Why readdir returns null and I/O error the next call to readdir after first call on a directory

Question

DIR *dir_ptr;
struct dirent *dir_entery;
dir_ptr = opendir("/tmp");

while (dir_ptr&&(dir_entery = readdir(dir_ptr))) {
   printf("%s \n", dir_entery->d_name);
}

printf("%s \n", strerror(errno));

gives this output:

file_name
dir_name
errno = Remote I/O error

in /tmp I have one dir and two files when get to readdir after the execution of opendir(dir) It exits the while and put this error:

errno = Remote I/O error

Why it fails to read the file after the dir in the /tmp directory?

errno 121 is EREMOTEIO (Remote I/O error). Perhaps there are permission problems on the target file system. Did you tried your program to run with root (supper user) permission, if you are in Ubuntu try with sudo — Grijesh Chauhan
I would assume that the 2nd file has unusual attributes, name, access rights, etc. Try runing in a lower directory. Maybe its choking on ".."? — chux - Reinstate Monica
What is the output of ls -la /tmp? Hide the actual file names if you need to keep them private. — jxh

alk alk · Accepted Answer · 2013-05-30T16:50:34

readdir() is not documented to return REREMOTEIO, so most likely sterror() gives misleading information.

Set errno to 0 before entering the while() loop, that is before calling readdir().

From man readdir:

If the end of the directory stream is reached, NULL is returned and errno is not changed. If an error occurs, NULL is returned and errno is set appropriately. To distinguish end of stream and from an error, set errno to zero before calling readdir() and then check the value of errno if NULL is returned.

To test these two cases when readdir() returns NULL you might like to modify your code like this:

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
#include <errno.h>

  ...

  DIR * dir_ptr = opendir("/tmp");
  if (NULL != dir_ptr)
  {
    perror("opendir() failed");
  }
  else
  {
    struct dirent * dir_entery;

    errno = 0; 
    while ((dir_entery = readdir(dir_ptr))) /* an extra pair of parenthesis here to silence GCC */
    {
      printf("%s\n", dir_entery->d_name);
    }

    if (0 != errno)
    {
      perror("readdir() failed");
    }
    else
    {
      printf("No more entries.\n");
    }
  }

  ...

Why readdir returns null and I/O error the next call to readdir after first call on a directory

1 Answers