消息通信 message
消息是进程间通信的一种重要方式，通常由客户端和服务器端组成。服务器以特定的键值创建一个消息队列，根据约定的消息格式填充要发送的消息和数据类型，把他插入消息队列。而在客户端，根据已知的键值和指定地点类型编码，接收对应的数据。和共享内存的方式相比，不用自己维护消息队列，可以集中精力关注数据传输，但不足的是，对于需要共享或者传输比较复杂数据结构的应用，它就显得不太灵活。
消息通信重要的函数组包括：
int msgget(key_t key, int msgflg);
The msgget() system call returns the System V message queue identifier associated with the value of the key argument. A
new message queue is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message queue with the given key
key exists, and IPC_CREAT is specified in msgflg.
If msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno
set to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)
Upon creation, the least significant bits of the argument msgflg define the permissions of the message queue. These per‐
mission bits have the same format and semantics as the permissions specified for the mode argument of open(2). (The exe‐
cute permissions are not used.)
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 specified 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 receiving process for message selection (see the description of msgrcv() below).
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
DESCRIPTION
msgctl() performs the control operation specified by cmd on the System V message queue with identifier msqid.
The msqid_ds data structure is defined in <sys/msg.h> as follows:
struct msqid_ds {
struct ipc_perm msg_perm; /* Ownership and permissions */
time_t msg_stime; /* Time of last msgsnd(2) */
time_t msg_rtime; /* Time of last msgrcv(2) */
time_t msg_ctime; /* Time of last change */
unsigned long __msg_cbytes; /* Current number of bytes in
queue (nonstandard) */
msgqnum_t msg_qnum; /* Current number of messages
in queue */
msglen_t msg_qbytes; /* Maximum number of bytes
allowed in queue */
pid_t msg_lspid; /* PID of last msgsnd(2) */
pid_t msg_lrpid; /* PID of last msgrcv(2) */
};
The ipc_perm structure is defined as follows (the highlighted fields are settable using IPC_SET):
struct ipc_perm {
key_t __key; /* Key supplied to msgget(2) */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions */
unsigned short __seq; /* Sequence number */
};
Valid values for cmd are:
IPC_STAT
Copy information from the kernel data structure associated with msqid into the msqid_ds structure pointed to by buf.
The caller must have read permission on the message queue.
IPC_SET
Write the values of some members of the msqid_ds structure pointed to by buf to the kernel data structure associated
with this message queue, updating also its msg_ctime member. The following members of the structure are updated:
msg_qbytes, msg_perm.uid, msg_perm.gid, and (the least significant 9 bits of) msg_perm.mode. The effective UID of
the calling process must match the owner (msg_perm.uid) or creator (msg_perm.cuid) of the message queue, or the
caller must be privileged. Appropriate privilege (Linux: the CAP_SYS_RESOURCE capability) is required to raise the
msg_qbytes value beyond the system parameter MSGMNB.
IPC_RMID
Immediately remove the message queue, awakening all waiting reader and writer processes (with an error return and
errno set to EIDRM). The calling process must have appropriate privileges or its effective user ID must be either
that of the creator or owner of the message queue. The third argument to msgctl() is ignored in this case.
IPC_INFO (Linux-specific)
Return information about system-wide message queue limits and parameters in the structure pointed to by buf. This
structure is of type msginfo (thus, a cast is required), defined in <sys/msg.h> if the _GNU_SOURCE feature test
macro is defined:
struct msginfo {
int msgpool; /* Size in kibibytes of buffer pool
used to hold message data;
unused within kernel */
int msgmap; /* Maximum number of entries in message
map; unused within kernel */
int msgmax; /* Maximum number of bytes that can be
written in a single message */
int msgmnb; /* Maximum number of bytes that can be
written to queue; used to initialize
msg_qbytes during queue creation
(msgget(2)) */
int msgmni; /* Maximum number of message queues */
int msgssz; /* Message segment size;
unused within kernel */
int msgtql; /* Maximum number of messages on all queues
in system; unused within kernel */
unsigned short int msgseg;
/* Maximum number of segments;
unused within kernel */
};
The msgmni, msgmax, and msgmnb settings can be changed via /proc files of the same name; see proc(5) for details.
MSG_INFO (Linux-specific)
Return a msginfo structure containing the same information as for IPC_INFO, except that the following fields are
returned with information about system resources consumed by message queues: the msgpool field returns the number of
message queues that currently exist on the system; the msgmap field returns the total number of messages in all
queues on the system; and the msgtql field returns the total number of bytes in all messages in all queues on the
system.
MSG_STAT (Linux-specific)
Return a msqid_ds structure as for IPC_STAT. However, the msqid argument is not a queue identifier, but instead an
index into the kernel's internal array that maintains information about all message queues on the system.
RETURN VALUE
On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful IPC_INFO or MSG_INFO operation returns the index of the
highest used entry in the kernel's internal array recording information about all message queues. (This information can be
used with repeated MSG_STAT operations to obtain information about all queues on the system.) A successful MSG_STAT opera‐
tion returns the identifier of the queue whose index was given in msqid.
On error, -1 is returned with errno indicating the error.
依赖的头文件包括：
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
下面以具体可用的一个基于消息通信的服务端和客户端的代码为例：
服务端：test_msg_svc.c
#include<stdio.h>
#include<assert.h>
#include<inttypes.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/stat.h>
#include <sys/msg.h>
#define MSG_FILE "test_msg_clnt"
#define BUFFER 255
#define PERM S_IRUSR|S_IWUSR
struct msgtype {
long mtype;
char buffer[BUFFER+1];
};
int main()
{
struct msgtype msg;
key_t key;
int msgid;
if((key=ftok(MSG_FILE,'a'))==-1)
{
{
fprintf(stderr,"Creat Key Error：%s\a\n",strerror(errno));
exit(1);
}
if((msgid=msgget(key,PERM|IPC_CREAT|IPC_EXCL))==-1)
//if((msgid=msgget(key,PERM|IPC_EXCL))==-1)
{
fprintf(stderr,"Creat Message Error：%s\a\n",strerror(errno));
exit(1);
}
while(1)
{
msgrcv(msgid,&msg,sizeof(struct msgtype),1,0);
fprintf(stderr,"Server Receive：%s\n",msg.buffer);
msg.mtype=2;
msgsnd(msgid,&msg,sizeof(struct msgtype),0);
memcpy(msg.buffer, "Hello", strlen("Hello") + 1);
msg.mtype=2;
msgsnd(msgid,&msg,sizeof(struct msgtype),0);
}
exit(0);
}
客户端： test_msg_clnt.c:
#include<stdio.h>
#include<assert.h>
#include<inttypes.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/stat.h>
#include <sys/msg.h>
#define MSG_FILE "test_msg_clnt"
#define BUFFER 255
#define PERM S_IRUSR|S_IWUSR
struct msgtype {
long mtype;
char buffer[BUFFER+1];
};
int main(int argc,char **argv)
{
struct msgtype msg;
key_t key;
int msgid;
if(argc!=2)
{
fprintf(stderr,"Usage：%s string\n\a",argv[0]);
exit(1);
}
if((key=ftok(MSG_FILE,'a'))==-1)
{
fprintf(stderr,"Creat Key Error：%s\a\n",strerror(errno));
exit(1);
}
if((msgid=msgget(key,PERM))==-1)
{
fprintf(stderr,"Creat Message Error：%s\a\n",strerror(errno));
exit(1);
}
msg.mtype=1;
strncpy(msg.buffer,argv[1],BUFFER);
msgsnd(msgid,&msg,sizeof(struct msgtype),0);
memset(&msg,'\0',sizeof(struct msgtype));
msgrcv(msgid,&msg,sizeof(struct msgtype),2,0);
fprintf(stdout,"Client receive：%s\n",msg.buffer);
msgrcv(msgid,&msg,sizeof(struct msgtype),2,0);
fprintf(stdout,"Client receive：%s\n",msg.buffer);
exit(0);
}
编译运行及其结果：
[kxyx@localhost testcases]$ gcc -o test_msg_svc test_msg_svc.c
[kxyx@localhost testcases]$ gcc -o test_msg_clnt test_msg_clnt.c
[kxyx@localhost testcases]$ ./test_msg_svc
Server Receive：HaHi
Server Receive：GdLuCK
[kxyx@localhost testcases]$ ./test_msg_clnt HaHi
Client receive：HaHi
Client receive：Hello
[kxyx@localhost testcases]$ ./test_msg_clnt GdLuCK
Client receive：GdLuCK
Client receive：Hello
