Shulou(Shulou.com)06/02 Report--

As mentioned last time, real-time communication cannot be achieved. Then it is OK to open two processes, one to listen for messages and one to wait for user input. So, let's review the concepts of process first.

Process structure

The process in linux contains PCB (process control block), program and the set of data structures manipulated by the program, which can be divided into "code segment", "data segment" and "stack segment".

Process status

Running status R (TASK_RUNNING)

Interruptible sleep state S (TASK_INTERRUPTIBLE)

Uninterruptible sleep state D (TASK_UNINTERRUPTIBLE)

Pause status T (TASK_STOPPED or TASK_TRACED)

Dead state Z (TASK_ZOMBIE)

Exit status X (TASK_DEAD)

Ps-aux looks at the process information and can see the status of each process:

Init process

The parent of all processes

The init process will never be terminated.

It is an ordinary user process (unlike the swap process, it is not a system process in the kernel), but it runs with superuser privileges.

Get process identity

# include

# include

Pid_t getpid (void); return: process ID of the calling process

Pid_t getppid (void); return: the parent process I D of the calling process

Uid_t getuid (void); return: actual user ID of the calling process

Uid_t geteuid (void); return: valid user ID of the calling process

Gid_t getgid (void); return: actual group ID of the calling process

Gid_t getegid (void); return: valid group ID of the calling process

Fork system call # include

# include

Pid_t fork (void); return: 0 in child process, I / D in parent process, error is-1

Note:

1. The child process obtained by using the fork function inherits the address space of the entire process from the parent process, including:

Process context, process stack, memory information, open file descriptor, signal control setting, process priority, process group number, current working directory, root directory, resource limit, control terminal, etc.

2. The difference between a child process and a parent process is:

1) the lock set by the parent process, and the child process does not inherit

2) the respective process ID and parent process ID are different

3) pending alarms for child processes are cleared

4) the pending signal set of the child process is set to an empty set.

3. After the fork system call, the parent and child processes will execute alternately.

4. If the parent process exits first, the child process has not exited. Then the parent process of the child process becomes the init process. (note: any process must have a parent)

5. If the child process exits first and the parent process has not quit, then the child process must wait until the parent process captures the exit state of the child process before it really ends, otherwise the child process will become a stiff process. (this is not good, so the process will take up memory resources all the time)

Solution:

1) wait function: blocks the parent process until a child process ends or until the process receives a specified signal. (however, this is not good)

2) signal (SIGCHLD,SIG_IGN). Indicates that the parent process ignores SIGCHLD. This signal is sent to the parent process when the child process exits and is received by the init process.

test

Next, we can continue our last Mini Program.

# include

# include

# include

# include "port.h" # include "readConfig.h" / / 1, host and virtual machine int main () {int fd,pid,child_id; char filename [20] = "serial.cfg"; char recbuf [100] = ""; char sendbuf [100] = ""; struct t_port port = {0}; / / get configuration information getMsg (filename,&port) from file / / Serial port initialization fd = portInit (port.devname,port.speed,port.data,port.parity,port.stop); / / Serial communication between host and virtual machine pid = fork (); if (pid =-1) {perror ("create process"); return 1;} else if (pid = = 0) / / child process receives message {child_id = getpid () While (read (fd,recbuf,100) > 0) / / strlen cannot be used here. Because the initial strlen (recbuf) = 0 {if (strcmp (recbuf, ")! = 0) {printf (" receive msg:% s\ n ", recbuf); putchar ('$'); fflush (stdout);} memset (recbuf,0100) }} else / / (main) parent process sends message {printf ("$"); while (1) {memset (sendbuf,0100); scanf ("% s", sendbuf) If (strcmp (sendbuf, "over") = = 0) / / but at this point, the child process is not finished and {break;} if (write (fd,sendbuf,strlen (sendbuf)) > 0) {printf ("$");}} close (fd) is still running in the background. Return 0;} / / 2, two serial ports of virtual machine / * int main (int argc,char * argv []) {int fd,pid; char receivebuf [100] = ""; char sendbuf [100] = ""; if (argc! = 2) {printf ("format error!\ n"); return 1 } if (strcmp (argv [1], "ttyS0") = = 0) {fd = portInit ("/ dev/ttyS0", 115200);} else if (strcmp (argv [1], "ttyS1") = = 0) {fd = portInit ("/ dev/ttyS1", 115200);} else {printf ("format error!\ n"); return 1;} pid = fork () If (pid =-1) {perror ("create process"); return 1 } else if (pid = = 0) / child process {/ / receive message while (read (fd,receivebuf,sizeof (receivebuf)) > 0) {if (strcmp (receivebuf, ")! = 0) {printf (" receive msg:% s\ n ", receivebuf);} memset (receivebuf,0100) }} else / / parent process {/ / send message while (1) {memset (sendbuf,0100); scanf ("% s", sendbuf); if (strcmp (sendbuf, "- over") = = 0) {break } write (fd,sendbuf,strlen (sendbuf)); printf ("send OK\ n");}} close (fd); return 0;} * /

Running result:

Did you find the problem? You're right. The parent process will die first.

When the program is running:

At the end of the program:

How to solve the problem? Say it next time.

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