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..581d518221aa9530cdeb2f99f43b2d84da06a3ee 100644
--- a/base/os_compat_android.cc
+++ b/base/os_compat_android.cc
@@ -4,22 +4,29 @@
 
 #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);
+  // Convert timeval to timespec.
+  // timeval.usec is guaranteed to be less than one million.

[Mark Mentovai, 2013/06/06 13:37:44]

Guaranteed by whom? Any old user code might call this function and supply bogus
data.

For a faithful reimplementation of futimes, you should check the range, 0 <=
tv_usec < 1000000 as glibc does, and set errno to EINVAL and return -1 if not in
range. Why? Because there’s no guarantee that user code actually limits tv_usec
to the right range, and if a value comes in that’s in the wrong range, you need
to catch it before integer multiplication might transform it into a tv_nsec
value that actually is in range.

[nilesh, 2013/06/06 19:03:14]

Thanks. Done.

+  struct timespec ts[2];
+  ts[0].tv_sec = tv[0].tv_sec;

[Mark Mentovai, 2013/06/06 13:37:44]

tv is allowed to be NULL, in which case you would pass NULL to utimensat in
place of ts. You should handle this to implement futimes fully.

[nilesh, 2013/06/06 19:03:14]

Done.

+  ts[0].tv_nsec = tv[0].tv_usec * 1000;

[Mark Mentovai, 2013/06/06 13:37:44]

Normally you could use TIMEVAL_TO_TIMESPEC(&tv, &ts); but I guess Bionic doesn’t
have that either.

[nilesh, 2013/06/06 19:03:14]

You are right. TIMEVAL_TO_TIMESPEC is not avialable in the ndk.

+  ts[1].tv_sec = tv[1].tv_sec;
+  ts[1].tv_nsec = tv[1].tv_usec * 1000;
+  return syscall(__NR_utimensat, fd, NULL, ts, 0);
 }
 
 // Android has only timegm64() and no timegm().