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     cap_rights_limit — limit capability rights


     Standard C Library (libc, -lc)


     #include <sys/capsicum.h>

     cap_rights_limit(int fd, const cap_rights_t *rights);


     When a file descriptor is created by a function such as accept(2), accept4(2), fhopen(2),
     kqueue(2), mq_open(2), open(2), openat(2), pdfork(2), pipe(2), shm_open(2), socket(2) or
     socketpair(2), it is assigned all capability rights.  Those rights can be reduced (but never
     expanded) by using the cap_rights_limit() system call.  Once capability rights are reduced,
     operations on the file descriptor will be limited to those permitted by rights.

     The rights argument should be prepared using cap_rights_init(3) family of functions.

     Capability rights assigned to a file descriptor can be obtained with the cap_rights_get(3)

     The complete list of the capability rights can be found in the rights(4) manual page.


     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and
     the global variable errno is set to indicate the error.


     The following example demonstrates how to limit file descriptor capability rights to allow
     reading only.

     cap_rights_t setrights;
     char buf[1];
     int fd;

     fd = open("/tmp/foo", O_RDWR);
     if (fd < 0)
             err(1, "open() failed");

     if (cap_enter() < 0)
             err(1, "cap_enter() failed");

     cap_rights_init(&setrights, CAP_READ);
     if (cap_rights_limit(fd, &setrights) < 0)
             err(1, "cap_rights_limit() failed");

     buf[0] = 'X';

     if (write(fd, buf, sizeof(buf)) > 0)
             errx(1, "write() succeeded!");

     if (read(fd, buf, sizeof(buf)) < 0)
             err(1, "read() failed");


     cap_rights_limit() succeeds unless:

     [EBADF]            The fd argument is not a valid active descriptor.

     [EINVAL]           An invalid right has been requested in rights.

     [ENOTCAPABLE]      The rights argument contains capability rights not present for the given
                        file descriptor.  Capability rights list can only be reduced, never


     accept(2), accept4(2), cap_enter(2), fhopen(2), kqueue(2), mq_open(2), open(2), openat(2),
     pdfork(2), pipe(2), read(2), shm_open(2), socket(2), socketpair(2), write(2),
     cap_rights_get(3), cap_rights_init(3), err(3), capsicum(4), rights(4)


     The cap_rights_limit() function first appeared in FreeBSD 8.3.  Support for capabilities and
     capabilities mode was developed as part of the TrustedBSD Project.


     This function was created by Pawel Jakub Dawidek <> under sponsorship of
     the FreeBSD Foundation.