unistd: introduce nice()

This commit implements nice() as specified in POSIX.1-2008.

Fixes #4932.

Change-Id: Ibd8d4636d9e3a8adf0f26a081d5b1180f0cbb839
Reviewed-on: https://review.haiku-os.org/c/863
Reviewed-by: Jérôme Duval <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/posix/unistd.h

@@ -246,6 +246,8 @@ extern pid_t	setpgrp(void);
 
 extern int		chroot(const char *path);
 
+extern int		nice(int incr);
+
 /* access permissions */
 extern gid_t	getegid(void);
 extern uid_t	geteuid(void);

src/system/libroot/posix/unistd/Jamfile

@@ -30,6 +30,7 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			lockf.cpp
 			lseek.c
 			mount.c
+			nice.c
 			pause.c
 			pipe.c
 			process.c

src/system/libroot/posix/unistd/nice.c

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2019, Leorize <[email protected]>. All rights reserved.
+ * Distributed under the terms of the MIT license.
+ */
+
+
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
+#include <limits.h>
+#include <sys/resource.h>
+#include <unistd.h>
+
+#include <sys/param.h> /* MAX(), MIN() */
+
+
+/* setpriority() is used for the implementation as they share the same
+ * restrictions as defined in POSIX.1-2008. However, some restrictions might not
+ * be implemented by Haiku's setpriority(). */
+int
+nice(int incr)
+{
+	int priority = incr;
+
+	/* avoids overflow by checking the bounds beforehand */
+	if (priority > -(2 * NZERO - 1) && priority < (2 * NZERO - 1))
+		priority += getpriority(PRIO_PROCESS, 0);
+
+	priority = MAX(priority, -NZERO);
+	priority = MIN(priority, NZERO - 1);
+
+	return setpriority(PRIO_PROCESS, 0, priority) != -1 ? priority : -1;
+}