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Top Forums Programming readdir and dynamic array memory corruption Post 302491516 by torbium on Thursday 27th of January 2011 04:34:26 PM
Old 01-27-2011
readdir and dynamic array memory corruption

Hi everyone

I am developing an utility.
At some part of it I read directory entries to a dynamic array: struct list
It stores pointers to items: list.entries,
which are structures: struct entry

If a number of files in a directory is greater then number of elements an array was initially allocated,
I reallocate memory for this array.
if (c > list_size)

And at this point something strange happens.
Pointers are correct.
While I can successfully allocate memory for new items
list.entries[c] = malloc(sizeof(struct entry));
list.entries[0]->pde->d_name is corrupted at some iteration, but always if list.entries was reallocated.

See the test code provided.

If I do not realloc list.entries everything goes fine.
I played with list_size values.
On Mac OS X (10.4.0 Darwin Kernel Version 10.4.0) memory is corrupted while list.entries[124] is processed.
On Ubuntu Linux 2.6.24-23-xen #1 SMP Wed Apr 1 23:47:10 UTC 2009 x86_64 GNU/Linux while list.entries[196] is processed.
If I don't use readdir and explicitly allocate
list.entries[c]->pde = malloc(sizeof(struct dirent));
memory is not corrupted.

What is wrong?
Where is the problem?
What is the best way to read dir entries to a dynamic array?

Code:
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dirent.h>
#include <math.h>
#include <pwd.h>
#include <dirent.h>
#include <unistd.h>

int
main (void)
{
    int i, c, ac;
    unsigned int list_size = 10;
    char * fname;
    
    DIR *pdir;
    struct dirent *pde;
    
    
    struct entry {
        struct stat st;
        struct dirent *pde;
    };
    
    struct list {
        int count;
        struct entry **entries;
    } list;
    
    struct entry **ppent;
    pdir = NULL;
    pde = NULL;
    
    pdir = opendir("/usr/bin");
    
    if ((list.entries = malloc(list_size * sizeof(struct entry *))) == NULL)
        return 1;
    
    c = 0;
    ac = 0; /* allocation counter */
    while ((pde = readdir(pdir)) != NULL) {
        if (c > list_size - 1) {
            list_size <<= 1;
            if ((list.entries = realloc(list.entries,
                                    list_size * sizeof(struct entry*))) == NULL) {
                perror("unable to realloc");
                return 1;
            }
            ac++;
        }

        list.entries[c] = malloc(sizeof(struct entry));
        list.entries[c]->pde = pde;

        if (strcmp(list.entries[0]->pde->d_name, ".") != 0) {
            printf("memory corruption. size of array %d items\n", c);
            printf("number of reallocations %d\n", ac);
            return 1;
        }
        
        printf("base address: %p pointer address: %p entry pointer value: %p name: %s\n",
               list.entries, &list.entries[c], list.entries[c], list.entries[c]->pde->d_name);
        c++;
    }
    
    return 0;
}

 

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readdir(3UCB)					     SunOS/BSD Compatibility Library Functions					     readdir(3UCB)

NAME
readdir - read a directory entry SYNOPSIS
/usr/ucb/cc [ flag ... ] file ... #include <sys/types.h> #include <sys/dir.h> struct direct *readdir(dirp) DIR *dirp; DESCRIPTION
The readdir() function returns a pointer to a structure representing the directory entry at the current position in the directory stream to which dirp refers, and positions the directory stream at the next entry, except on read-only file systems. It returns a NULL pointer upon reaching the end of the directory stream, or upon detecting an invalid location in the directory. The readdir() function shall not return directory entries containing empty names. It is unspecified whether entries are returned for dot (.) or dot-dot (..). The pointer returned by readdir() points to data that may be overwritten by another call to readdir() on the same directory stream. This data shall not be overwritten by another call to readdir() on a different directory stream. The readdir() function may buffer several directory entries per actual read operation. The readdir() function marks for update the st_atime field of the directory each time the directory is actu- ally read. RETURN VALUES
The readdir() function returns NULL on failure and sets errno to indicate the error. ERRORS
The readdir() function will fail if one or more of the following are true: EAGAIN Mandatory file/record locking was set, O_NDELAY or O_NONBLOCK was set, and there was a blocking record lock. EAGAIN Total amount of system memory available when reading using raw I/O is temporarily insufficient. EAGAIN No data is waiting to be read on a file associated with a tty device and O_NONBLOCK was set. EAGAIN No message is waiting to be read on a stream and O_NDELAY or O_NONBLOCK was set. EBADF The file descriptor determined by the DIR stream is no longer valid. This results if the DIR stream has been closed. EBADMSG Message waiting to be read on a stream is not a data message. EDEADLK The read() was going to go to sleep and cause a deadlock to occur. EFAULT buf points to an illegal address. EINTR A signal was caught during the read() or readv() function. EINVAL Attempted to read from a stream linked to a multiplexor. EIO A physical I/O error has occurred, or the process is in a background process group and is attempting to read from its control- ling terminal, and either the process is ignoring or blocking the SIGTTIN signal or the process group of the process is orphaned. ENOENT The current file pointer for the directory is not located at a valid entry. ENOLCK The system record lock table was full, so the read() or readv() could not go to sleep until the blocking record lock was removed. ENOLINK fildes is on a remote machine and the link to that machine is no longer active. ENXIO The device associated with fildes is a block special or character special file and the value of the file pointer is out of range. EOVERFLOW The value of the direct structure member d_ino cannot be represented in an ino_t. USAGE
The readdir() function has a transitional interface for 64-bit file offsets. See lf64(5). SEE ALSO
cc(1B), getdents(2), readdir(3C), scandir(3UCB), lf64(5) NOTES
Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the sys- tem libraries or in multi-thread applications is unsupported. SunOS 5.11 30 Oct 2007 readdir(3UCB)
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