System V 消息队列概述

System V消息队列使用消息队列标识符标识，对于内核中的每个消息队列，内核维护一个定义在<sys/msg.h>头文件中的信息结构。

struct msqid_ds
{
struct msqid_ds {
struct ipc_perm msg_perm;
struct msg *msg_first; /* first message on queue,unused */
struct msg *msg_last; /* last message in queue,unused */
__kernel_time_t msg_stime; /* last msgsnd time */
__kernel_time_t msg_rtime; /* last msgrcv time */
__kernel_time_t msg_ctime; /* last change time */
unsigned long msg_lcbytes; /* Reuse junk fields for 32 bit */
unsigned long msg_lqbytes; /* ditto */
unsigned short msg_cbytes; /* current number of bytes on queue */
unsigned short msg_qnum; /* number of messages in queue */
unsigned short msg_qbytes; /* max number of bytes on queue */
__kernel_ipc_pid_t msg_lspid; /* pid of last msgsnd */
__kernel_ipc_pid_t msg_lrpid; /* last receive pid */

};

msgget函数：

创建一个新的消息队列或者访问一个已存在的消息队列。

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

int msgget(key_t key, int msgflg);

key是通过ftok的返回值，也可以是常值IPC_PRIVATE。oflag值是读写权限值的组合，它还可以与IPC_CREAT和IPC_EXCL按位或。

msgsnd和msgrcv函数：

打开一个消息队列后，使用此函数往其上放置一个消息和读取一个消息

#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);

msgflg可以是0也可以是IPC_NOWAIT。

对于type的选项：

为0的话，返回队列中的第一个消息

为大于0的话，返回类型值为type的第一个消息。

小于0的话，返回类型值小于type参数绝对值的消息中类型值最小的。

msgctl函数：

提供在一个消息队列上的各种控制操作

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

例子：

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

int main(int argc, char **argv)
{
int c, oflag, mqid;
oflag = IPC_CREAT | 00666;

while ((c = getopt(argc, argv, "e")) != -1) {
switch(c) {
case 'e':
oflag |= IPC_EXCL;
break;
}
}
printf("the optind is %d, the argc is %d\n", optind, argc);
if (optind != argc - 1) {
printf("usage: msgcreate [-e] <pathname>");
exit(0);
}
printf("argv[optind] is %s \n",argv[optind]);
if ((mqid = msgget(ftok(argv[optind], 0), oflag)) < 0); {
perror("msgget error\n");
exit(0);
}
exit(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/msg.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <fcntl.h>
#include <errno.h>

struct msgbuf {
long mtype;
char mtext[1];
};

int main(int argc, char **argv)
{
int mqid;
size_t len;
long type;
struct msgbuf *ptr;
int n;

if (argc != 4) {
printf("usage: msgsnd <pathmane> <#bytes> <type>");
exit(0);
}

len = atoi(argv[2]);
type = atoi(argv[3]);
if ((mqid = msgget(ftok(argv[1],0), 00200)) == -1) {
perror("msgget error");
exit(0);
}

ptr = calloc(sizeof(long) + len, sizeof(char));
ptr->mtype = type;

n = msgsnd(mqid, ptr, len, 0);
if (n < 0)
perror("msgsend erro");
printf("the size of send is %d\n", n);

exit(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/msg.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <fcntl.h>

#define MAXMSG (8192 + sizeof(long))

struct msgbuf {
long mtype;
char mtext[1];
};

int main(int argc, char **argv)
{
int c, flag, mqid;
long type;
ssize_t n;
struct msgbuf *buff;

type = flag = 0;

while ((getopt(argc, argv, "nt:")) != -1) {
switch(c) {
case 'n':
flag |= IPC_NOWAIT;
break;
case 't':
type = atol(optarg);
break;
}
}

if (optind != argc - 1) {
printf("usage: msgrcv [-n] [-t type] <pathname>");
exit(0);
}

if ((mqid = msgget(ftok(argv[optind], 0), O_RDONLY)) == -1) {
printf("msgget error");
exit(0);
}

buff = malloc(MAXMSG);

n = msgrcv(mqid, buff, MAXMSG, type, flag);
printf("read %d bytes, type = %ld\n", n, buff->mtype);

exit(0);
}