无法使用c中的多个管道从进程读取消息

Question

Im creating pipes for multiple children. The code catches a signal fro the keyboard and then the child should sent a message to the father with its result. The handling of the signal works fine but i cant sent the result to the father. I am i doing something wrong here?

The code of the father process is :

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>

pid_t *childs; //array for storing childs pids
int number_of_childs;//variable for the number of childs
int exit_count=0;
int *fd;

/*Handler for the SIGINT signal*/

void control_handler(int sig)
{
if (sig==SIGINT){

    int j,bytes;
    char message[512];
    /*Sending SIGUSR1*/

    for (j=0;j<number_of_childs;j++)
    {
    kill(childs[j],SIGUSR1);
    }

    /*Reading from PIPES*/

    for (j=0;j<number_of_childs;j++)
    {   
    close(fd[(2*j)+1]);  
    bytes = read(fd[(2*j)], message, sizeof(message));
    printf("Read from: %s\n",message);
    close(fd[2*j]);
    }
}

if (sig==SIGUSR2)
{
exit_count++;
}

}

main (int argc,char *argv[]){

int i,child_status;
char cast[512];
char cast2[512];
char cast3[512];
int pid;
number_of_childs=atoi(argv[1]);

signal(SIGINT,control_handler);
signal(SIGUSR2,control_handler);

/*Creating array for children pipes*/
fd=(int*)malloc((2*number_of_childs)*sizeof(int));


/*array that holds the pids for every child used in sending signals*/
childs=malloc(number_of_childs*sizeof (pid_t));



for (i=0;i<number_of_childs;i++){
pid=fork();
    /*Create pipes to communicate with all children*/

    if(pipe(fd+(2*i))==-1)
    {
    perror("pipe");exit(1);
    }

    /*Fathers code goes here*/

    if(pid!=0)
        {
        printf("Parent process: PID= %d,PPID=%d, CPID=%d \n",getpid(),getppid(),pid);
        childs[i]=pid; // Keep all your childs in an array
        printf("Child:%d\n",childs[i]); 
        }

    /*If you are a child*/

        else 
        {
        /*Change the code for the childs and set the time of execution*/
        sprintf(cast,"%d",i+1); // make the time char
        sprintf(cast2,"%d",(2*i)+1);    //make the pipe char
        sprintf(cast3,"%d",number_of_childs);   
        execl("./Child.out","",cast,cast2,cast3,NULL);
        }
    }   
        /*Father should never terminate*/               
        while (exit_count!=number_of_childs);
        printf("Father pospastex!!\n");

}

the code for the child children is :

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <time.h> 
#include <string.h>
#define WRITE 0
#define READ 1


/*Global declerations*/

int alarmflag=0;
double result=0;
int count=0;
int global_pipe;
int *fd;

/*Handler for the alarm and SIGUSR1 signal*/
void signal_handler (int sig)
{

if(sig==SIGALRM)
{
printf("Im child with pid:%d im going to die my value is %lf \n",getpid(),result);
alarmflag=1;
}

if(sig==SIGUSR1)
{
count++;
char message[512];

    if(count==1)
    {
    close(fd[global_pipe-1]);
    sprintf(message,"%d,%lf",getpid(),result);
    write(fd[global_pipe],message,strlen(message)+1);
    close(fd[global_pipe]); 
    //printf("PID:%d report: %lf\n",getpid(),result);
    }
    if(count==2)
    {
    close(fd[global_pipe-1]);
    sprintf(message,"%d,%lf",getpid(),result);
    write(fd[global_pipe],message,strlen(message)+1);
    close(fd[global_pipe]); 
    //printf("PID:%d report2 : %lf\n",getpid(),result);
    //kill(getppid(),SIGUSR2);
    //exit(0);
    }
}
if(sig==SIGINT)
{
/*Do nothing*/ 
}

}


double p_calculation ()
{
 int i=2;
 result=3;
 double prosimo=-1;

 while(!alarmflag)
    {
 prosimo=prosimo*(-1);
 result=result+(prosimo*(4/((double)i*((double)i+1)*((double)i+2))));
 i=i+2;
    }

}

main(int argc,char *argv[])
{
int pipe;
int size_fd;

size_fd=(atoi(argv[3]));
pipe=(atoi(argv[2]));
global_pipe=pipe;

fd=(int*)malloc(size_fd*sizeof(int));

/*handling signals*/
signal(SIGALRM,signal_handler);
signal(SIGUSR1,signal_handler);
signal(SIGINT,signal_handler);

/*Notify for execution time*/
printf("PID : %d with PPID : %d executing for %d seconds \n",getpid(),getppid(),atoi(argv[1]));
//printf("pipe:%d\n",pipe);
/*end this after the value passed as argument*/
alarm(atoi(argv[1]));
p_calculation();

/*Notify for finish*/

printf("Done!!!\n");

}

Answer 1

There are a number of problems related to your pipes:

• you call pipe AFTER you call fork , so you end up with two independent pipes (one in the child and one in the parent). The parent listens to the one it creates (and noone is writing to), so it never sees anything.

• you pass the index in your global fd array to the child rather than the file descriptor of the pipe. The child has its own global fd array that contains random garbage, so you are essentially writing to a random file descriptor rather than the pipe.

• you don't close the unneeded pipe ends in the various processes they exist in, so you cna never reliably get EOFs

If you search for pipe+child here on stackoverflow, you'll see a large number of questions with example code trying to do things similar to what you are doing -- you may find it useful to read through those questions and answers.