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本文详细介绍了 setuid 函数的功能及使用方法，该函数用于设置当前进程的有效用户ID。如果调用者的有效用户ID为root，则同时设置真实和保存的用户ID。在Linux下，setuid 实现了 POSIX 版本的特性，允许程序暂时放弃所有权限，执行非特权操作后以安全方式重新获取原始的有效用户ID。

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地址：http://www.linuxmanpages.com/man2/setuid.2.php

NAME

setuid - set user identity

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

int setuid(uid_t uid);

DESCRIPTION

setuid sets the effective user ID of the current process. If the effective userid of the caller is root, the real and saved user ID's are also set.

Under Linux, setuid is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a setuid (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage the original effective user ID in a secure manner.

If the user is root or the program is setuid root, special care must be taken. The setuid function checks the effective uid of the caller and if it is the superuser, all process related user ID's are set to uid. After this has occurred, it is impossible for the program to regain root privileges.

Thus, a setuid-root program wishing to temporarily drop root privileges, assume the identity of a non-root user, and then regain root privileges afterwards cannot use setuid. You can accomplish this with the (non-POSIX, BSD) call seteuid.

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

EPERM

The user is not the super-user, and uid does not match the real or saved user ID of the calling process.

EAGAIN

The uid does not match the current uid and uid brings process over it's NPROC rlimit.

CONFORMING TO

SVr4, SVID, POSIX.1. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs. SVr4 documents an additional EINVAL error condition.

LINUX-SPECIFIC REMARKS

Linux has the concept of filesystem user ID, normally equal to the effective user ID. The setuid call also sets the filesystem user ID of the current process. See setfsuid(2).

If uid is different from the old effective uid, the process will be forbidden from leaving core dumps.