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
     
      中的设置用户ID的方法的作用#include 
      
       #include 
       
        int real = 0;          // 实际用户IDint effective = 0;     // 有效用户IDint saved = 0;         // 保存的设置用户IDvoid showid();  // 打印所有IDinline 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。