Do I need to Allocate Memory for a Dirent Structure

Question

• Platform: Windows XP Service Pack 3
• Compiler: Code::Blocks version 12.11

I am currently writing a program that will recursively delete a given directory using the POSIX directory functions. But I am having a problem with readdir() and its counterpart the dirent structure. I read in readdir's documentation that multiple calls to the function will overwrite the data held in the structure returned by the function. So I thought readdir() must allcoate memmory for the struct itself and then simply reassign the pointer address to the structure that captures it's return value. I tested this theory and I was correct readdir() allocated memmory for it's member d_name. The problem I am having is that readdir returns a NULL pointer when the directory stream is empty, so I use a while loop with the condition (dirent_ptr != NULL) to iterate the entire directory. But because readdir() will handle the memmory allocation of the structure I simply declare a dirent structure and let readdir() do its job. Unfourtatnly for some reason dirent structures are initialized to NULL(or it might be my complier) so my loop never starts because it's conditional statement is not initialy true. So I guess my question is what am I doing wrong here?

Here are important variable declarations and the included librarys. Please note that all of these variables are declared globaly.

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <sys/stat.h>
#include <string.h>

int recursive_delete(const char *path);
int file_delete(const char *path, int file_size);

struct dirent *direntp;
struct stat *statp;

struct switches
{
    int verbose;
    int no_prompt;
    int continue_if_error;
    int files_only;
}; struct switches switches;

Rather than resolving relative paths I simply cd to the path given as an argument and then I use the . and .. wildcards to move threw the directorys so that relative paths(d_names) are valid. Also the switches structure simply contains command line switches and should be ignored and I am aware of errors in the following code but unfourtantly I cannot fix them because I cannot get past the above mentioned problem.

int recursive_delete(const char *path)
{
    DIR *dirp;
    int return_value = 0;
    int recursive_return_value = 0;

    if((chdir(path)) == -1)
    {
        perror("ERROR(3)");
        return 1;
    }
    printf("CDED to \"%s\"\n", path);

    dirp = opendir(".");
    if(dirp == NULL)
    {
        perror("ERROR(4)");
        return 1;
    }
    printf("OPENED \"%s\"\n", path);

    while(direntp != NULL)
    {
        direntp = readdir(dirp);
        if( (direntp == NULL) && (errno != 0) )
        {
            perror("ERROR(5)");
            return 1;
        }
        printf("READ \"%s\" FROM \"%s\"\n", direntp->d_name, path);

        if( (strcmp(direntp->d_name, ".")!=0) && (strcmp(direntp->d_name, "..")!=0) )
        {
            if((stat(direntp->d_name, statp)) == -1)
            {
                perror("ERROR(6)");
                return 1;
            }
            printf("STATED \"%s\"\n", direntp->d_name);

            if(S_ISREG(statp->st_mode))
            {
                printf("DELETING \"...\\%s\\%s\"\n", path, direntp->d_name);
                return_value += file_delete(direntp->d_name, statp->st_size);
                if( (!switches.continue_if_error) && (return_value != 0) )
                {
                    break;
                }
            }
            else if(S_ISDIR(statp->st_mode))
            {
                printf("\n\n\nCALLING RECURSIVE DELETE with \"%s\"\n", direntp->d_name);
                recursive_return_value = recursive_delete(direntp->d_name);
                return_value += recursive_return_value;

                if( (!switches.continue_if_error) && (recursive_return_value != 0) )
                {
                    break;
                }

                if( (!switches.files_only) && (recursive_return_value == 0) )
                {
                    if((chdir("..")) == -1)
                    {
                        perror("ERROR(6)");
                        return 1;
                    }
                    printf("CDED BACK TO \"%s\" FROM \"%s\"\n", path, direntp->d_name);

                    if((rmdir(direntp->d_name)) == -1)
                    {
                        perror("ERROR(7)");
                        return 1;
                    }

                    if(switches.verbose)
                    {
                        printf("DELETED DIRECTORY \"...\\%s\\\"\n\n\n", direntp->d_name);
                    }
                }
            }
        }
    }

    return return_value;
}

Answer 1

From the code attached, it's not clear where direntp is being initialized (before the while loop). Possibly try something like:

direntp = readdir(dirp);
while(direntp != NULL)
{
    // all your work here

    direntp = readdir(dirp);
}

This pattern ensures that direntp is initialized and updated for your while loop. However, on second glance of your code I'm not exactly sure what the while loop is supposed to be doing in the first place. How is direntp or dirp changing in your loop?

It's possible that you can just get away with an if test (instead of the while) and just let the recursive call handle the "looping" effect...

Answer 2

Your code structure should look something like thhis (with most error checks omitted for clarity):

int recursive_delete(const char *path)
{
  DIR* dirp = NULL;
  int return_value = 0;
  char* initial_cur_dir = malloc(1000);

  getcwd(initial_cur_dir, 1000);
  chdir(path);
  dirp = opendir(".");

  while (dirp != NULL)
  {
    struct dirent* direntp;
    struct stat stat;

    direntp = readdir(dirp);

    if (direntp == NULL)
      break;

    stat(direntp->d_name, &stat);

    if (S_ISDIR(statp->st_mode))
    {
      if (strcmp(direntp->d_name, ".") && strcmp(direntp->d_name, ".."))
      {
        return_value += recursive_delete(direntp->d_name);
      }
    }
    else if (S_ISREG(statp->st_mode))
    {
      unlink(direntp->d_name);
    }
  }

  if (initial_cur_dir != NULL)
  {
    chdir(initial_cur_dir);
    rmdir(path);
  }

ErrorLabel: // you should goto here when an error is detected

  if (dirp != NULL)
  {
    closedir(dirp);
  }

  if (initial_cur_dir != NULL)
  {
    chdir(initial_cur_dir);
    free(initial_cur_dir);
  }

  return return_value;
}