就是模拟setuid seteuid setreuid setresuid，感觉代码比书上大段的文字好记，就写成代码形式了。

// setuid.cc: 模拟<unistd.h>中的设置用户ID的方法的作用

#include <stdio.h>

#include <errno.h>

int real = 0;          // 实际用户ID

int effective = 0;     // 有效用户ID

int saved = 0;         // 保存的设置用户ID

void showid();  // 打印所有ID

inline bool hasPrivilege() { return effective == 0; }

#define EINVAL_RETURN { errno = EINVAL; return -1; }

#define EPERM_RETURN { errno = EPERM; return -1; }

int setuid(int uid)  // 修改所有用户ID

{

if (uid < 0)

EINVAL_RETURN;

if (!hasPrivilege() && uid != real && uid != saved)

EPERM_RETURN;

if (hasPrivilege())

real = effective = saved = uid;

else

effective = uid;

return 0;

}

int seteuid(int euid)  // 修改有效用户ID

{

if (euid < 0)

EINVAL_RETURN;

if (!hasPrivilege() && euid != real && euid != saved)

EPERM_RETURN;

effective = euid;

return 0;

}

int setreuid(int ruid, int euid)  // 修改实际/有效用户ID

{

if (ruid < -1 || euid < -1)

EINVAL_RETURN;

if (!hasPrivilege())

{

if (ruid != -1 && ruid != real && ruid != effective)

EPERM_RETURN;

if (euid != -1 && euid != real && euid != effective && euid != saved)

EPERM_RETURN;

}

real = (ruid != -1) ? ruid : real;

effective = (euid != -1) ? euid : effective;

if (ruid != -1 || effective != real)

saved = effective;

return 0;

}

// 非SUSv3规范, 其他UNIX实现对其也鲜有支持

int setresuid(int ruid, int euid, int suid)  // 修改实际/有效/保存用户ID

{

if (ruid < -1 || euid < -1 || suid < -1)

EINVAL_RETURN;

if (!hasPrivilege())

{

if (ruid != -1 && ruid != real && ruid != effective && ruid != saved)

EPERM_RETURN;

if (euid != -1 && euid != real && euid != effective && euid != saved)

EPERM_RETURN;

if (suid != -1 && suid != real && suid != effective && suid != saved)

EPERM_RETURN;

}

real = (ruid != -1) ? ruid : real;

effective = (euid != -1) ? euid : effective;

saved = (suid != -1) ? suid : saved;

return 0;

}

int main()

{

real = 1000;

// 下面4句只能执行其中1句

//    setuid(2000);

setreuid(-1, 2000);

//    seteuid(2000);

//    setresuid(-1, 2000, 3000);

showid();

return 0;

}

void showid()

{

printf("实际用户ID:       %4d\n", real);

printf("有效用户ID:       %4d\n", effective);

printf("保存的设置用户ID: %4d\n", saved);

}

main函数是TLPI第9章习题第1道的运行结果，然后模拟了一遍功能，后面几道也很简单就能做出来了。以后忘记的话看遍代码就能很快记起来了。