add error checking for Linux get_cpu_time()

Author: davidpanderson

lib/procinfo.h

@@ -83,7 +83,7 @@ extern double process_tree_cpu_time(int pid);
     // get the CPU time of the given process and its descendants
 
 extern double total_cpu_time();
-    // total CPU time, as reported by OS
+    // total user-mode CPU time, as reported by OS
 
 extern double boinc_related_cpu_time(PROC_MAP&, bool using_vbox);
     // total CPU of current BOINC processes, low-priority processes,

lib/procinfo_unix.cpp

@@ -273,14 +273,25 @@ double total_cpu_time() {
     static double scale;
     if (!f) {
         f = fopen("/proc/stat", "r");
+        if (!f) {
+            fprintf(stderr, "can't open /proc/stat\n");
+            return 0;
+        }
         long hz = sysconf(_SC_CLK_TCK);
         scale = 1./hz;
     } else {
         fflush(f);
         rewind(f);
     }
-    if (!fgets(buf, 256, f)) return 0;
+    if (!fgets(buf, 256, f)) {
+        fprintf(stderr, "can't read /proc/stat\n");
+        return 0;
+    }
     double user, nice;
-    sscanf(buf, "cpu %lf %lf", &user, &nice);
+    int n = sscanf(buf, "cpu %lf %lf", &user, &nice);
+    if (n != 2) {
+        fprintf(stderr, "can't parse /proc/stat: %s\n", buf);
+        return 0;
+    }
     return (user+nice)*scale;
 }