Msgget(2) returns 0 - a workaround fix Post: 302906930

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Top Forums Programming Msgget(2) returns 0 - a workaround fix Post 302906930 by jim mcnamara on Tuesday 24th of June 2014 02:55:44 PM

06-24-2014

Registered User

msgid is NOT a message it is a message queue id. (A shared (IPC) memory object, not an individual message) A return of msgid ==0 means success. Any number >-1 == success.

Since you really did not post much code --
You should be calling msgrcv like this (note infinite loop is NOT required):

Code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

struct my_msgbuf 
{
 long mtype;
 char mtext[200];
};

int main(void)
{
   struct my_msgbuf buf;
   int msqid=0;
   key_t key;
   if ((key = ftok("[your key code goes here]", 'B')) =< -1)  /* same key as other program  edit:changed to =< */
   {
     perror("ftok");
     exit(1);
   }
   if ((msqid = msgget(key, 0666)) == -1)  /* connect to the queue */
   {
      perror("msgget");
      exit(1);
   }
   printf("Ready to receive messages\n");
   for(;;) 
   { 
      if (msgrcv(msqid, &buf, sizeof(buf.mtext), 0, 0) == -1)
      {
        perror("msgrcv");
        exit(1);
      }
      printf("%s\n", buf.mtext);
   }
   return 0;
}

The above snippet works correctly, I use it in other code....

Last edited by jim mcnamara; 06-24-2014 at 11:25 PM..

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ftok

FTOK(3) 						     Linux Programmer's Manual							   FTOK(3)

NAME

       ftok - convert a pathname and a project identifier to a System V IPC key

SYNOPSIS

       # include <sys/types.h>
       # include <sys/ipc.h>

       key_t ftok(const char *pathname, int proj_id);

DESCRIPTION

       The  ftok  function  uses  the  identity of the file named by the given pathname (which must refer to an existing, accessible file) and the
       least significant 8 bits of proj_id (which must be nonzero) to generate a key_t type System V IPC key, suitable	for  use  with	msgget(2),
       semget(2), or shmget(2).

       The  resulting  value  is  the  same  for all pathnames that name the same file, when the same value of proj_id is used. The value returned
       should be different when the (simultaneously existing) files or the project IDs differ.

RETURN VALUE

       On success the generated key_t value is returned. On failure -1 is returned, with errno indicating the error  as  for  the  stat(2)  system
       call.

CONFORMING TO

       XPG4

NOTES

       Under libc4 and libc5 (and under SunOS 4.x) the prototype was
	      key_t ftok(char *pathname, char proj_id);
       Today proj_id is an int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behaviour is said to
       be undefined when proj_id is zero.

       Of course no guarantee can be given that the resulting key_t is unique. Typically, a best effort attempt combines the given  proj_id  byte,
       the  lower 16 bits of the i-node number, and the lower 8 bits of the device number into a 32-bit result.  Collisions may easily happen, for
       example between files on /dev/hda1 and files on /dev/sda1.

SEE ALSO

       ipc(5), msgget(2), semget(2), shmget(2), stat(2)

Linux 2.4							    2001-11-28								   FTOK(3)