C Reading from pipe blocks before expiration

The parent process creates N children, each of which replaces exec. There is a relationship between the parent and exec through the pipe array ( int pipefd [N] [2]; )

exec writes these commands to the pipe:

char msg[50];
sprintf( msg, "\tsent from pid: %d, pi= %f", getpid(), pi);
printf("%s\n",msg);
write(i1, msg, strlen(msg)+1);

and the parent reads using this data:

for (i=0;i<N;i++) {
   close(pipefd[i][1]);  // close the write end of the pipe in the parent

   read(pipefd[i][0], buffer, sizeof(buffer));
   printf("\n-C-\n");

   if (buffer[0] == '\t'){
     printf("%s\n",buffer);
   }
   int j;
   for (j=0; j<100; j++) {
      buffer[j]='\n';
   }
   close(pipefd[i][0]);
}

Now the problem is that only after the child is completed, the reading breaks up and prints a buffer. What I want to do is print a buffer right after exec writes to the pipe.

Below is the code:

Parent file:

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

#define N 5
pid_t *pid;
int pipefd[N][2];

int flag = 0;
int count_ctrl_c = 0;
void signal_handler(int sig){
    signal(sig, SIG_IGN);
    printf("\n");

    flag = 1;
    signal(SIGINT, signal_handler);
}

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


    int i;
    for (i = 0; i < N; i++) {
        pipe(pipefd[i]);
    }

    int parent_pid = getpid();
    pid= (pid_t *)malloc(N * sizeof(pid_t));

    for (i = 0; i < N; i++) {
        pid[i] = fork();
        if (pid[i] == 0) //The parent process will keep looping
        {

            char b[50];
            sprintf( b, "%d", i+1);

            char i0[50];
            sprintf( i0, "%d", pipefd[i][0]);

            char i1[50];
            sprintf( i1, "%d", pipefd[i][1]);

            char par_id[50];
            sprintf( par_id, "%d", parent_pid);

            execl("***the/path/to/exec/calculate***", b,i0,i1,par_id,NULL);
        }
    }

    if (parent_pid == getpid()) {
        signal(SIGINT, signal_handler);


        while(1){
            if (flag){

                printf("\n-A-\n");
                char buffer[100];
                int i;
                for (i=0;i<N;i++) {
                    // Apostellei to shma SIGUSR2 se ola ta paidia tou
                    kill(pid[i], SIGUSR2);
                }

                printf("\n-B-\n");
                for (i=0;i<N;i++) {
                    close(pipefd[i][1]);  // close the write end of the pipe in the parent

                    read(pipefd[i][0], buffer, sizeof(buffer));
                    printf("\n-C-\n");
                    if (buffer[0] == '\t'){
                        printf("%s\n",buffer);
                    }
                    int j;
                    for (j=0; j<100; j++) {
                        buffer[j]='\n';
                    }
                    close(pipefd[i][0]);
                }
                //exit(0);
                printf("finished reading\n");

                flag = 0;
                count_ctrl_c++;
                if (count_ctrl_c == 2) {
                    exit(0);
                }
            }

        }
    }
}

calculate.c

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

#define N 5

int i0,i1,parent_pid;

int flag = 0;
int time_to_term = 0;
double pi;

void signal_handler2(int sig);
void signal_handler(int sig);

void signal_handler2(int sig){
    signal(sig, SIG_IGN);
    signal(SIGALRM, SIG_IGN);
    flag = 1;
    signal(SIGUSR2, signal_handler2);
    signal(SIGALRM, signal_handler);
}

void signal_handler(int sig){
    signal(sig, SIG_IGN);
    pid_t pid = getpid();
    printf("time: %d, pid: %d, pi: %1.10f\n", time_to_term, pid, pi);
    exit(0);
}

int main(int argc, char *argv[]) {
    int pid;
    signal(SIGINT, SIG_IGN);
    signal(SIGUSR2, signal_handler2);
    signal(SIGALRM, signal_handler);

    time_to_term = atoi(argv[0]);   
    alarm(time_to_term);

    i0 = atoi(argv[1]);
    i1 = atoi(argv[2]);
    parent_pid = atoi(argv[3]);

    double mul = 1.0;
    double par = 2.0;
    pi = 3.0;

    while(1){


        pi = pi + (mul * (4.0 / (par * (par + 1.0) * (par + 2.0))));
        mul = mul * (-1.0);
        par += 2;

        sleep(1);

         if (flag) {
            signal(SIGALRM, SIG_IGN);
            close(i0);

            char msg[50];
            sprintf( msg, "\tsent from pid: %d, pi= %f", getpid(), pi);
             printf("%s\n",msg);
            write(i1, msg, strlen(msg)+1);

            close(i1);
            flag = 0;

            signal(SIGALRM, signal_handler);
            //exit(0);


        }
    }

}