Android: Provide futimes() implementation using utimensat()

base/os_compat_android.cc

Index: base/os_compat_android.cc
diff --git a/base/os_compat_android.cc b/base/os_compat_android.cc
index c4ea463d1781a4fba039e323e90bf39ee276b52d..81cd8127e437c72506955a7eb154c9ed723f63b3 100644
--- a/base/os_compat_android.cc
+++ b/base/os_compat_android.cc
@@ -4,22 +4,22 @@
 
 #include "base/os_compat_android.h"
 
+#include <asm/unistd.h>
 #include <errno.h>
 #include <math.h>
 #include <sys/stat.h>
+#include <sys/syscall.h>
 #include <time64.h>
 
 #include "base/rand_util.h"
 #include "base/stringprintf.h"
 #include "base/strings/string_piece.h"
 
+extern "C" {
 // There is no futimes() avaiable in Bionic, so we provide our own
 // implementation until it is there.
-extern "C" {
-
 int futimes(int fd, const struct timeval tv[2]) {
-  const std::string fd_path = base::StringPrintf("/proc/self/fd/%d", fd);
-  return utimes(fd_path.c_str(), tv);
+  return syscall(__NR_utimensat, fd, NULL, tv, 0);

[Mark Mentovai, 2013/06/06 00:50:59]

utimensat can be used to implement futimens. The time argument is a timespec[2].
A timespec specifies seconds and nanoseconds.

futimes’ time argument is a timeval[2]. A timeval specifies seconds and
microseconds.

timeval and timespec are both the same width and have the same alignment, so you
may not have noticed the problem unless you actually looked at the fractional
second portion of the times set on files, but your implementation confuses
nanoseconds and microseconds.

[nilesh, 2013/06/06 01:57:36]

Thanks for catching this. I am now converting timeval to timespec.

I tried adding a test for this, but looks like the fractional value is ignored
on most filesystems. PlatformFile.TouchGetInfoPlatformFile says that "ext2/ext3
and HPS/HPS+ seem to have a timestamp granularity of 1s." and hence only checks
for the seconds value in tests.

Please take another look.

 }
 
 // Android has only timegm64() and no timegm().