msgrcv(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
   specified 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
   receiving  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
   immediately.  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  modified 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
     handler.)

   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
   specified,  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
          considered 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
     system 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
     handler.)

   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
   values:

   EACCES The calling process  does  not  have  write  permission  on  the
          message 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
          MSGMAX).

   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
   values:

   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 4.09 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/.





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