linux ipc.为什么msgrcv总是阻塞

Question

客户端readline写入共享内存，并向服务器发送消息。服务器获取消息并从共享内存中读取。

但是服务器不能正确输出服务器没有输出任何内容，我不知道为什么。

手册页上说：如果没有请求类型的消息可用，并且在msgflg中未指定IPC_NOWAIT，则调用过程将被阻止，直到出现以下条件之一

但是服务器总是被阻塞。

我用gdb调试它，发现std:：cout不工作

调试上下文

Breakpoint 1, main () at shared_mem_server.cpp:35
35      sem_init(reinterpret_cast<sem_t*>(shm),0,1);
(gdb) p shm
$1 = 0x7ffff7ff6000 ""
(gdb) x/10w 0x7ffff7ff6000
0x7ffff7ff6000: 0   0   0   0
0x7ffff7ff6010: 0   0   0   0
0x7ffff7ff6020: 0   0
(gdb) s
__new_sem_init (sem=0x7ffff7ff6000, pshared=0, value=1) at sem_init.c:31
31  sem_init.c: No such file or directory.
(gdb) return
Make __new_sem_init return now? (y or n) n
Not confirmed
(gdb) finish
Run till exit from #0  __new_sem_init (sem=0x7ffff7ff6000, pshared=0, value=1)
at sem_init.c:31
main () at shared_mem_server.cpp:37
37          msgrcv(msgid,&msg,256,ret_type,0);
Value returned is $2 = 0
(gdb) x/10w 0x7ffff7ff6000
0x7ffff7ff6000: 1   0   0   0
0x7ffff7ff6010: 0   0   0   0
0x7ffff7ff6020: 0   0
(gdb) p sem_sz
$3 = 32
(gdb) n
38          sem_p(reinterpret_cast<sem_t*>(shm));
(gdb) n
39          if(shm + sem_sz == "q")
(gdb) x/10w 0x7ffff7ff6000
0x7ffff7ff6000: 0   0   0   0
0x7ffff7ff6010: 0   0   0   0
0x7ffff7ff6020: 3355185 0
(gdb) x/12w 0x7ffff7ff6000
0x7ffff7ff6000: 0   0   0   0
0x7ffff7ff6010: 0   0   0   0
0x7ffff7ff6020: 3355185 0   0   0
(gdb) n
41          std::cout << "shared memory " << shm + sem_sz;
(gdb) n
42          sem_v(reinterpret_cast<sem_t*>(shm));
(gdb) q

以下是代码

服务器代码：

#include <iostream>
#include <sys/shm.h>
#include <sys/msg.h>
#include "error.h"
#include "sempv.h"
const int SHM_SIZE=1024;
struct msg_form{
long  msg_type;
char msg_text[256];
};
int main(){
key_t key;
int shmid,msgid,ret_type = 888,sem_sz = sizeof(sem_t);
char *shm;

msg_form msg;
if((key = ftok(".",'v')) < 0)
unix_error("ftok error");

if((shmid = shmget(key,SHM_SIZE,IPC_CREAT|0666)) == -1)
unix_error("create shared memory error");
if((shm = (char*)shmat(shmid,0,0)) == (void*)-1){
unix_error("attach shared memeory error");
}
if((msgid = msgget(key,IPC_CREAT|07777)) == -1)
unix_error("msgget error");
sem_init(reinterpret_cast<sem_t*>(shm),0,1);
while(true){
msgrcv(msgid,&msg,256,ret_type,0);
sem_p(reinterpret_cast<sem_t*>(shm));
if(shm + sem_sz == "q")
break;
std::cout << "shared memory " << shm + sem_sz;
sem_v(reinterpret_cast<sem_t*>(shm));
}
shmdt(shm);

shmctl(shmid,IPC_RMID,0);
shmctl(msgid,IPC_RMID,0);
return 0;
}

客户端代码

#include <iostream>
#include <sys/shm.h>
#include <sys/msg.h>
#include "error.h"
#include "sempv.h"
#include <string>
using std::string;
const int SHM_SIZE=1024;
struct msg_form{
long  msg_type;
char msg_text[256];
};
int main(){
key_t key;
int shmid,msgid,sem_sz = sizeof(sem_t);
char *shm;

int err;
msg_form msg;
string s;
if((key = ftok(".",'v')) < 0)
unix_error("ftok error");

if((shmid = shmget(key,SHM_SIZE,0)) == -1)
unix_error("shmget error");
if((shm = (char*)shmat(shmid,0,0)) == (void*)-1){
unix_error("attach shared memeory error");
}
if((msgid = msgget(key,0777)) == -1)
unix_error("msgget error");
std::cout << "key is " << key << std::endl;

while(getline(std::cin,s)){
sem_p(reinterpret_cast<sem_t*>(shm));
memset(shm+sem_sz,0,SHM_SIZE-sem_sz);
memcpy(shm+sem_sz,s.c_str(),s.size());

msg.msg_type = 888;
sprintf(msg.msg_text,"shared memory write signal");
if((err =  msgsnd(msgid,&msg,sizeof(msg.msg_text),0)) == -1)
unix_error("msgsnd error");
sem_v(reinterpret_cast<sem_t*>(shm));
//std::cout << "message sendn";
}
return 0;
}

Answer 1

一般备注：

• System V IPC已弃用，对于任何新项目，建议使用POSIX对应项(man 7 shm_overview和man 7 mq_overview(

• 客户端/服务器同步较弱：当服务器读取共享内存段时，客户端可能会覆盖该段。您应该使用互斥体来读取/写入共享内存段(当一个正在读取/写入时，另一个被阻止(：参见man 7 sem_overview

• 由于您没有在服务器中进行任何清理，请确保在应用程序的每次尝试之间，使用外壳下的ipcs/ipcrm删除队列和共享内存标识符

在服务器中：

• 错误检查是错误的：如果shmget((/msget返回-1并且errno等于EEXIST，则继续，但没有得到任何shm/msg标识符，因为它是-1

在客户端：

• 为了健壮性，最好使用snprintf((而不是sprintf((来强制检查缓冲区的边界

• 客户端不应将IPC_CREAT用于msgget((。通常，服务器的角色是在服务器/客户端应用程序中创建资源

这是您修改后的代码的C版本：

服务器：

#include <stdio.h>
#include <sys/shm.h>
#include <sys/msg.h>
#include <stdlib.h>
#include <semaphore.h>

const int SHM_SIZE=1024;
struct msg_form{
long  msg_type;
char msg_text[256];
};

static void unix_error(const char *str)
{
fprintf(stderr, "%sn", str);
exit(1);
}

int main(){
key_t key;
int shmid,msgid,ret_type = 888,sem_sz = sizeof(sem_t);
char *shm;

struct msg_form msg;
if((key = ftok(".",'v')) < 0)
unix_error("ftok error");

if((shmid = shmget(key,SHM_SIZE,IPC_CREAT|0666)) == -1)
unix_error("create shared memory error");
if((shm = (char *)shmat(shmid,0,0)) == (void*)-1){
unix_error("attach shared memory error");
}
if((msgid = msgget(key,IPC_CREAT|07777)) == -1)
unix_error("msgget error");
sem_init((sem_t *)(shm),1,1);
while(1){
msgrcv(msgid,&msg,256,ret_type,0);
sem_wait((sem_t *)(shm));
if (*(shm + sem_sz) == 'q' && *(shm + sem_sz + 1) == 'n') {
sem_post((sem_t *)(shm));
break;
}
printf("shared memory: %s", shm + sem_sz);
sem_post((sem_t *)(shm));
}
shmdt(shm);

shmctl(shmid,IPC_RMID,0);
msgctl(msgid,IPC_RMID,0);
return 0;
}

客户：

#include <stdio.h>
#include <sys/shm.h>
#include <sys/msg.h>
#include <stdlib.h>
#include <semaphore.h>
#include <string.h>

const int SHM_SIZE=1024;
struct msg_form{
long  msg_type;
char msg_text[256];
};

static void unix_error(const char *str)
{
fprintf(stderr, "%sn", str);
exit(1);
}

int main(){
key_t key;
int shmid,msgid,sem_sz = sizeof(sem_t);
char *shm;

int err;
struct msg_form msg;
char s[256];
if((key = ftok(".",'v')) < 0)
unix_error("ftok error");

if((shmid = shmget(key,SHM_SIZE,0)) == -1)
unix_error("shmget error");
if((shm = (char*)shmat(shmid,0,0)) == (void*)-1){
unix_error("attach shared memeory error");
}
if((msgid = msgget(key,0777)) == -1)
unix_error("msgget error");
printf("key is 0x%xn", (int)key);

while(fgets(s, sizeof(s) - 1, stdin)) {
sem_wait((sem_t *)(shm));
memset(shm+sem_sz,0,SHM_SIZE-sem_sz);
memcpy(shm+sem_sz,s, strlen(s));

msg.msg_type = 888;
snprintf(msg.msg_text, 256, "shared memory write signal");
if((err =  msgsnd(msgid,&msg,sizeof(msg.msg_text),0)) == -1) {
sem_post((sem_t *)(shm));
unix_error("msgsnd error");
}
sem_post((sem_t *)(shm));
}
return 0;
}