在C中使用系统调用来比较2个用户输入的密码

Question

我是C语言的学习者，我需要使用shellforge编译代码，因此不能使用任何库函数，因为它们会提供错误。

有人可以帮我吗？

到目前为止，这是我所拥有的，在比较两个密码并将其写入文件之前，我需要帮助。为实现此目的而对我的代码进行的任何帮助/修改，并附带解释，将不胜感激。

#include <stdio.h>

int main()
{
 int fd;
 char username;
 char kbinput[KBINPUT_SIZE];
 char kbinputpassone[KBINPUTPASSONE_SIZE];
 char kbinputpasstwo[KBINPUTPASSTWO_SIZE];
 char passwordone;
 char passwordtwo;
 int count = 0;

 int ret;
 fd = open("test.txt", O_CREAT | O_RDWR,0666);
 if(fd == -1)
 {
  printf("Error!");
 exit(1);
 }

 write(1,"Enter the username",18);
do
{
   read(0, &username, 1);
  if (username == '\n')
  break;
  kbinput[count++] = username;
 }
while (count < KBINPUT_SIZE - 1);
kbinput[count] = 0;
write(fd,username,10);


 write(1,"Enter the password",18);
  count =0;
 do
 {
   read(0, &passwordone, 1);
  if (passwordone == '\n')
  break;
  kbinputpassone[count++] = passwordone;
 }
  while (count < KBINPUTPASSONE_SIZE - 1);
  kbinputpassone[count] = 0;


  write(1,"Enter the password again",25);
   count=0;
  do
  {
  read(0, &passwordtwo, 1);
  if (passwordtwo == '\n')
  break;
  kbinputpasstwo[count++] = passwordtwo;
 }
 while (count < KBINPUTPASSTWO_SIZE - 1);
 kbinputpasstwo[count] = 0;


 write(1,"The Passwords match",20);
 write(fd, passwordone, 15);

 return 0;

}