
}

void error(char *format, ...) {
  va_list arg;

  va_start (arg, format);
  (void) vfprintf (stderr, format, arg);
  va_end (arg);

  free_ressources();

  exit(EXIT_FAILURE);
}

void free_ressources(void) {
  (void) printf("Freeing all ressources...
");
  if(sockfd != -1) close(sockfd);
  if(connfd != -1) close(connfd);
  freeaddrinfo(ai);
}
#endif // COMMON_H

So here is the client

#include "common.h"

static char *port = DEFAULT_PORT;
static char *host = DEFAULT_HOST;

int main(int argc, char *argv[]) {
  int s;
  struct addrinfo hints, *rp;
  memset(&ai, 0, sizeof(hints));
  hints.ai_family = AF_INET;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags = AI_PASSIVE;

  s = getaddrinfo(host, port, &hints, &ai);
  if(s != 0) error("Couldn't get address info...
");

  for (rp=ai; rp != NULL; rp=rp->ai_next) {
      sockfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
      if(sockfd == -1) continue;
      if((connfd = connect(sockfd, rp->ai_addr, rp->ai_addrlen)) != -1) break;
      close(sockfd);
  }

  if(rp == NULL) error("Couldn't connect...
");

  uint16_t msg = 0xF0F0;
  uint8_t msg_r = 0x00;
  for(int i = 0; i < 10; ++i) {
    (void) fprintf(stdout, "Sending message %#X
", msg);
    send(sockfd, &msg, 2, MSG_CONFIRM);
    recv(connfd, &msg_r, 1, MSG_CONFIRM);
    (void) fprintf(stdout, "Recieved from Server %#X
", msg_r);
    msg+=1;
  }
  free_ressources();
}

And here is the server

#include "common.h"

static char *port = DEFAULT_PORT;

int main(int argc, char *argv[]) {
  struct addrinfo hints;
  memset(&ai, 0, sizeof(hints));
  hints.ai_family = AF_INET;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags = AI_PASSIVE;

  int res = getaddrinfo(NULL, port, &hints, &ai);
  if(res != 0) error("Failed to get addr info: %s
", gai_strerror(res));

  sockfd = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
  if(sockfd == -1) error("Couldn't create a socket
");

  int val = 1;
  res = setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof val);
  if(res == -1) error("Socket options couldn't be set
");

  res = bind(sockfd, ai->ai_addr, ai->ai_addrlen);
  if(res == -1) error("Socket binding failed
");

  res = listen(sockfd, 1);
  if(res == -1) error("Listener setup failed
");

  connfd = accept(sockfd, NULL, NULL);
  if(connfd == -1) error("Connect failed
");

  uint8_t msg_s = 0x00;
  uint16_t msg = 0x0000;
  for(int i = 0; i < 10; ++i) {
    msg_s+=1;
    recv(connfd, &msg, 2, MSG_CONFIRM);   
    (void) fprintf(stdout, "Recieved from Client %#X
", msg);
    (void) fprintf(stdout, "Sending %#X
", msg_s);
    send(sockfd, &msg_s, 1, MSG_CONFIRM);
  }
  free_ressources();
}

Ok so i have several problems with this.

If i do a server client application in one direction, like only client sends and only server recieves, everything goes smooth and expected as always. However things change when i mix it up like here. I understand that TCP is a stream protocol without fixed "package" amounts, but i also can understand how to implement that with just needing 2 bytes max. I might try to just recieve 1 byte at a time and do that in a loop if i need 2, but why though? Why is everything fine at onelane mode and goes hell in mixed?
I tried to use recv and send accordingly to its man pages so use them with the socket. However i don't know why, but whenever i try to use recv with the sock_fd it just won't work, but conn_fd on the otherside does. But i see everywhere that i clearly should use sock_fd.

This is my output when i mix up the recv and send

OUTPUT FROM SERVER

Recieved from Client 0XF0F0
Sending 0X1

OUTPUT FROM CLIENT

Sending message 0XF0F0
Recieved from Server 0
Sending message 0XF0F1
Recieved from Server 0
Sending message 0XF0F2
Recieved from Server 0
Sending message 0XF0F3
Recieved from Server 0
Sending message 0XF0F4
Recieved from Server 0
Sending message 0XF0F5
Recieved from Server 0
Sending message 0XF0F6
Recieved from Server 0
Sending message 0XF0F7
Recieved from Server 0
Sending message 0XF0F8
Recieved from Server 0
Sending message 0XF0F9
Recieved from Server 0
Freeing all ressources...

So in conclusion, i know maybe it's a stupid and simple question, but why is it like that? Why does the server crash at sending the very first message, whereas the client seams to throw out his messages into the void.

Thanks for any answers in advance, i really need to solve this.

Stargateur · Accepted Answer

You have some minors issue and some big issue:

Small:

You don't affect ai_protocol in hints, but it's require by getaddrinfo() when hints is not NULL
You use the return of connect() as a file descriptor but it's a error code number. You must use the file descriptor return by socket() in a client, or by accept() in a server.
You use the wrong file descriptor when you use send() in the server. You SHOULD not send data on the file descriptor that handle connection in a SOCK_STREAM socket type.
getaddrinfo() return a list, you should iterate on it, but you assume that there is always one result, but they could be zero or more than one.
hints is initialized, you invoke undefined behavior with memset() because sizeof(struct addrinfo) > sizeof(struct addrinfo *) so you are out of bound when you write zero on ai (GLOBAL... GLOBAL EVERYWHERE)

Big:

You use global without reason
You declare more than one variable by line.
You use global without reason
You don't verify all your system call error !
You use global without reason

// server and client
struct addrinfo hints = {
  .ai_family = AF_INET,
  .ai_socktype = SOCK_STREAM,
  .ai_flags = AI_PASSIVE,
  .ai_protocol = IPPROTO_TCP,
};
// server
if (send(connfd, &msg_s, 1, MSG_CONFIRM) == -1) {
  // error
}
// client
if (recv(sockfd, &msg_r, 1, MSG_CONFIRM) == -1) {
  // error
}

send and recv are not consistent

Answers (2)

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