msgop(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       msgrcv, msgsnd - System V message queue operations

SYNOPSIS         top

       #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         top

       The msgsnd() and msgrcv() system calls are used 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         top

       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         top

       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 in the user namespace that governs its IPC
              namespace.

       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)
              Both MSG_COPY and IPC_NOWAIT were specified in msgflg, and
              this kernel was configured without
              CONFIG_CHECKPOINT_RESTORE.

CONFORMING TO         top

       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         top

       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         top

       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.

EXAMPLES         top

       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, &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, &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         top

       msgctl(2), msgget(2), capabilities(7), mq_overview(7), sysvipc(7)

COLOPHON         top

       This page is part of release 5.10 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                          2020-11-01                       MSGOP(2)

Pages that refer to this page: ipcs(1)lsipc(1)pcp-ipcs(1)ipc(2)msgctl(2)msgget(2)syscalls(2)capabilities(7)mq_overview(7)signal(7)sysvipc(7)