Various System V IPC pages: Improve wording describing RETURN VALUE and setting of 'errno'

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/msgctl.2

@@ -331,9 +331,9 @@ or
 operation returns the identifier of the queue whose index was given in
 .IR msqid .
 .PP
-On error, \-1 is returned with
+On failure, \-1 is returned and
 .I errno
-indicating the error.
+is set to indicate the error.
 .SH ERRORS
 On failure,
 .I errno

man2/msgget.2

@@ -144,11 +144,12 @@ If the message queue already exists the permissions are
 verified, and a check is made to see if it is marked for
 destruction.
 .SH RETURN VALUE
-If successful, the return value will be the message queue identifier (a
+On success,
-nonnegative integer), otherwise \-1
+.BR msgget ()
-with
+returns the message queue identifier (a nonnegative integer).
+On failure, \-1 is returned, and
 .I errno
-indicating the error.
+is set to indicate the error.
 .SH ERRORS
 On failure,
 .I errno

man2/msgop.2

@@ -336,11 +336,7 @@ is decremented by 1.
 .I msg_rtime
 is set to the current time.
 .SH RETURN VALUE
-On failure both functions return \-1
+On success,
-with
-.I errno
-indicating the error,
-otherwise
 .BR msgsnd ()
 returns 0
 and
@@ -348,6 +344,9 @@ and
 returns the number of bytes actually copied into the
 .I mtext
 array.
+On failure, both functions return \-1, and set
+.I errno
+to indicate the error.
 .SH ERRORS
 When
 .BR msgsnd ()

man2/semctl.2

@@ -395,14 +395,9 @@ If the changes to semaphore values would permit blocked
 calls in other processes to proceed, then those processes are woken up.
 The calling process must have alter permission on the semaphore set.
 .SH RETURN VALUE
-On failure,
+On success,
 .BR semctl ()
-returns \-1
+returns a nonnegative value depending on
-with
-.I errno
-indicating the error.
-.PP
-Otherwise, the system call returns a nonnegative value depending on
 .I cmd
 as follows:
 .TP
@@ -447,6 +442,12 @@ as for
 All other
 .I cmd
 values return 0 on success.
+.PP
+On failure,
+.BR semctl ()
+returns \-1 and sets
+.I errno
+to indicate the error.
 .SH ERRORS
 On failure,
 .I errno

man2/semget.2

@@ -155,11 +155,12 @@ If the semaphore set already exists, the permissions are
 verified.
 .\" and a check is made to see if it is marked for destruction.
 .SH RETURN VALUE
-If successful, the return value will be the semaphore set identifier
+On sucess,
-(a nonnegative integer), otherwise, \-1
+.BR semget ()
-is returned, with
+ireturns the semaphore set identifier (a nonnegative integer).
+On failure, \-1 is returned, and
 .I errno
-indicating the error.
+is set to indicate the error.
 .SH ERRORS
 On failure,
 .I errno

man2/semop.2

@@ -297,15 +297,14 @@ the contents of
 .IR timeout
 are left unchanged.
 .SH RETURN VALUE
-If successful,
+On success,
 .BR semop ()
 and
 .BR semtimedop ()
-return 0;
+return 0.
-otherwise they return \-1
+On failure, they return \-1, and set
-with
 .I errno
-indicating the error.
+to indicate the error.
 .SH ERRORS
 On failure,
 .I errno