MSGOP(2)		   Linux Programmer's Manual		      MSGOP(2)



NAME
       msgrcv, msgsnd - System V message queue operations

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

       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

       ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
		      int msgflg);

DESCRIPTION
       The  msgsnd() and msgrcv() system calls are used, respectively, to send
       messages to, and receive messages from, a System V message queue.   The
       calling	process	 must  have  write  permission on the message queue in
       order to send a message, and read permission to receive a message.

       The msgp argument is a pointer to a  caller-defined  structure  of  the
       following general form:

	   struct msgbuf {
	       long mtype;	 /* message type, must be > 0 */
	       char mtext[1];	 /* message data */
	   };

       The  mtext  field is an array (or other structure) whose size is speci-
       fied by msgsz, a nonnegative integer value.  Messages  of  zero	length
       (i.e.,  no  mtext  field)  are  permitted.  The mtype field must have a
       strictly positive integer value.	 This value can be used by the receiv-
       ing  process  for  message  selection  (see the description of msgrcv()
       below).

   msgsnd()
       The msgsnd() system call appends a copy of the message  pointed	to  by
       msgp to the message queue whose identifier is specified by msqid.

       If  sufficient space is available in the queue, msgsnd() succeeds imme-
       diately.	 The queue capacity is governed by the msg_qbytes field in the
       associated data structure for the message queue.	 During queue creation
       this field is initialized to MSGMNB bytes, but this limit can be	 modi-
       fied  using  msgctl(2).	 A  message  queue is considered to be full if
       either of the following conditions is true:

       * Adding a new message to the queue would cause	the  total  number  of
	 bytes in the queue to exceed the queue's maximum size (the msg_qbytes
	 field).

       * Adding another message to the queue would cause the total  number  of
	 messages  in  the  queue  to  exceed  the  queue's  maximum size (the
	 msg_qbytes field).  This check is necessary to prevent	 an  unlimited
	 number	 of  zero-length messages being placed on the queue.  Although
	 such messages contain no data,	 they  nevertheless  consume  (locked)
	 kernel memory.

       If  insufficient	 space	is  available  in  the queue, then the default
       behavior of msgsnd() is to block until  space  becomes  available.   If
       IPC_NOWAIT is specified in msgflg, then the call instead fails with the
       error EAGAIN.

       A blocked msgsnd() call may also fail if:

       * the queue is removed, in which case the system call fails with	 errno
	 set to EIDRM; or

       * a  signal  is	caught, in which case the system call fails with errno
	 set  to  EINTR;see  signal(7).	  (msgsnd()  is	 never	 automatically
	 restarted  after being interrupted by a signal handler, regardless of
	 the setting of the SA_RESTART flag when establishing  a  signal  han-
	 dler.)

       Upon  successful completion the message queue data structure is updated
       as follows:

	      msg_lspid is set to the process ID of the calling process.

	      msg_qnum is incremented by 1.

	      msg_stime is set to the current time.

   msgrcv()
       The msgrcv() system call removes a message from the queue specified  by
       msqid and places it in the buffer pointed to by msgp.

       The  argument  msgsz specifies the maximum size in bytes for the member
       mtext of the structure pointed to by the msgp argument.	If the message
       text  has  length  greater  than	 msgsz,	 then  the behavior depends on
       whether MSG_NOERROR is specified in msgflg.  If MSG_NOERROR  is	speci-
       fied,  then  the message text will be truncated (and the truncated part
       will be lost); if MSG_NOERROR is not specified, then the message	 isn't
       removed	from  the  queue  and  the system call fails returning -1 with
       errno set to E2BIG.

       Unless MSG_COPY is specified in msgflg (see below), the msgtyp argument
       specifies the type of message requested, as follows:

       * If msgtyp is 0, then the first message in the queue is read.

       * If  msgtyp  is greater than 0, then the first message in the queue of
	 type msgtyp is read, unless MSG_EXCEPT was specified  in  msgflg,  in
	 which case the first message in the queue of type not equal to msgtyp
	 will be read.

       * If msgtyp is less than 0, then the first message in  the  queue  with
	 the  lowest  type  less than or equal to the absolute value of msgtyp
	 will be read.

       The msgflg argument is a bit mask constructed by ORing together zero or
       more of the following flags:

       IPC_NOWAIT
	      Return immediately if no message of the requested type is in the
	      queue.  The system call fails with errno set to ENOMSG.

       MSG_COPY (since Linux 3.8)
	      Nondestructively fetch a copy of	the  message  at  the  ordinal
	      position	in the queue specified by msgtyp (messages are consid-
	      ered to be numbered starting at 0).

	      This flag must be specified in conjunction with IPC_NOWAIT, with
	      the  result  that, if there is no message available at the given
	      position, the call fails	immediately  with  the	error  ENOMSG.
	      Because  they  alter  the	 meaning of msgtyp in orthogonal ways,
	      MSG_COPY and MSG_EXCEPT may not both be specified in msgflg.

	      The MSG_COPY flag was added for the implementation of the kernel
	      checkpoint-restore  facility and is available only if the kernel
	      was built with the CONFIG_CHECKPOINT_RESTORE option.

       MSG_EXCEPT
	      Used with msgtyp greater than 0 to read the first message in the
	      queue with message type that differs from msgtyp.

       MSG_NOERROR
	      To truncate the message text if longer than msgsz bytes.

       If  no  message of the requested type is available and IPC_NOWAIT isn't
       specified in msgflg, the calling process is blocked until  one  of  the
       following conditions occurs:

       * A message of the desired type is placed in the queue.

       * The message queue is removed from the system.	In this case, the sys-
	 tem call fails with errno set to EIDRM.

       * The calling process catches a signal.	In this case, the system  call
	 fails	with  errno  set  to  EINTR.  (msgrcv() is never automatically
	 restarted after being interrupted by a signal handler, regardless  of
	 the  setting  of  the SA_RESTART flag when establishing a signal han-
	 dler.)

       Upon successful completion the message queue data structure is  updated
       as follows:

	      msg_lrpid is set to the process ID of the calling process.

	      msg_qnum is decremented by 1.

	      msg_rtime is set to the current time.

RETURN VALUE
       On  failure  both  functions return -1 with errno indicating the error,
       otherwise msgsnd() returns 0 and msgrcv() returns the number  of	 bytes
       actually copied into the mtext array.

ERRORS
       When  msgsnd() fails, errno will be set to one among the following val-
       ues:

       EACCES The calling process does not have write permission on  the  mes-
	      sage queue, and does not have the CAP_IPC_OWNER capability.

       EAGAIN The  message  can't  be sent due to the msg_qbytes limit for the
	      queue and IPC_NOWAIT was specified in msgflg.

       EFAULT The address pointed to by msgp isn't accessible.

       EIDRM  The message queue was removed.

       EINTR  Sleeping on a full message queue condition, the process caught a
	      signal.

       EINVAL Invalid  msqid  value,  or  nonpositive  mtype value, or invalid
	      msgsz value (less than 0 or greater than the system  value  MSG-
	      MAX).

       ENOMEM The  system  does	 not  have enough memory to make a copy of the
	      message pointed to by msgp.

       When msgrcv() fails, errno will be set to one among the following  val-
       ues:

       E2BIG  The  message  text  length is greater than msgsz and MSG_NOERROR
	      isn't specified in msgflg.

       EACCES The calling process does not have read permission on the message
	      queue,  and  does	 not  have the CAP_IPC_OWNER capability in the
	      user namespace that governs its IPC namespace.

       EFAULT The address pointed to by msgp isn't accessible.

       EIDRM  While the process was sleeping to receive a message, the message
	      queue was removed.

       EINTR  While the process was sleeping to receive a message, the process
	      caught a signal; see signal(7).

       EINVAL msqid was invalid, or msgsz was less than 0.

       EINVAL (since Linux 3.14)
	      msgflg specified MSG_COPY, but not IPC_NOWAIT.

       EINVAL (since Linux 3.14)
	      msgflg specified both MSG_COPY and MSG_EXCEPT.

       ENOMSG IPC_NOWAIT was  specified	 in  msgflg  and  no  message  of  the
	      requested type existed on the message queue.

       ENOMSG IPC_NOWAIT  and  MSG_COPY were specified in msgflg and the queue
	      contains less than msgtyp messages.

       ENOSYS (since Linux 3.8)
	      MSG_COPY was specified in msgflg, and this kernel was configured
	      without CONFIG_CHECKPOINT_RESTORE.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

       The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their definitions
       can be obtained by defining the _GNU_SOURCE feature test macro.

NOTES
       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on	 Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of these header files, and the SVID also documented their
       inclusion.   Applications  intended  to be portable to such old systems
       may need to include these header files.

       The msgp argument is declared as struct msgbuf * in glibc 2.0 and  2.1.
       It  is  declared as void * in glibc 2.2 and later, as required by SUSv2
       and SUSv3.

       The following limits on message queue  resources	 affect	 the  msgsnd()
       call:

       MSGMAX Maximum  size  of	 a message text, in bytes (default value: 8192
	      bytes).  On Linux, this limit  can  be  read  and	 modified  via
	      /proc/sys/kernel/msgmax.

       MSGMNB Maximum  number  of  bytes  that	can be held in a message queue
	      (default value: 16384 bytes).  On Linux, this limit can be  read
	      and  modified via /proc/sys/kernel/msgmnb.  A privileged process
	      (Linux: a process	 with  the  CAP_SYS_RESOURCE  capability)  can
	      increase	the  size  of  a message queue beyond MSGMNB using the
	      msgctl(2) IPC_SET operation.

       The implementation has no intrinsic system-wide limits on the number of
       message	headers	 (MSGTQL)  and the number of bytes in the message pool
       (MSGPOOL).

BUGS
       In Linux 3.13 and earlier, if msgrcv() was  called  with	 the  MSG_COPY
       flag, but without IPC_NOWAIT, and the message queue contained less than
       msgtyp messages, then the call would block until the  next  message  is
       written	to  the queue.	At that point, the call would return a copy of
       the message, regardless of whether that	message	 was  at  the  ordinal
       position msgtyp.	 This bug is fixed in Linux 3.14.

       Specifying  both	 MSG_COPY  and MSC_EXCEPT in msgflg is a logical error
       (since these flags impose different  interpretations  on	 msgtyp).   In
       Linux 3.13 and earlier, this error was not diagnosed by msgrcv().  This
       bug is fixed in Linux 3.14.

EXAMPLE
       The program below demonstrates the use of msgsnd() and msgrcv().

       The example program is first run with the -s option to send  a  message
       and then run again with the -r option to receive a message.

       The following shell session shows a sample run of the program:

	   $ ./a.out -s
	   sent: a message at Wed Mar  4 16:25:45 2015

	   $ ./a.out -r
	   message received: a message at Wed Mar  4 16:25:45 2015

   Program source

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

       struct msgbuf {
	   long mtype;
	   char mtext[80];
       };

       static void
       usage(char *prog_name, char *msg)
       {
	   if (msg != NULL)
	       fputs(msg, stderr);

	   fprintf(stderr, "Usage: %s [options]\n", prog_name);
	   fprintf(stderr, "Options are:\n");
	   fprintf(stderr, "-s	      send message using msgsnd()\n");
	   fprintf(stderr, "-r	      read message using msgrcv()\n");
	   fprintf(stderr, "-t	      message type (default is 1)\n");
	   fprintf(stderr, "-k	      message queue key (default is 1234)\n");
	   exit(EXIT_FAILURE);
       }

       static void
       send_msg(int qid, int msgtype)
       {
	   struct msgbuf msg;
	   time_t t;

	   msg.mtype = msgtype;

	   time(&t);
	   snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
		   ctime(&t));

	   if (msgsnd(qid, (void *) &msg, sizeof(msg.mtext),
		       IPC_NOWAIT) == -1) {
	       perror("msgsnd error");
	       exit(EXIT_FAILURE);
	   }
	   printf("sent: %s\n", msg.mtext);
       }

       static void
       get_msg(int qid, int msgtype)
       {
	   struct msgbuf msg;

	   if (msgrcv(qid, (void *) &msg, sizeof(msg.mtext), msgtype,
		      MSG_NOERROR | IPC_NOWAIT) == -1) {
	       if (errno != ENOMSG) {
		   perror("msgrcv");
		   exit(EXIT_FAILURE);
	       }
	       printf("No message available for msgrcv()\n");
	   } else
	       printf("message received: %s\n", msg.mtext);
       }

       int
       main(int argc, char *argv[])
       {
	   int qid, opt;
	   int mode = 0;	       /* 1 = send, 2 = receive */
	   int msgtype = 1;
	   int msgkey = 1234;

	   while ((opt = getopt(argc, argv, "srt:k:")) != -1) {
	       switch (opt) {
	       case 's':
		   mode = 1;
		   break;
	       case 'r':
		   mode = 2;
		   break;
	       case 't':
		   msgtype = atoi(optarg);
		   if (msgtype <= 0)
		       usage(argv[0], "-t option must be greater than 0\n");
		   break;
	       case 'k':
		   msgkey = atoi(optarg);
		   break;
	       default:
		   usage(argv[0], "Unrecognized option\n");
	       }
	   }

	   if (mode == 0)
	       usage(argv[0], "must use either -s or -r option\n");

	   qid = msgget(msgkey, IPC_CREAT | 0666);

	   if (qid == -1) {
	       perror("msgget");
	       exit(EXIT_FAILURE);
	   }

	   if (mode == 2)
	       get_msg(qid, msgtype);
	   else
	       send_msg(qid, msgtype);

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			      MSGOP(2)