Msgrcv does not receive messages however they have been sent properly

Question

I'm writing a basic server program which has to receive two types of messages from clients (first message is of type 1 and second is of type 2). It seems that it doesn't see messeges from clients that have been sent properly (msgsnd doesn't return -1). I read that it may be caused by too big message, but my messages are like a few characters long and my buffer is 100. I didn't find any differences in structures I create, msgget also doesn't return any error. Here's the server program code:

struct msgbuf
{
  long type;
  char text[100];
}m1;

int main()
{
  int id;
  if (id = msgget(1998,0666|IPC_EXCL)==-1) {
     printf("msgget error");
  }
  char first_msg[100][100];
  char second_msg[100][100];
  int counter = 0;
  while (counter < 100) {
     msgrcv(id,&m1,sizeof(m1.text),1,0);
     printf("%s",m1.text);
     strcpy(first_msg[counter], m1.text);
     msgrcv(id,&m1,sizeof(m1.text),2,0);
     printf("%s",m1.text);
     strcpy(second_msg[counter],m1.text);
     counter++;
  }
  return 0;
}

And a client code:

struct msgbuf
{
   long type;
   char text[100];
};

int main()
{
   struct msgbuf m1,m2;
   int id;
   if ((id = msgget(1998,0666|IPC_EXCL)) == -1) {
     printf("error");
   }
   char first_msg[90];
   scanf("%s",&first_msg[0]);
   char second_msg[90];
   scanf("%s",&second_msg[0]);
   m1.type = 1;
   strcpy(m1.text,first_msg);
   if (msgsnd(id,&m1,sizeof(m1.text),0) == -1) {
     perror("msgsnd failed");
  }
  m2.type = 2;
  strcpy(m2.text,second_msg);
  if (msgsnd(id,&m2,sizeof(m2.text),0)==-1) {
    perror("msgsnd2 failed");
  }
  return 0;
}

Answer 1

This is a nasty bug caused by an operator precedence problem.

In client, you have correctly written:

if ((id = msgget(1998,0666|IPC_EXCL)) == -1) {

and parens ensure that you first assign to id and then compare to -1.

But in server, you only have:

if (id = msgget(1998,0666|IPC_EXCL)==-1) {

Unfortunately it actually reads if (id = (msgget(1998,0666|IPC_EXCL)==-1)), because the comparison has higher precedence than the assignment!

The fix is trivial: just add parens in server.c as they are in client.c