Just a note....

implementation by signal

#include <apue.h>

static volatile sig_atomic_t sigflag;

static sigset_t newmask,oldmask,zeromask;

static void sig_usr(int signo)
{
        sigflag = 1;
}

void TELL_WAIT(void)
{
        struct sigaction act_usr1,act_usr2;

        sigemptyset(act_usr1.sa_mask);
        sigemptyset(act_usr2.sa_mask);

        act_usr1.flags = 0;
        act_usr2.flags = 0;

        act_usr1.sa_handler = sig_usr;
        act_usr2.sa_handler = sig_usr;

        if(sigaction(SIGUSR1,&act_usr1,NULL) < 0)
        {
                printf("sigaction error
");
        }

        if(sigaction(SIGUSR2,&act_usr2,NULL) < 0)
        {
                printf("sigaction error
");
        }

        sigemptyset(&zeromask);
        sigemptyset(&newmask);

        sigaddset(&newmask,SIGUSR1);
        sigaddset(&newmask,SIGUSR2);

        if(sigprocmask(SIG_BLOCK,&newmask,&oldmask) < 0)
        {
                printf("sigprocmask error
");
        }

}

void TELL_PARENT(void)
{
        kill(pid,SIGUSR2);
}


void WAIT_PARENT(void)
{
        while(sigflag == 0)
        {
                sigsuspend(&zeromask);
        }


        sigflag = 0;


        if(sigprocmask(SIG_SETMASK,&oldmask,NULL) < 0)
        {
                printf("sigprocmask error
");
        }
}


void TELL_CHILD(pid_t pid)
{
        kill(pid,SIGUSR1);
}


void WAIT_CHILD(void)
{
        while(sigflag == 0)
        {
                sigsuspend(&zeromask);
        }


        sigflag = 0;


        if(sigprocmask(SIG_SETMASK,&oldmask,NULL) < 0)
        {
                err_sys("SIG_SETMAKS error
");
        }
}

implementation by pipe

#include <stdio.h>
#include <fcntl.h>

static pfd1[2],pfd2[2];

void TELL_WAIT(void)
{
        if(pipe(fpd1) < 0 || pipe(pfd2) < 0)
        {
                printf("pipe error
");
        }
}

void TELL_PARENT(void)//tell to parent
{
        if(write(pfd2[1],"c",1) != 1)
        {
                printf("write error
");
        }
}

void WAIT_PARENT(void)//wait for parent
{
        char c;

        if(read(pfd1[0],&c,1) != 1)
        {
                printf("read error
");
        }

        if(c != 'p')
        {
                printf("WAIT_PARENT: incorrect data
");
        }
}

void TELL_CHILD(pid_t pid)//tell to child
{
        if(write(pfd1[1],"p",1) != 1)
        {
                printf("write error
");
        }
}

void WAIT_CHILD(void)//wait for child
{
        char c;
        if(read(pdf2[0],&c,1) != 1)
        {
                printf("read error
");
        }

        if(c != 'c')
        {
                printf("WAIT_CHILD : incorrect data
");
        }
}