fix: Open /dev/null only once and close it at the end

Before this patch, we would open and close /dev/null every
iteration.

cbench.c

@@ -18,17 +18,10 @@ static void as_timespec(uint64_t ns, struct timespec* ts) {
     ts->tv_nsec = ns % (uint64_t)1000000000L;
 }
 
-uint64_t run_individual_benchmark(char** argv)
+uint64_t run_individual_benchmark(char** argv, int fd)
 {
     struct timespec start, end;
 
-    int fd = open("/dev/null", O_WRONLY);
-    if(fd < 0)
-    {
-        perror("open");
-        return (uint64_t)-1;
-    }
-
     if(clock_gettime(CLOCK_MONOTONIC, &start) != 0)
     {
         perror("clock_gettime");
@@ -50,7 +43,6 @@ uint64_t run_individual_benchmark(char** argv)
         exit(1);
     }
 
-    close(fd);
     int status;
     pid_t result = waitpid(child, &status, 0);
 
@@ -112,11 +104,19 @@ int main(int argc, char** argv)
     uint64_t min = UINT64_MAX;
     uint64_t max = 0;
 
+    int fd = open("/dev/null", O_WRONLY);
+    if(fd < 0)
+    {
+        perror("open");
+        return 1;
+    }
+
     for(int i = 0; i < iterations; i++)
     {
-        uint64_t ns = run_individual_benchmark(argv+1);
+        uint64_t ns = run_individual_benchmark(argv+1, fd);
         if(ns == (uint64_t)-1)
         {
+            close(fd);
             return 1;
         }
 
@@ -124,6 +124,7 @@ int main(int argc, char** argv)
         if(ns < min) min = ns;
         if(ns > max) max = ns;
     }
+    close(fd);
 
     uint64_t average = total / (uint64_t)iterations;