#include <cstdio>#include <sys/stat.h>#include <cerrno>#include <fcntl.h>#include <unistd.h>#include <iostream>#include <stdlib.h>using namespace std;int main(int argc, char *argv[]){   int fd;   int n;   const char*mark = "This is child process write.";   char buf[50];   if (argc != 2)    {      cout << "Usage: child <filename> " << endl;      exit(0);    }   fd = atoi(argv[1]);     if(fd < 0)    {      perror("file descriptor error");      exit(1);    }    n = write(fd,mark, strlen(mark));   if (n == -1)     {      perror("child write mark error");      exit(1);    }   lseek(fd,0,SEEK_SET);      n = read(fd, buf, sizeof(buf));   if (n == -1)     {     perror("read error");     exit(1);    }      buf[n]='\0';   cout << "Read the message is: "<< buf << endl;   close(fd);   }