The readdir_r() function is thread-safe and shall return values in a user-supplied buffer instead of possibly using a static data area that may be overwritten by each call.

The readdir_r() function returns values in a user-supplied buffer, but does not allow the size of the buffer to be specified by the caller. If {NAME_MAX} is indeterminate, there is no way for an application to know how large the buffer needs to be and readdir_r() cannot safely be used.

The readdir_r() function returns values in a user-supplied buffer instead of possibly using a static data area that may be overwritten by each call. Either the {NAME_MAX} compile-time constant or the corresponding pathconf() option can be used to determine the maximum sizes of returned pathnames. However, since the size of a filename has no limit on some filesystem types, there is no way to reliably allocate a buffer large enough to hold a filename being returned by readdir_r(). Therefore, readdir_r() has been marked obsolescent and readdir() is now required to be thread safe as long as there are no concurrent calls to it on a single directory stream.

Historically, readdir() returned a pointer to an internal static buffer that was overwritten by each call. The readdir_r() function was added as a thread-safe alternative that returns values in a user-supplied buffer. However, it does not allow the size of the buffer to be specified by the caller, and so is only usable if {NAME_MAX} is a compile-time constant or fpathconf() with _SC_NAME_MAX returns a value other than -1. If {NAME_MAX} is indeterminate (indicated by fpathconf() returning -1), there is no way to reliably allocate a buffer large enough to hold a filename being returned by readdir_r(). Therefore, readdir_r() has been marked obsolescent and readdir() is now required to be thread safe as long as there are no concurrent calls to it on a single directory stream.