diff --git a/lib/libc/sys/semctl.2 b/lib/libc/sys/semctl.2
--- a/lib/libc/sys/semctl.2
+++ b/lib/libc/sys/semctl.2
@@ -23,7 +23,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 9, 2020
+.Dd October 2, 2023
 .Dt SEMCTL 2
 .Os
 .Sh NAME
@@ -79,6 +79,15 @@
 .Fa "struct semid_ds" ,
 storing it in the memory pointed to by
 .Fa arg.buf .
+.It Dv SEM_STAT
+Return a
+.Fa "semid_ds"
+structure as for
+.Dv IPC_STAT .
+However, for this command we assume
+.Fa semid
+is an array index in the kernel's internal array that maintains information
+about all semaphore sets on the system rather than an id of a semaphore set.
 .It Dv IPC_SET
 Changes the
 .Fa sem_perm.uid ,
@@ -161,7 +170,14 @@
 or
 .Dv GETZCNT ,
 .Fn semctl
-returns the corresponding value; otherwise, 0 is returned.
+returns the corresponding value, when
+.Fa cmd
+is
+.Dv SEM_STAT
+.Fn semctl
+returns the identifier of the semaphore set whose index was given in
+.Fa semid ;
+otherwise, 0 is returned.
 On failure, -1 is returned, and
 .Va errno
 is set to indicate the error.