text: #include "kernel/types.h"
  #include "kernel/stat.h"
  #include "user/user.h"
  
  char buf[512];
  
  void cat(int fd){
    int n;
  
    while((n = read(fd, buf, sizeof(buf))) > 0) {
      if (write(1, buf, n) != n) {
        fprintf(2, "cat: write error\n");
        exit(1);
      }
    }
    if(n < 0){
      fprintf(2, "cat: read error\n");
      exit(1);
    }
  }
  
  int main(int argc, char *argv[]){
    int fd, i;
  
    if(argc <= 1){
      cat(0);
      exit(0);
    }
  
    for(i = 1; i < argc; i++){
      if((fd = open(argv[i], 0)) < 0){
        fprintf(2, "cat: cannot open %s\n", argv[i]);
        exit(1);
      }
      cat(fd);
      close(fd);
    }
    exit(0);
  }
  