6.087 Practical Programming in C, lec14
Linux inter process communication
Preliminaries
• Each process has its own address space. Therefore, individualprocesses cannot communicate unlike threads.
• Interprocess communication: Linux/Unix provides several waysto allow communications
-
signal
pipes
FIFO queues
shared memory
semaphores
sockets
<signals.h>
• Unix/Linux allows us to handle exceptions that arise duringexecution (e.g., interrupt, floating point error, segmentationfault etc.).
• A process recieves a signal when such a condition occurs.
void (∗signal( int sig,void(∗handler)(int )))( int )
• determines how subsequent signals will be handled.
• pre-defined behavior: SIG_DFL (default), SIG_IGN (ignore)
• returns the previous handler.
这里的exceptions应该就是运行时异常吧，从signal的类型可以得到运行时异常的类别，这个在实际应用中和工作面试中都会经常用到哦：）
Valid signals:
SIGABRT abnormal termination
SIGFPE floating point error
SIGILL illegal instruction
SIGINT interrupt
SIGSEGV segmentation fault
SIGTERM termination request
SIGBUS bus error
SIGQUIT quit
The two signals SIGSTOP,SIGKILL cannot be handled.
<signal.h>
int raise( int sig) can be used to send signal sig to the program.
Notes:
• There can be race conditions.
• signal handler itself can beinterrupted.
• use of non-reentrant functionsunsafe.
• sigprocmask can be used toprevent interruptions.
• handler is reset each time it iscalled.
signal实际上存在共享数据：操作系统中的signal记录，因此线程同步的问题也存在于signal中。
Example
#include <stdio.h>
void sigproc()
{
printf("you have pressed ctrl−c \n");
}
void quitproc()
{
printf("ctrl−\\ pressed toquit");
exit(0); ∗normal exit status∗/
}
main ( )
{
signal(SIGINT, sigproc);
signal(SIGQUIT, quitproc);
printf(‘‘ctrl−c disabled use ctrl−\\ to quit\n’’);
for( ; ; ); /∗infinite loop∗/
}
signal方法的作用是定义接受到某种信号时对应的操作。
Fork
pid_t fork (void)
• fork() is a system call to createa new process
• In the child process, it returns 0
• In the parent process, it returnsthe PID (process id) of the child.
• The child PID can be used to sendsignals to the child process.
• returns -1 on failure (invalidPID)
原来创建一个子进程是如此简单，只需复制自己即可。操作系统自身应该是所有进程的祖先进程。
Example
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
int main(){
pid_t pid =fork();
int i;
if(pid){
for(i=0;i<5; i++){
sleep(2);
}
printf("parent process: %d\n", i);
}
运行结果：
childprocess: 0
parent process: 0
child process: 1
child process: 2
parent process: 1
child process: 3
child process: 4
parent process: 2
parent process: 3
parent process: 4
Fork
• fork() makes a full copy of the parents address space.
• pid_t getpid() returns PID of the current process.
• pid_t getppid() returns PID of the parent process.
• wait( int∗) is used to wait for the child to finish.
• waitpid () is used to wait for a specific child.
Zombies:
• the child process can exit before the parent
• stray process is marked as <defunct>
• preap can be used to reap zombie processes.
子进程可以在父进程结束之前结束，父进程能否在子进程之前结束呢？我很怀疑，因为子进程有父进程的PID，如果父进程没了，那么这个PID就为空，看起来不怎么舒服。
Pipes
Pipes are used in unix to redirect output of one command toanother. Pipes also allow parent processes to communicate with itschildren.
Examples
• ls | more - displays results of lsone screen at a time
• cat file.txt | sort -displayscontents of file.txt in sorted order
int pipe(int FILEDES[2])
• A pipe can be thought of as a pairof file descriptors
• no physical file is associatedwith the file descriptor
• one end is opened in write mode.
• other end is opened in read mode.
管道在UNIX中是一个十分重要的工具，实现简单但功能强大，很符合UNIX的style。其实现可以看作用一根两端为fd(filedescriptor)的管道将两个进程连接起来，管道前面的进程会有输出到一个文件，而后面的进程则会读取一个文件。
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
int main(void)
{
int pfds[2];
char buf[30];
if (pipe(pfds) == -1) {
perror("pipe");
exit(1);
}
printf("writing to file descriptor#%d\n", pfds[1]);
write(pfds[1], "test", 5);
printf("reading from filedescriptor #%d\n", pfds[0]);
read(pfds[0], buf, 5);
printf("read \"%s\"\n",buf);
return 0;
}
FIFO
• FIFO queues may be thought of as named pipes.
• Multiple processes can read and write from a FIFO.
• Unlike pipes, the processes can be unrelated.
• FIFOs can be created using mknod system call.
intmknod (const char ∗path,mode_t mode,dev_t dev)
• <sys/stat.h> contains the declaration for mknod.
• mknod used to create special files - devices,fifos etc.
• mode can have special bits such as S_IFIFO | 0644
• dev is interpreted based on the mode.
Example: mknod("myfifo", S_IFIFO | 0644 , 0);
FIFO和管道本质上都是使用读写同一个文件的方法实现线程间的通信。
Example
