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man:msgop

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 4.16 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 2017-09-15 MSGOP(2)

/data/webs/external/dokuwiki/data/pages/man/msgop.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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