READDIR_R(3) Library Functions Manual READDIR_R(3)

readdir readdir_r — read a directory

#include <dirent.h>

struct dirent *readdir(DIR *dirp);
[OB] int readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
       struct dirent **restrict result);

The type , which is defined in the <dirent.h> header, represents a , which is an ordered sequence of all the directory entries in a particular directory. Directory entries represent files; files may be removed from a directory or added to a directory asynchronously to the operation of readdir ().

The readdir () function shall return a pointer to a structure representing the directory entry at the current position in the directory stream specified by the argument dirp, and position the directory stream at the next entry. It shall return a null pointer upon reaching the end of the directory stream. The structure dirent defined in the <dirent.h> header describes a directory entry. The value of the structure's d_ino member shall be set to the file serial number of the file named by the d_name member. If the d_name member names a symbolic link, the value of the d_ino member shall be set to the file serial number of the symbolic link itself. The d_name member shall be a filename string, and (if not dot or dot-dot) shall contain the same byte sequence as the last pathname component of the string used to create the directory entry, plus the terminating <NUL> byte.

The readdir () function shall not return directory entries containing empty names. If entries for dot or dot-dot exist, one entry shall be returned for dot and one entry shall be returned for dot-dot; otherwise, they shall not be returned.

The application shall not modify the structure to which the return value of readdir () points, nor any storage areas pointed to by pointers within the structure. The returned pointer, and pointers within the structure, might be invalidated or the structure or the storage areas might be overwritten by a subsequent call to readdir () on the same directory stream. They shall not be affected by a call to readdir () on a different directory stream. The returned pointer, and pointers within the structure, might also be invalidated if the calling thread is terminated.

If a file is removed from or added to the directory after the most recent call to opendir(3) or rewinddir(3), whether a subsequent call to readdir () returns an entry for that file is unspecified.

The readdir () function may buffer several directory entries per actual read operation; readdir () shall mark for update the last data access timestamp of the directory each time the directory is actually read.

After a call to fork(3), either the parent or child (but not both) may continue processing the directory stream using readdir (), rewinddir(3), or seekdir(3). If both the parent and child processes use these functions, the result is undefined.

The readdir () function need not be thread-safe if concurrent calls are made for the same directory stream.

Applications wishing to check for error situations should set errno to 0 before calling readdir (). If errno is set to non-zero on return, an error occurred.

[OB] The readdir_r () function shall initialize the dirent structure referenced by entry to represent the directory entry at the current position in the directory stream referred to by dirp, store a pointer to this structure at the location referenced by result, and position the directory stream at the next entry.

The storage pointed to by entry shall be large enough for a dirent with an array of d_name members containing at least {NAME_MAX}+1 elements.

Upon successful return, the pointer returned at * result shall have the same value as the argument entry. Upon reaching the end of the directory stream, this pointer shall have the value NULL.

The readdir_r () function shall not return directory entries containing empty names.

If a file is removed from or added to the directory after the most recent call to opendir(3) or rewinddir(3), whether a subsequent call to readdir_r () returns an entry for that file is unspecified.

The readdir_r () function may buffer several directory entries per actual read operation; readdir_r () shall mark for update the last data access timestamp of the directory each time the directory is actually read.

Upon successful completion, readdir () shall return a pointer to an object of type struct dirent. When an error is encountered, a null pointer shall be returned and errno shall be set to indicate the error. When the end of the directory is encountered, a null pointer shall be returned and errno is not changed.

[OB] If successful, the readdir_r () function shall return zero; otherwise, an error number shall be returned to indicate the error.

The readdir () [OB] and readdir_r () functions shall fail if:

One of the values in the structure to be returned cannot be represented correctly.
Insufficient memory is available.

The readdir () [OB] and readdir_r () functions may fail if:

The dirp argument does not refer to an open directory stream.
The current position of the directory stream is invalid.

The following sample program searches the current directory for each of the arguments supplied on the command line.

#include <dirent.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>


static void lookup(const char *arg)
{
    DIR *dirp;
    struct dirent *dp;


    if ((dirp = opendir(".")) == NULL) {
        perror("couldn't open '.'");
        return;
    }


    do {
        errno = 0;
        if ((dp = readdir(dirp)) != NULL) {
            if (strcmp(dp->d_name, arg) != 0)
                continue;


            (void) printf("found %s\n", arg);
            (void) closedir(dirp);
                return;


        }
    } while (dp != NULL);


    if (errno != 0)
        perror("error reading directory");
    else
        (void) printf("failed to find %s\n", arg);
    (void) closedir(dirp);
    return;
}


int main(int argc, char *argv[])
{
    int i;
    for (i = 1; i < argc; i++)
        lookup(argv[i]);
    return (0);
}

The readdir () function should be used in conjunction with opendir(3), closedir(3), and rewinddir(3) to examine the contents of the directory.

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 returned value of readdir () merely a directory entry. No equivalence should be inferred.

Historical implementations of readdir () obtain multiple directory entries on a single read operation, which permits subsequent readdir () operations to operate from the buffered information. Any wording that required each successful readdir () operation to mark the directory last data access timestamp for update would disallow such historical performance-oriented implementations.

When returning a directory entry for the root of a mounted file system, some historical implementations of readdir () returned the file serial number of the underlying mount point, rather than of the root of the mounted file system. This behavior is considered to be a bug, since the underlying file serial number has no significance to applications.

Since readdir () returns NULL when it detects an error and when the end of the directory is encountered, an application that needs to tell the difference must set errno to zero before the call and check it if NULL is returned. Since the function must not change errno in the second case and must set it to a non-zero value in the first case, a zero errno after a call returning NULL indicates end-of-directory; otherwise, an error.

Routines to deal with this problem more directly were proposed:

int derror (dirp)
DIR *dirp;


void clearderr (dirp)
DIR *dirp;

The first would indicate whether an error had occurred, and the second would clear the error indication. The simpler method involving errno was adopted instead by requiring that readdir () not change errno when end-of-directory is encountered.

An error or signal indicating that a directory has changed while open was considered but rejected.

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(3) with _SC_NAME_MAX returns a value other than -1. If {NAME_MAX} is indeterminate (indicated by fpathconf(3) 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.

Conforming file systems are required to store filenames unaltered from how they were created (via open(3), link(3), mkdir(3), mkfifo(3), rename(3), etc.). By definition, a filename string does not include a <slash>, even if a trailing <slash> was present in the pathname presented to mkdir(3) when creating a sub-directory.

However, there are non-conforming file systems where filenames are converted to a canonical representation before a directory entry is created, such that it is possible to create a file using one string, then perform opendir(3) and a readdir () loop and not encounter the same string, because readdir () returns the canonical form of the string instead. Such non-conforming file systems also have the issue that multiple filenames can resolve to the same directory entry, with potentially confusing results. This standard cannot mandate the behavior of non-conforming file systems, and strictly conforming applications need not worry about dealing with such file systems, but it is a concern for developers of portable applications. Therefore, this standard recommends that file system implementations that perform canonicalization of filenames should reject attempts to create a directory entry with a non-canonical filename using the [EILSEQ] error. However, if a directory entry already exists, it is reasonable for a file system to permit accessing that file via a non-canonical filename.

The readdir_r () function may be removed in a future version.

closedir(3), dirfd(3), exec(3), fdopendir(3), fstatat(3), posix_getdents(3), rewinddir(3), symlink(3)

XBD <dirent.h>, <sys/types.h>

First released in Issue 2.

Large File Summit extensions are added.

The readdir_r () function is included for alignment with the POSIX Threads Extension.

A note indicating that the readdir () function need not be reentrant is added to the DESCRIPTION.

The readdir_r () function is marked as part of the Thread-Safe Functions option.

The Open Group Corrigendum U026/7 is applied, correcting the prototype for readdir_r ().

The Open Group Corrigendum U026/8 is applied, clarifying the wording of the successful return for the readdir_r () function.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

  • The requirement to include <sys/types.h> has been removed. Although <sys/types.h> was required for conforming implementations of previous POSIX specifications, it was not required for UNIX applications.
  • A statement is added to the DESCRIPTION indicating the disposition of certain fields in struct dirent when an entry refers to a symbolic link.
  • The [EOVERFLOW] mandatory error condition is added. This change is to support large files.
  • The [ENOENT] optional error condition is added.

The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

The keyword is added to the readdir_r () prototype for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

IEEE Std 1003.1-2001 (“POSIX.1”)/Cor 1-2002, item XSH/TC1/D6/50 is applied, replacing the EXAMPLES section with a new example.

Austin Group Interpretation 1003.1-2001 #059 is applied, updating the ERRORS section.

Austin Group Interpretation 1003.1-2001 #156 is applied.

The readdir_r () function is moved from the Thread-Safe Functions option to the Base.

Changes are made related to support for finegrained timestamps.

The value of the d_ino member is no longer unspecified for symbolic links.

SD5-XSH-ERN-193 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0486 [75] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0304 [656] is applied.

Austin Group Defect 293 is applied, adding a requirement that d_name contains the same byte sequence as the last pathname component of the string used to create the directory entry.

Austin Group Defects 696 and 1664 are applied, making readdir_r () obsolescent, requiring readdir () to be thread-safe except when concurrent calls are made for the same directory stream, and adding the [ENOMEM] error.

Austin Group Defect 697 is applied, adding posix_getdents(3) to the SEE ALSO section.

June 14, 2024 posix.fail