英文:
C library message queue didn't read all messages
问题
我运行这段代码：
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/ipc.h>#include <sys/msg.h>#include <sys/types.h>// 为消息定义一个结构体struct message {long msg_type;char msg_text[100];};int main() {key_t key;int msg_id;struct message msg;// 为消息队列生成一个键key = ftok("progfile", 'A');// 创建消息队列msg_id = msgget(key, 0666 | IPC_CREAT);// 准备要发送的消息struct message messages[3] = {{1, "This is message with priority 1"},{2, "This is message with priority 2"},{3, "This is message with priority 3"}};// 发送消息for (int i = 0; i < 3; i++) {msgsnd(msg_id, &messages[i], sizeof(messages[i]), 0);}// 接收消息for (int i = 1; i <= 3; i++) {memset(&msg, 0, sizeof msg);if (msgrcv(msg_id, &msg, sizeof(msg), 0, IPC_NOWAIT) != -1) {printf("Received message: %s\n", msg.msg_text);} else {printf("No message in the queue\n");}}// 销毁消息队列msgctl(msg_id, IPC_RMID, NULL);return 0;}
我预期的输出应该是打印出所有三条消息，但实际上只打印出：
Received message: This is message with priority 1No message in the queueNo message in the queue
而且 msgctl 没有运行，因为当我运行 ipcs 时，即使这段代码已经运行完毕，我仍然可以看到消息队列仍然存在。
根据我的阅读，msgrcv(msg_id, &msg, sizeof(msg), 0, IPC_NOWAIT) 应该读取并删除队列中的第一条可用消息，类型参数设置为 0。但实际上，它只读取了一条消息，然后无法读取任何消息，这让我感到困惑。我使用 gdb 进行了调试，并发现即使消息队列仍然有消息，msgrcv 也无法检索消息。另一件奇怪的事情是，如果我将此代码拆分为两个文件，并在两个进程中运行发送和接收逻辑，接收进程可以读取所有 3 条消息。我不知道其中的差异是什么。谢谢您的任何帮助！
英文:
I'm running this code:
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/ipc.h>#include <sys/msg.h>#include <sys/types.h>// Define a structure for our messagesstruct message {long msg_type;char msg_text[100];};int main() {key_t key;int msg_id;struct message msg;// Generate a key for the message queuekey = ftok("progfile", 'A');// Create the message queuemsg_id = msgget(key, 0666 | IPC_CREAT);// Prepare messages to sendstruct message messages[3] = {{1, "This is message with priority 1"},{2, "This is message with priority 2"},{3, "This is message with priority 3"}};// Send the messagesfor (int i = 0; i < 3; i++) {msgsnd(msg_id, &messages[i], sizeof(messages[i]), 0);}// Receive the messagesfor (int i = 1; i <= 3; i++) {memset(&msg, 0, sizeof msg);if (msgrcv(msg_id, &msg, sizeof(msg), 0, IPC_NOWAIT) != -1) {printf("Received message: %s\n", msg.msg_text);} else {printf("No message in the queue\n");}}// Destroy the message queuemsgctl(msg_id, IPC_RMID, NULL);return 0;}
What I expect is it prints out all three messages, but actually it only prints:
Received message: This is a message with priority 1No message in the queueNo message in the queue
And msgctl did not run because when I ipcs, I can still see the message queue alive even after this code finished.
From what I read, msgrcv(msg_id, &msg, sizeof(msg), 0, IPC_NOWAIT) will read and remove the 1st message available, with the type arg set to 0. But actually, it reads only 1 message and then it cannot read any message, which confused me. I debugged using gdb, and found that even when the message queue still has messages, msgrcv won't retrieve the message. The other strange thing is, if I split this code into two files and run the send and read logic in two processes, the read process can read all 3 messages. I don't know what is the difference here. Thank you for any help!
答案1
得分: 2
错误出在msgsnd和msgrcv，它们应该是
msgsnd(msg_id, &messages[i], sizeof(messages[i].msg_text), 0);
和
if (msgrcv(msg_id, &msg, sizeof(msg.msg_text), 0, IPC_NOWAIT) != -1)
第三个参数msgsz应该只包含消息的msg_text的大小。
英文:
The error is in msgsnd and msgrcv, they should be
msgsnd(msg_id, &messages[i], sizeof(messages[i].msg_text), 0);
and
if (msgrcv(msg_id, &msg, sizeof(msg.msg_text), 0, IPC_NOWAIT) != -1)
The 3rd arg msgsz should only contain the size of the msg_text of the message.
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