[英]Signal from parent to its child processes in Linux
I will write shortly what i want to do with my code and what's the real output. 我将很快写出我想用我的代码做什么以及真正的输出是什么。 I have a parent process which creates two child processes. 我有一个创建两个子进程的父进程。 Each of the two process read 100 bytes from a different file, child 1 read from "child1.txt", child 2 read from "child2.txt". 两个进程中的每个进程均从不同文件读取100字节,子进程1从“ child1.txt”读取,子进程2从“ child2.txt”读取。 I want to force to execute child 2 before child 1 through the use of signals. 我想通过使用信号强制在子1之前执行子2。
I created the two child process and paused them (pause end when they receive a signal). 我创建了两个子进程并暂停了它们(当它们收到信号时,暂停结束)。 So I sent a signal to the child 2, then i sent a signal to the child 1. In this way I expect they will do what i want but they don't (it shows always child1.txt content)and i do not understand why. 因此,我向孩子2发送了一个信号,然后我向孩子1发送了一个信号。这样,我希望他们会做我想要的,但他们不做(它始终显示child1.txt的内容),我听不懂为什么。
This is the code 这是代码
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <fcntl.h>
#include <wait.h>
void my_handler(int signo);
int main(int argc, char *argv[]){
signal(SIGCHLD,my_handler);
pid_t pid[2]; //here is where I save child pid
char buffer[100];
for (int i=0;i<2;i++){
if((pid[i]=fork())==0){
signal(SIGUSR1,my_handler);
int fd,nbyte;
pause();
if(i==0){
fd=open("Child1.txt",O_RDONLY);
}else{
fd=open("Child2.txt",O_RDONLY);
}
nbyte=read(fd,buffer,100);
buffer[nbyte-1]='\0';
write(STDOUT_FILENO,buffer,strlen(buffer));
exit(1);
}
sprintf(buffer,"Child %d\n",(int)pid[i]);
write(STDOUT_FILENO,buffer,strlen(buffer));
}
kill(pid[1],SIGUSR1); //I send a signal to unpause child2
wait(NULL); // wait child 2 to terminate
kill(pid[0],SIGUSR1); //unpause child 1
wait(NULL); // wait child 1 to terminate
}
void my_handler(int signo){
}
the following proposed code:: 下面的建议代码:
and now the proposed code: 现在建议的代码:
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <fcntl.h>
#include <wait.h>
#define BUF_SIZE 100
#define NUM_CHILDREN 2
void my_handler(int signo);
int main( void )
{
//signal(SIGCHLD,my_handler);
pid_t pid[ NUM_CHILDREN ] = {0};
char buffer[ BUF_SIZE +1];
for ( int i=0; i<NUM_CHILDREN; i++ )
{
if((pid[i]=fork())==0)
{ // then child
signal(SIGUSR1,my_handler);
int fd;
ssize_t nbyte;
pause();
if(i==0)
{
if( (fd=open("child1.txt",O_RDONLY) ) == -1 )
{
perror( "open to read child1.txt failed" );
exit( EXIT_FAILURE );
}
}
else
{
if( (fd=open("child2.txt",O_RDONLY) ) == -1 )
{
perror( "open to read child2.txt failed" );
exit( EXIT_FAILURE );
}
}
if( (nbyte=read(fd,buffer,100) ) ==-1 || nbyte == 0 )
{
perror( "read failed" );
exit( EXIT_FAILURE );
}
buffer[nbyte]='\0';
write(STDOUT_FILENO,buffer,strlen(buffer));
fflush( stdout );
close( fd );
exit( EXIT_SUCCESS );
}
else if( pid[i] > 0 )
{ // then parent
sprintf( buffer,"Child %d PID %d\n", i, (int)pid[i] );
write( STDOUT_FILENO, buffer, strlen(buffer) );
fflush( stdout );
}
else
{ // fork failed
perror( "fork failed" );
}
}
sleep(1); // allow all child processes to be ready for signal
if( pid[0] > 0 )
{
kill(pid[0],SIGUSR1); //send a signal to unpause child 0
wait( NULL );
}
if( pid[1] > 0 )
{
kill(pid[1],SIGUSR1); //unpause child 1
wait( NULL );
}
}
void my_handler(int signo)
{
(void)signo;
}
contents of child1.txt (note I used all lower case for the file name) child1.txt的内容(请注意,我使用所有小写字母作为文件名)
123 123
contents of child2.txt (note I used all lower case for the file name) child2.txt的内容(请注意,我使用所有小写字母作为文件名)
456 456
a run of the code results in: 运行这些代码会导致:
Child 0 6630
Child 1 6631
123
456
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