#include <sys/signalfd.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

static void
transfer_line(FILE* from) {
    char buffer[4096];
    memset(buffer, 0, sizeof(buffer));
    fgets(buffer, sizeof(buffer), from);
    fputs(buffer, stdout);
    fflush(stdout);
}

int main(int argc, char** argv) {
    FILE* files[argc-1];
    for (int i = 1; i < argc; i++) {
        files[i - 1] = fopen(argv[i], "r");
    }
    sigset_t block, catch;
    sigfillset(&block);
    sigemptyset(&catch);

    for (int i = 0; i < argc; i++) {
        sigaddset(&catch, SIGRTMIN + i);
    }
    sigprocmask(SIG_BLOCK, &block, NULL);
    int fd = signalfd(-1, &catch, 0);
    struct signalfd_siginfo si;
    int index;

    for(;;) {
        read(fd, &si, sizeof(si));
        index = si.ssi_signo - SIGRTMIN;
        if (0 == index) {
            break;
        } else {
            transfer_line(files[index-1]);
        }
    }
    return 0;
}