Unix IPC之Posix消息队列(2)

/* Query status and attributes of message queue MQDES.  */
extern int mq_getattr (mqd_t __mqdes, struct mq_attr *__mqstat)
  __THROW __nonnull ((2));

/* Set attributes associated with message queue MQDES and if OMQSTAT is
   not NULL also query its old attributes.  */
extern int mq_setattr (mqd_t __mqdes,
               __const struct mq_attr *__restrict __mqstat,
               struct mq_attr *__restrict __omqstat)
  __THROW __nonnull ((2));

struct mq_attr {
    long    mq_flags;    /* message queue flags            */
    long    mq_maxmsg;    /* maximum number of messages        */
    long    mq_msgsize;    /* maximum message size            */
    long    mq_curmsgs;    /* number of messages currently queued    */
    long    __reserved[4];    /* ignored for input, zeroed for output */
};

程序1(mqgetattr.c)：获取一个消息队列的默认属性。(以大写开头的函数都是对应函数的包裹函数，仅仅在里面添加了出错信息)

// mqgetattr.c
#include    "unpipc.h"

int
main(int argc, char **argv)
{
    mqd_t    mqd;
    struct mq_attr    attr;

    if (argc != 2)
        err_quit("usage: mqgetattr <name>");

    mqd = Mq_open(argv[1], O_RDONLY);

    Mq_getattr(mqd, &attr);
    printf("max #msgs = %ld, max #bytes/msg = %ld, "
           "#currently on queue = %ld
",
           attr.mq_maxmsg, attr.mq_msgsize, attr.mq_curmsgs);

    Mq_close(mqd);
    exit(0);
}

运行结果：

[dell@localhost pxmsg]$ ./mqcreate1 /hello.1234
[dell@localhost pxmsg]$ ./mqgetattr /hello.1234
max #msgs = 10, max #bytes/msg = 8192, #currently on queue = 0
[dell@localhost pxmsg]$ 

[dell@localhost pxmsg]$ ls -l /tmp/mqueue/hello.1234 
-rw-r--r--. 1 dell dell 80 8月  12 21:03 /tmp/mqueue/hello.1234

即：80KB = 10 * 8192B

程序2：指定消息队列的最大消息个数及每个消息的最大长度。

#include    "unpipc.h"

struct mq_attr  attr;   /* mq_maxmsg and mq_msgsize both init to 0 */

int
main(int argc, char **argv)
{
    int     c, flags;
    mqd_t   mqd;

    flags = O_RDWR | O_CREAT;
    while ( (c = Getopt(argc, argv, "em:z:")) != -1)
    {
        switch (c)
        {
            case 'e':
                flags |= O_EXCL;
                break;

            case 'm':
                attr.mq_maxmsg = atol(optarg);
                break;

            case 'z':
                attr.mq_msgsize = atol(optarg);
                break;
        }
    }
    if (optind != argc - 1)
        err_quit("usage: mqcreate [ -e ] [ -m maxmsg -z msgsize ] <name>");

    if ((attr.mq_maxmsg != 0 && attr.mq_msgsize == 0) ||
        (attr.mq_maxmsg == 0 && attr.mq_msgsize != 0))
        err_quit("must specify both -m maxmsg and -z msgsize");

    mqd = Mq_open(argv[optind], flags, FILE_MODE,
                  (attr.mq_maxmsg != 0) ? &attr : NULL);

    Mq_close(mqd);
    exit(0);
}

运行结果：

[dell@localhost pxmsg]$ cat /proc/sys/fs/mqueue/msg_max
10
[dell@localhost pxmsg]$ cat /proc/sys/fs/mqueue/msgsize_max
8192
[dell@localhost pxmsg]$ ./mqcreate -m 5 -z 512 /hello.123
[dell@localhost pxmsg]$ ./mqgetattr /hello.123
max #msgs = 5, max #bytes/msg = 512, #currently on queue = 0
[dell@localhost pxmsg]$ ls -l /tmp/mqueue/hello.123
-rw-r--r--. 1 dell dell 80 8月  12 21:26 /tmp/mqueue/hello.123
[dell@localhost pxmsg]$ 

说明：这里设置时不能超过系统设定的参数。