#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>


/*the loop run by the "worker process" to execute 'system' commands*/
static int worker_loop(int fFile)
{
        if (fFile < 0) return -1;

        FILE *command_file = fdopen(fFile, "r+");

        if (!command_file) return -1;

        static char command_buffer[1024];

        while (fgets(command_buffer, sizeof command_buffer, command_file))
        {
        int outcome = system(command_buffer);
        fprintf(command_file, "%i\n", outcome);
        if (outcome < 0) break;
        }

        fclose(command_file);
        return 0;
}


/*a replacement for 'system' in the main process*/
static int fake_system(FILE *fFile, const char *cCommand)
{
        static char return_buffer[16];

        if (!fFile) return -1;
        fprintf(fFile, "%s\n", cCommand);

        if (fgets(return_buffer, sizeof return_buffer, fFile))
        return strtol(return_buffer, NULL, 10); /*(assumes only digits are read)*/

        else return -1;
}


int main(int argc, char *argv[])
{
        /*create a pair of sockets for IPC between the main and worker processes*/

        int duplex[2] = { -1, -1 };

        if (socketpair(PF_LOCAL, SOCK_STREAM, 0, duplex) == -1)
        {
        fprintf(stderr, "%s: couldn't create socket pair: %s\n", argv[0], strerror(errno));
        return 1;
        }

        fcntl(duplex[0], F_SETFD, fcntl(duplex[0], F_GETFD) | FD_CLOEXEC);
        fcntl(duplex[1], F_SETFD, fcntl(duplex[1], F_GETFD) | FD_CLOEXEC);


        /*create the worker process up front while memory usage is low*/
        pid_t worker = fork();


        if (worker < 0)
        {
        fprintf(stderr, "%s: couldn't create worker process: %s\n", argv[0], strerror(errno));
        close(duplex[0]);
        close(duplex[1]);
        return 1;
        }


        /*execute the worker-process loop*/
        if (worker == 0)
        {
        close(duplex[0]);
        return worker_loop(duplex[1]);
        }


        /*cleanup/preparation in the main process*/

        close(duplex[1]);

        FILE *worker_file = fdopen(duplex[0], "r+");
        if (!worker_file) return 1;

        int outcome;


        /*proceed with the rest of the program and call 'fake_system' as needed*/

        while ((outcome = fake_system(worker_file, "echo the time is: `date`")) >= 0)
        {
        fprintf(stderr, "[outcome=%i]\n", outcome);
        sleep(5);
        }

        fclose(worker_file);
        return 0;
}