void seekdir(DIR *dirp, long loc);
The seekdir() function shall set the position of the next readdir() operation on the directory stream specified by dirp to the position specified by loc. The value of loc should have been returned from an earlier call to telldir(). The new position reverts to the one associated with the directory stream when telldir() was performed.
If the value of loc was not obtained from an earlier call to telldir(), or if a call to rewinddir() occurred between the call to telldir() and the call to seekdir(), the results of subsequent calls to readdir() are unspecified.
The seekdir() function shall not return a value.
No errors are defined.
The following sections are informative.
The original standard developers perceived that there were restrictions on the use of the seekdir() and telldir() functions related to implementation details, and for that reason these functions need not be supported on all POSIX-conforming systems. They are required on implementations supporting the XSI extension.
One of the perceived problems of implementation is that returning to a given point in a directory is quite difficult to describe formally, in spite of its intuitive appeal, when systems that use B-trees, hashing functions, or other similar mechanisms to order their directories are considered. The definition of seekdir() and telldir() does not specify whether, when using these interfaces, a given directory entry will be seen at all, or more than once.
On systems not supporting these functions, their capability can sometimes be accomplished by saving a filename found by readdir() and later using rewinddir() and a loop on readdir() to relocate the position from which the filename was saved.
opendir() , readdir() , telldir() , the Base Definitions volume of IEEE Std 1003.1-2001, <dirent.h>, <stdio.h>, <sys/types.h>