Warning with nftw

Hmm. Your code works for me. Check your include paths, maybe? Though this is a system header, so it should be pretty hard to miss this. Or were you accidentally compiling a version that didn't have the #include <ftw.h> line?

$ gcc -o ftw ftw.c
$ ./ftw my-directory
File 10
File 11
File 16
File 16
File 16
File 16
File 16
... etc ...

edit: The test above was done on Mac OS X. In a (now deleted) comment the OP mentioned he was on Debian, for which the man page mentions that #define _XOPEN_SOURCE 500 is necessary, as Juliano points out.

Linux, for some reason, still uses SUSv1 for this API, where nftw() is still considered an extension.

From the Linux manual page, the include has to be:

#define _XOPEN_SOURCE 500
#include <ftw.h>