消息队列的系统限制

作者：冯老师,华清远见嵌入式学院讲师。

消息队列是System V的IPC对象的一种，用于进程间通信，会受到系统的限制，本文主要描述了三个限制。第一：议个消息的最大长度；第二：消息队列的最大容量；第三：最大消息队列数。

一、 一个消息的最大长度

示例程序如下：

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

        #define N 8192

        typedef struct
        {
                long mtype;
                char mtext[N];
        }MSG;

        int main()
        {
                key_t key;
                int msgid;
                MSG buf;

                if ((key = ftok(".", 5)) == -1)
                {
                        perror("ftok");
                        exit(-1);
                }

                if ((msgid = msgget(key, 0666 | IPC_CREAT)) == -1)
                {
                        perror("msgget");
                        exit(-1);
                }

                memset(&buf, 'a', sizeof(buf));
                buf.mtype = 100;

                if (-1 == msgsnd(msgid, &buf, N, 0))
                {
                        perror("msgsnd 1");
                        exit(-1);
                }

                printf("**\n");

                return 0;
        }

程序执行结果如下：

        linux@ubuntu:~/process/fifth$ ./a.out
        msgsnd 1: Invalid argument

当N为8192时，可以成功执行，打印出’**”，当改成N为8193时，就出现了上面的错误，因此，消息队列中，每个消息的正文的最大长度为8192.

二、 消息队列的最大容量

示例程序如下：

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

        #define N 8192

        typedef struct
        {
                long mtype;
                char mtext[N];
        }MSG;

        int main()
        {
                key_t key;
                int msgid;
                MSG buf;

                if ((key = ftok(".", 5)) == -1)
                {
                        perror("ftok");
                        exit(-1);
                }

                if ((msgid = msgget(key, 0666 | IPC_CREAT)) == -1)
                {
                        perror("msgget");
                        exit(-1);
                }

                memset(&buf, 'a', sizeof(buf));
                buf.mtype = 100;
                if (-1 == msgsnd(msgid, &buf, N, 0))
                {
                        perror("msgsnd 1");
                        exit(-1);
                }

                if (-1 == msgsnd(msgid, &buf, N, 0))
                {
                        perror("msgsnd 2");
                        exit(-1);
                }

                if (-1 == msgsnd(msgid, &buf, 1, 0))
                {
                        perror("msgsnd 3");
                        exit(-1);
                }

                return 0;
        }

程序执行结果如下：

        linux@ubuntu:~/process/fifth$ ipcs -q
        ------------------ Message Queues --------------------
        key                      msqid       owner       perms      used-bytes   messages
        0x0501451d     131076     linux          666           16384            2

可以看出，一个消息队列中的所有消息最多占16384字节。超过这个值后，发送函数msgsnd阻塞。

三、 最大消息队列数

示例程序如下：

        #include <stdio.h>
        #include <sys/ipc.h>
        #include <sys/msg.h>
        #include <stdlib.h>
        #include <errno.h>
        #include <string.h>
        #include <strings.h>
        #define N 8192
        typedef struct
                {
                        long mtype;
                        char mtext[N];
                }MSG;

                int main()
                {
                        key_t key;
                        int msgid, i, n = 0;
                        MSG buf;

                        for (i = 1; ; i++)
                        {
                        if ((key = ftok(".", i)) == -1)
                        {
                                fprintf(stderr, "ftok i=%d--%s\n", i, strerror(errno));
                                //perror("ftok");
                                exit(-1);
                        }

                        if ((msgid = msgget(key, 0666 | IPC_CREAT | IPC_EXCL)) == -1)
                        {
                                if (errno == EEXIST)
                                break;
                                //fprintf(stderr, "msgget i=%d--%s\n", i, strerror(errno));
                                //perror("msgget");
                                exit(-1);
                        }
                        n++;

                }
                printf("n=%d msgid=%d\n", n, msgid);
                return 0;
        }

程序执行结果，n为256.

该程序说明了，最多可以创建256个消息队列。

文章来源：华清远见嵌入式学院，原文地址：http://www.embedu.org/Column/Column716.htm

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