chown, fchown, fchownat, and lchown Functions

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本文详细介绍了Linux系统下通过chown、fchown、lchown及fchownat等系统调用实现文件所有者和组别变更的方法。阐述了这些函数在处理符号链接时的行为差异，并解释了_POSIX_CHOWN_RESTRICTED标志对所有权变更的影响。

chown, fchown, lchown, fchownat - change ownership of a file
用于改变user ID and group ID，如果某一个为-1，相应的ID保持不变。
原型如下

#include <unistd.h>

int chown(const char *pathname, uid_t owner, gid_t group);
int fchown(int fd, uid_t owner, gid_t group);
int lchown(const char *pathname, uid_t owner, gid_t group);

#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>

int fchownat(int dirfd, const char *pathname, uid_t owner, gid_t group, int flags);
//Returns： 0 if OK， -1 on error

这些系统调用在引用的文件是symbolic link时，才有所不同。在该情况下，lchown and fchownat（使用AT_SYMLINK_NOFOLLOW flag set）会改变链接文件本身的所有者，而不是链接文件指向的文件。
fchown function behaves like either chown or lchown when the pathname argument is absolute or when the fd argument has the value AT_FDCWD and the pathname argument is relative.In these cases, fchownat acts like lchown if the
AT_SYMLINK_NOFOLLOW flag is set in the flag argument, or it acts likechown if the AT_SYMLINK_NOFOLLOWflag is clear. When the fd argument is set to the file descriptor of an open directory and the pathname argument is a relative pathname, fchownat evaluates(评价) the pathname relative to the open directory.

Linux enforced the restriction that only the superuser can change the ownership of a file.

If _POSIX_CHOWN_RESTRICTED is in effect for the specified file, then
1. Only a superuser process can change the user ID of the file.
2. A nonsuperuser process can change the group ID of the file if the process owns the file (the effective user ID equals the user ID of the file), owner is specified as −1 or equals the user ID of the file, and group equals either the effective group ID of the process or one of the process’s supplementary group IDs.

This means that when _POSIX_CHOWN_RESTRICTED is in effect, you can’t change the user ID of your files. You can change the group ID of files that you own, but only to groups that you belong to.

If these functions are called by a process other than a superuser process, on successful return, both the set-user-ID and the set-group-ID bits are cleared.