selftests/sched: fix warn_unused_result build warns

Fix the following warns by adding return check and error handling.

gcc -O2 -Wall -g -I./ -isystem .../tools/testing/selftests/../../../usr/include -Wl,-rpath=./      cs_prctl_test.c -lpthread -o .../tools/testing/selftests/sched/cs_prctl_test
cs_prctl_test.c: In function ‘create_processes’:
cs_prctl_test.c:187:17: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  187 |                 read(proc[i].pfd[0], &proc[i].thr_tids, sizeof(int) * proc[i].num_threads);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cs_prctl_test.c: In function ‘child_func_process’:
cs_prctl_test.c:159:9: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  159 |         write(ca->pfd[1], &ca->thr_tids, sizeof(int) * ca->num_threads);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/sched/cs_prctl_test.c

@@ -27,6 +27,7 @@
 #include <sys/prctl.h>
 #include <unistd.h>
 #include <time.h>
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -151,12 +152,17 @@ static void create_threads(int num_threads, int thr_tids[])
 static int child_func_process(void *arg)
 {
 	struct child_args *ca = (struct child_args *)arg;
+	int ret;
 
 	close(ca->pfd[0]);
 
 	create_threads(ca->num_threads, ca->thr_tids);
 
-	write(ca->pfd[1], &ca->thr_tids, sizeof(int) * ca->num_threads);
+	ret = write(ca->pfd[1], &ca->thr_tids, sizeof(int) * ca->num_threads);
+	if (ret == -1)
+		printf("write failed on pfd[%d] - error (%s)\n",
+			ca->pfd[1], strerror(errno));
+
 	close(ca->pfd[1]);
 
 	while (1)
@@ -169,7 +175,7 @@ static unsigned char child_func_process_stack[STACK_SIZE];
 void create_processes(int num_processes, int num_threads, struct child_args proc[])
 {
 	pid_t cpid;
-	int i;
+	int i, ret;
 
 	for (i = 0; i < num_processes; ++i) {
 		proc[i].num_threads = num_threads;
@@ -184,7 +190,10 @@ void create_processes(int num_processes, int num_threads, struct child_args proc
 	}
 
 	for (i = 0; i < num_processes; ++i) {
-		read(proc[i].pfd[0], &proc[i].thr_tids, sizeof(int) * proc[i].num_threads);
+		ret = read(proc[i].pfd[0], &proc[i].thr_tids, sizeof(int) * proc[i].num_threads);
+		if (ret == -1)
+			printf("read failed on proc[%d].pfd[0] error (%s)\n",
+				i, strerror(errno));
 		close(proc[i].pfd[0]);
 	}
 }