readdir_r(3) Library Functions Manual readdir_r(3)

readdir_r - read a directory

Standard C library (libc, -lc)

#include <dirent.h>
[[deprecated]] int readdir_r(DIR *restrict dirp,
                             struct dirent *restrict entry,
                             struct dirent **restrict result);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

readdir_r():

    _POSIX_C_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

This function is deprecated; use readdir(3) instead.

The readdir_r() function was invented as a reentrant version of readdir(3). It reads the next directory entry from the directory stream dirp, and returns it in the caller-allocated buffer pointed to by entry. For details of the dirent structure, see readdir(3).

A pointer to the returned buffer is placed in *result; if the end of the directory stream was encountered, then NULL is instead returned in *result.

It is recommended that applications use readdir(3) instead of readdir_r(). Furthermore, since glibc 2.24, glibc deprecates readdir_r(). The reasons are as follows:

•
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'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.

The readdir_r() function returns 0 on success. On error, it returns a positive error number (listed under ERRORS). If the end of the directory stream is reached, readdir_r() returns 0, and returns NULL in *result.

Invalid directory stream descriptor dirp.
A directory entry whose name was too long to be read was encountered.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
readdir_r () Thread safety MT-Safe

POSIX.1-2008.

POSIX.1-2001.

readdir(3)

2023-10-31 Linux man-pages 6.7