On systems where NAME_MAX is undefined, calling readdir_r() may be unsafe because the interface does not allow the caller to specify the length of the buffer used for the returned directory entry.

On some systems, readdir_r() can_zsingle_quotesz_t read directory entries with very long names. When the glibc implementation encounters such a name, readdir_r() fails with the error ENAMETOOLONG after the final directory entry has been read. On some other systems, readdir_r() may return a success status, but the returned d_name field may not be null terminated or may be truncated.

In the current POSIX.1 specification (POSIX.1-2008), readdir(3) is not required to be thread-safe. However, in modern implementations (including the glibc implementation), concurrent calls to readdir(3) that specify different directory streams are thread-safe. Therefore, the use of readdir_r() is generally unnecessary in multithreaded programs. In cases where multiple threads must read from the same directory stream, using readdir(3) with external synchronization is still preferable to the use of readdir_r(), for the reasons given in the points above.

It is expected that a future version of POSIX.1 will make readdir_r() obsolete, and require that readdir(3) be thread-safe when concurrently employed on different directory streams.