cap_t cap_get_file(const char *path_p);
int cap_set_file(const char *path_p, cap_t cap_p);
cap_t cap_get_fd(int fd);
int cap_set_fd(int fd, cap_t caps);
uid_t cap_get_nsowner(cap_t caps);
int cap_set_nsowner(cap_t caps, uid_t rootuid);
cap_set_file() and cap_set_fd() set the values for all capability flags for all capabilities for the pathname pointed to by path_p or the file open on descriptor fd, with the capability state identified by cap_p. The new capability state of the file is completely determined by the contents of cap_p. A NULL value for cap_p is used to indicate that capabilities for the file should be deleted. For these functions to succeed, the calling process must have the CAP_SETFCAP capability in its effective set and either the effective user ID of the process must match the file owner or the calling process must have the CAP_FOWNER capability in its effective capability set. The effects of writing the capability state to any file type other than a regular file are undefined.
A capability set held in memory can be associated with the root user ID in use in a specific user namespace. It is possible to get and set this value (in the memory copy) with cap_get_nsowner() and cap_set_nsowner() respectively. The root user ID is ignored by the libcap library in all cases other than when the capability is written to a file. Only if the value is non-zero will the library attempt to include it in the written file capability set.
cap_set_file() and cap_set_fd() return zero on success, and -1 on failure.
On Linux, the file Effective set is a single bit. If it is enabled, then all Permitted capabilities are enabled in the Effective set of the calling process when the file is executed; otherwise, no capabilities are enabled in the process's Effective set following an execve(2). Because the file Effective set is a single bit, if any capability is enabled in the Effective set of the cap_t given to cap_set_file() or cap_set_fd(), then all capabilities whose Permitted or Inheritable flag is enabled must also have the Effective flag enabled. Conversely, if the Effective bit is enabled on a file, then the cap_t returned by cap_get_file() and cap_get_fd() will have the Effective flag enabled for each capability that has the Permitted or Inheritable flag enabled.