I usb i.mx6 board with android 4.2.2. it's usb disk auto mount to /mnt/udisk dir. I find a a trick question,
this is code in native app
int main(void)
{
int a = 0;
FILE * fd = fopen("/data/test/out.txt", "a+");
fprintf(fd, "uid = %d, gid = %d, euid = %d, egid = %d\n", getuid(), getgid(), geteuid(), getegid());
a = umount("/mnt/udisk");
if(a != 0)
fprintf(fd,"%s\n",strerror(errno));
fprintf(fd,"umount = %d\n", a);
fclose(fd);
//sleep(60);
return 0;
}
this is the java app
ProcessBuilder process = new ProcessBuilder("su", "-c", "/data/test/testumount");
Process p = process.start();
p.waitFor();
I got a out.txt from native app.
uid = 0, gid = 0, euid = 0, egid = 0
umount = 0
You see the umount return 0, but /mnt/udisk not umount. So , why? the native app's uid = 0, it is root.
