mq_receive:消息太长

Question

我正在使用队列实现两个进程之间的通信.问题是,当我调用函数mq_receive时,我收到此错误:消息太长.

我做了以下事情:

struct mq_attr attr;

long size = attr.mq_msgsize;
.... // initializing the queue "/gateway"

int rc = mq_receive(gateway, buffer, size, &prio);

如果我打印大小值,我得到size = 1,而当我打印相同的大小但从另一个程序(通过相同的机制获得),我得到一些不长的整数(-1217186280)...

我怎么能解决这个错误？....所以当size = 1时,我认为说"消息太长"是正确的,但为什么是1？

PS我也尝试过:

int rc = mq_receive(gateway, buffer, sizeof(buffer), &prio);

但没有结果.

Answer 1

您似乎需要更仔细地阅读文档.当你打电话时,mq_receive你应该传递目标缓冲区的大小. 此大小必须大于队列的mq_msgsize属性.此外,您似乎在队列属性初始化中出现错误,导致mq_receive无法进行正确的调用.这是标准的消息队列会话:

1. 填充mq_attrstruct(doc):

   struct mq_attr attr;  
   attr.mq_flags = 0;  
   attr.mq_maxmsg = 10;  
   attr.mq_msgsize = 33;  
   attr.mq_curmsgs = 0;  
   

2. mq_open在主进程(doc)中创建队列:

   mqd_t queue = mq_open(qname, O_CREAT|O_RDWR, 0644, &attr);
   

3. 在编写进程中打开队列进行编写:

   mqd_t queue = mq_open(qname, O_WRONLY);
   

   并发送一些文字.文本的长度必须小于mq_msgsize队列的属性(doc):

   mq_send(queue, "some message", strlen("some message")+1, 1);
   

4. 在读者进程中打开队列进行阅读:

   mqd_t queue = mq_open(qname, O_RDONLY);
   

   然后分配缓冲区和接收消息.缓冲区*的大小必须大于mq_msgsize队列的属性.这里我们创建50字节缓冲区mq_msgsize == 33(doc):

   char rcvmsg[50];
   int iret = mq_receive(queue, rcvmsg, 50, NULL);
   

还要记住,你应该使用%ldprint long而不是%d.

Answer 2

在调试实时POSIX队列时,您应该从一个可以工作的示例程序开始,然后从那里开始.一旦运行了示例程序,就应该确保自己的代码遵循所有步骤.

以下程序已在Ubuntu 11.04下成功测试过:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <mqueue.h>

#define MQNAME "/pax"
#define MQMESG "Hello there!"

static mqd_t serverUp (void) {
    int rc;
    mqd_t svrHndl;
    struct mq_attr mqAttr;

    printf ("Bringing up server.\n");
    rc = mq_unlink (MQNAME);
    if (rc < 0) {
        printf ("   Warning %d (%s) on server mq_unlink.\n",
            errno, strerror (errno));
    }

    mqAttr.mq_maxmsg = 10;
    mqAttr.mq_msgsize = 1024;
    svrHndl = mq_open (MQNAME, O_RDWR|O_CREAT, S_IWUSR|S_IRUSR, &mqAttr);
    if (svrHndl < 0) {
        printf ("   Error %d (%s) on server mq_open.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Server opened mqd_t of %d.\n", svrHndl);
    return svrHndl;
}

static void serverReceive (mqd_t svrHndl) {
    int rc;
    char buffer[2048];
    printf ("Server receiving on mqd_t %d.\n", svrHndl);
    rc = mq_receive (svrHndl, buffer, sizeof (buffer), NULL);
    if (rc < 0) {
        printf ("   Error %d (%s) on server mq_receive.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Received [%s].\n", buffer);
}

static void serverDown (mqd_t svrHndl) {
    printf ("Bringing down server with mqd_t %d.\n", svrHndl);
    mq_close (svrHndl);
}
static void clientSend (void) {
    mqd_t cliHndl;
    int rc;
    printf ("Client sending.\n");
    cliHndl = mq_open (MQNAME, O_RDWR);
    if (cliHndl < 0) {
        printf ("   Error %d (%s) on client mq_open.\n",
            errno, strerror (errno));
        exit (1);
    }
    printf ("   Client opened mqd_t of %d.\n", cliHndl);

    rc = mq_send (cliHndl, MQMESG, sizeof (MQMESG), 1);
    if (rc < 0) {
        printf ("   Error %d (%s) on client mq_send.\n",
            errno, strerror (errno));
        exit (1);
    }

    mq_close (cliHndl);
}

int main (void) {
    mqd_t svrHndl;

    svrHndl = serverUp ();
    clientSend ();
    serverReceive (svrHndl);
    serverDown (svrHndl);

    return 0;
}

我系统的输出是:

Bringing up server.
   Server opened mqd_t of 3.
Client sending.
   Client opened mqd_t of 4.
Server receiving on mqd_t 3.
   Received [Hello there!].
Bringing down server with mqd_t 3.

Answer 3

在再次运行程序之前,请不要忘记取消链接消息队列.如果您不取消链接,它仍将使用旧的邮件队列设置.使用Ctrl + C结束程序时会发生这种情况.我认为将以下代码放在程序的开头是个好主意:

if(mq_unlink(QUEUENAME) == 0)
  fprintf(stdout, "Message queue %s removed from system.\n", QUEUENAME);

另一种形式(C++样式),用于检查实际错误(如权限)并忽略队列已存在或不存在的情况:

int rc = mq_unlink(name.c_str());
if (rc != 0 && errno != ENOENT)
      THROW_ERRNO_EXCEPTION();  

// ENOENT is the status code if the queue doesn't exist, which is not an error
// if you are immediately going to create it.