fork()이후 자식 프로세스끼리의 시그널 보내기

#include <stdio.h>
#include <signal.h>
 
#define NUM_CHILD 3
 
void sig_usr1(int signo)
{
  printf("%d: I Received SIGUSR1(%d)\n\n", getpid(), SIGUSR1);
}
 
pid_t find_next_sibling()
{
    pid_t child[NUM_CHILD];
    char buf[16];
    char cmd[32];
 
    sprintf(cmd, "pgrep -P %d", getppid());
    FILE *fp = popen(cmd, "r");
    if(NULL == fp) {
        perror("popen() failed");
        return -1;
    }
 
    pid_t mine = getpid();
    int j=0, me=-1;
    while(fgets(buf, 16, fp)) {
        pid_t pid = (pid_t)atoi(buf);
        child[j++] = pid;
        if(pid == mine)
            me = pid;
    }
    pclose(fp);
 
    if(me != -1)
        return child[(me+1)%NUM_CHILD];
    else
        return 0;
}
 
int main()
{
    int i;
 
    signal(SIGUSR1, (void *)sig_usr1);
 
    for(i=0; i<NUM_CHILD; i++) {
        if(fork() ==  0)                /* break if child process */
            break;
    }
 
    pid_t pid_next;
    if(i < NUM_CHILD) {
        sleep(1);                       /* wait until every child process be created */
        pid_next = find_next_sibling();
 
        while(i-- > 0)                  /* make timing difference for each process */
            sleep(1);
 
        if(pid_next) {
            printf("%d: I'm sending SIGUSR1(%d) to process %d\n", getpid(), SIGUSR1, pid_next);
            kill(pid_next, SIGUSR1);
        }
    }
 
    for(i=0; i<10; i++)
        sleep(1);
 
    return 0;
}

$ ./sample2 &
[1] 24920
24921: I'm sending SIGUSR1(10) to process 24922
24922: I Received SIGUSR1(10)
 
24922: I'm sending SIGUSR1(10) to process 24923
24923: I Received SIGUSR1(10)
 
24923: I'm sending SIGUSR1(10) to process 24921
24921: I Received SIGUSR1(10)
$ pstree -p 24920
sample2(24920)─┬─sample2(24921)
                ├─sample2(24922)
                └─sample2(24923)