UNIX Filesystems: Evolution, Design, and Implementation

Changing File Ownership

When a file is created, the user and group IDs are set to those of the caller. Occasionally it is useful to change ownership of a file or change the group in which the file resides. Only the root user can change the ownership of a file although any user can change the file's group ID to another group in which the user resides.

There are three calls that can be used to change the file's user and group as shown below:

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

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

The difference between chown() and lchown() is that the lchown() system call operates on the symbolic link specified rather than the file to which it points.

Table 2.1 Permissions Passed to chmod()

PERMISSION	DESCRIPTION
S_IRWXU	Read, write, execute/search by owner
S_IRUSR	Read permission by owner
S_IWUSR	Write permission by owner
S_IXUSR	Execute/search permission by owner
S_IRWXG	Read, write, execute/search by group
S_IRGRP	Read permission by group
S_IWGRP	Write permission by group
S_IXGRP	Execute/search permission by group
S_IRWXO	Read, write, execute/search by others
S_IROTH	Read permission by others
S_IWOTH	Write permission by others
S_IXOTH	Execute/search permission by others
S_ISUID	Set-user-ID on execution
S_ISGID	Set-group-ID on execution
S_ISVTX	On directories, set the restricted deletion flag

In addition ...