c - 在 C 中使用 Netcat 时数据丢失

Question

我有一个用于嵌入式协议(protocol)之一的 C 软件。在此，我需要将回调函数中传入的一些事件重定向到 TCP 服务器。于是，我想到了使用Netcat和系统命令。每当回调函数触发时，如果我保留一个字符串并将其转发到 netcat，那么我就会在 TCP 服务器上获取数据。但是，如果我使用回调函数中的参数构建命令，则只有第一次在 TCP 服务器上获取数据时才能看到连接成功和断开连接消息。

我的C代码是

static void zwp_avi_interfaces_alarm_report_handler(zwifd_p ifd, zwalrm_p alarm_info, time_t ts)
{

    zwp_avi_interfaces_alarm_state_t *state;
    zwifd_p desc_interface;

    char systemcomm[1500];

    char tcp_buf[100];
    sprintf(tcp_buf,"echo 'node id: %d alarm type: %d alarm_event: %d", ifd->nodeid, alarm_info->ex_event, alarm_info->ex_has_sequence);
    strcat(systemcomm,tcp_buf);
    strcat(systemcomm," ' | netcat localhost 9091");
    system(systemcomm);

}

TCP 服务器上的输出是

A new connection has been established.
Data received from client: node id: 7 alarm type: 22 alarm_event: 0 

Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client
A new connection has been established.
Closing connection with the client

在代码中，如果我将最后一个系统函数调用替换为 be

system("echo 'alarm_event' | netcat localhost 9091");

然后我得到这个回调触发了多少次。 TCP服务器输出如下

A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Closing connection with the client
A new connection has been established.
Data received from client: alarm_event

Answer 1

systemcomm 未初始化，因此 strcat(systemcomm,tcp_buf); 是未定义的行为。一切皆有可能。我们甚至可能无法理论上推测什么样的初始垃圾会导致此类症状。

如果您坚持使用系统，只需执行一个

    sprintf(systemcomm, "echo '....' | netcat localhost 9091, ....);