CODEWITHSUNDEEP
c
happy coding
happy coding

Reputation: 27

About POSIX message queue and invalid argument

I tried to write a message queue that included both the server and the client but when I run the server process, it shows that msgsnd was wrong. The error is Invalid argument! What is wrong?

The max length of message is defined 1024. Apparently this space is big enough.

// msgq.h

#define PATH "/tmp"
#define ID 666
#define MSG_MAX_LEN 1024

struct message{
    long mtype;
    char mtext[MSG_MAX_LEN];
};


// msgq.c


#include "msgq.h"
#include <stdlib.h> 
#include <stdio.h> 
#include <string.h> 

int init_msg_queue() {
  key_t KEY = ftok(PATH, ID);
  if(KEY==(key_t)-1){
    printf("[ LOG ERROR ] ftok faild\n"); 
    return -1;
  }

  // IPC_CREAT|IPC_EXCL|0666
  int msqid = msgget(KEY,IPC_CREAT|0666);
  if (msqid < 0){
    printf("[ LOG ERROR ] msgget faild\n"); 
    return -1;
  }
  return msqid;
}


int send_msg(char *msg,int msqid,long type) { 
  struct message message;
  message.mtype=type;
  size_t len=strlen(msg);
  if(len>MSG_MAX_LEN){
    printf("[ LOG ERROR ] Buffer not big enough\n");
    return -2;
  }
  strcpy(message.mtext, msg);
  int res=msgsnd(msqid,&message,(size_t)len,0);
  if (res==-1){
    perror("msg send failed");
    return -1;
  }
  return 0; 
}

this is the server.c
include msgq.h and try send message

// server.c 

#include "msgq.h"

int main(int args, char *argv[]) {
  int qid = init_msg_queue();
  if (qid == -1) {
    printf("创建/打开队列失败\n");
    return 1;
  }
  while (1) {
    send_msg("hello world!", qid, 0);
    sleep(2);
  }
  return 0;
}

Upvotes: 1

Views: 180

Answers (1)

dash-o
dash-o

Reputation: 14432

Using 'strace ./a.out', it shows:

msgsnd(0, {0, "hello world!"}, 12, 0) = -1 EINVAL (Invalid argument)

Showing two issues:

  • The 'sockfd' is zero. The sockfd is created using the 'ftok' on "/tmp" directory. As per ftok man page: 'The ftok() function uses the identity of the file named by the given pathname (which must refer to an existing, accessible file' (and not a directory).
  • The 'messagetype' attribute is 0, whereas the man page states'The mtype field must have a strictly positive integer value.'

Fixing: - Change PATH to "/tmp/my-file" (or other file name), which must be a valid, existing, accessible file - In server.c , modify message type to '1' (or other positive integer)

Upvotes: 1

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