perf tools: Fine tune readn function

Added a 'left' variable to make the flow clearer, and added a debug
check for the return value - returning 'n' is more obvious.

Added small comment for readn.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Original-patch-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1385634619-8129-4-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
index 9440481e9092a104e068788b7c6ef66584fc8af6..6ea0b4ae956936942f566ecca6cf34b65174b8be 100644 (file)
--- a/tools/perf/util/util.c
+++ b/tools/perf/util/util.c
@@ -6,6 +6,7 @@
 #endif
 #include <stdio.h>
 #include <stdlib.h>
+#include <linux/kernel.h>
 
 /*
  * XXX We need to find a better place for these things...
@@ -151,21 +152,26 @@ unsigned long convert_unit(unsigned long value, char *unit)
        return value;
 }
 
+/*
+ * Read exactly 'n' bytes or return an error.
+ */
 ssize_t readn(int fd, void *buf, size_t n)
 {
        void *buf_start = buf;
+       size_t left = n;
 
-       while (n) {
-               ssize_t ret = read(fd, buf, n);
+       while (left) {
+               ssize_t ret = read(fd, buf, left);
 
                if (ret <= 0)
                        return ret;
 
-               n -= ret;
-               buf += ret;
+               left -= ret;
+               buf  += ret;
        }
 
-       return buf - buf_start;
+       BUG_ON((size_t)(buf - buf_start) != n);
+       return n;
 }
 
 size_t hex_width(u64 v)