lstat: Bad Address

Question

When I run this code through several entries of a directory, it usually returns 0, but in two directories (one Windows one in Unix) I keep getting a "Bad Address" error message. lstat and relative_path are not null and relative_path does point to a valid directory. This code worked on other directories. The directories don't have any RWX restrictions. Prior to this I was able to call opendir and readdir without getting errors or NULL responses on these problem directories. Valgrind shows no memory leaks or other errors.

struct stat *this_lstat; 
...
DIR *dir = opendir(path); 
...
dptr = readdir(dir);
...

Note: relative_path == "./bin/Debug" at this point.

int return_code = lstat(relative_path, this_lstat);

if (return_code < 0) {
        fprintf(stderr, "find: ");
        perror(dptr->d_name); //printf("3"); //Error reading the file or directory
        return NULL;
}

UPDATE: I added checks on malloc and realloc and now I get segmentation error on the lstat line itself. The error handling code is not called. Here is the stacktrace: Program received signal SIGSEGV, Segmentation fault.

0xb76f2779 in ?? () from /lib/tls/i686/cmov/libc.so.6 
(gdb) backtrace
#0  0xb76f2779 in ?? () from /lib/tls/i686/cmov/libc.so.6
#1  0xb76ead47 in __lxstat () from /lib/tls/i686/cmov/libc.so.6
#2  0x0804959c in lstat ()
#3  0x0804903c in walk_directory_tree (path=0x805b058 "./testmine",
   findme=0x0, type_str=0x0, base_dir_searched=1, dirs_later_array=0x805b0d0)
   at pfind.c:266
#4  0x08049106 in walk_directory_tree (path=0xbfa5cca3 ".", findme=0x0,
   type_str=0x0, base_dir_searched=1, dirs_later_array=0x804b018)
   at pfind.c:282
#5  0x08048c2a in main (ac=2, av=0xbfa5c8a4) at pfind.c:143

Most obvious thing to check is that this_lstat is actually sensible, but if valgrind is working ok, then it sounds like it is! Bad Address is not one of the listed errors in the man page I looked at, could it be that something earlier in the code is causing a problem and it is just showing up here for some odd reason? — John3136
What is the exact error message? If the program is crashing, run it under a debugger and generate a stack trace when it crashes (backtrace command in gdb or kb command in MS debuggers, including Visual Studio's "Command window"). If that doesn't tell you what the problem is, post the trace to your question. we'll probably also need to see the code that sets this_lstat. — Michael Burr
I added checks for null after each malloc and realloc which I was missing. I didn't get any errors from those checks but now my code is getting segmentation error on the lstat() call itself. The error handling code is not called. Still no errors with Valgrind. Also, I didn't see any code that sets the this_lstat value. As I understand it, that variable should be set by the lstat call. — user994165

Adam Rosenfield Adam Rosenfield · Accepted Answer · 2012-02-27T02:56:08

From what I can tell, the error is being caused by your failure to initialize the this_lstat that gets passed as the second parameter to lstat.

The error string "Bad address" corresponds to the error code EFAULT, which comes from passing an invalid pointer to a system call. So, either the path name being passed to lstat doesn't point to a valid null-terminated string in readable memory, or the struct stat being passed as the second parameter doesn't point to valid writable memory.

You seem to be passing an unitialized pointer, which is almost certainly pointing to invalid memory. Valgrind doesn't complain because up until the system call, you haven't done anything wrong—only when the kernel tries to access the memory does it realize its invalid.

To fix this, either allocate memory for the struct stat with malloc, or just pass a pointer to a variable on the stack instead of using a pointer:

struct stat this_lstat;
lstat(..., &this_lstat);

lstat: Bad Address

1 Answers