readdir() is not context-safe. The data in the pointer it returns gets overwritten every time you call it. If you opened two directories at the same time like this:

Code:

struct dirent *p1, *p2;
DIR *d1, *d2;
d1=popen("/bin");
d2=popen("/dev");
p1=readdir(d1);
p2=readdir(d2);

...both p1 and p2 would point to the same data: Whatever the last readdir() found.

readdir_r() avoids this by writing data to wherever you tell it instead of giving you a pointer to its own memory.

Code:

struct dirent p1, p2;
DIR *d1, *d2;
d1=popen("/bin");
d2=popen("/dev");
readdir_r(d1, &p1);
readdir(d2, &p2);

This way, p1 and p2 both contain the expected data, instead of just containing whatever the last value read by readdir() was.