struct dirent in C d_name[256] and NAME_MAX definition

Question

I am confused by the use of d_name[256] and NAME_MAX in struct dirent definition. Does d_name[256] mean that the filename length can be atmost 256 character? Then it also mentions NAME_MAX (quoted in the bottom). So, my question is how NAME_MAX relates here and where can I find NAME_MAX value and definition?

In man readdir struct dirent is defined as following.

struct dirent {
    ino_t          d_ino;       /* inode number */
    off_t          d_off;       /* not an offset; see NOTES */
    unsigned short d_reclen;    /* length of this record */
    unsigned char  d_type;      /* type of file; not supported
                                    by all filesystem types */
    char           d_name[256]; /* filename */
};

It also asserts that

The only fields in the dirent structure that are mandated by POSIX.1 are: d_name[], of unspecified size, with at most NAME_MAX characters preceding the terminating null byte ('\0'); and (as an XSI exten‐ sion) d_ino. The other fields are unstandardized, and not present on all systems; see NOTES below for some further details.

Answer 1

NAME_MAX is declared in limits.h. You can also use pathconf() or fpathconf() to get the per-filesystem limit.

long max = pathconf(pathname, _PC_NAME_MAX);

Since the structure has this hard-coded to 256, it can't actually handle filesystems with longer filenames. So NAME_MAX will necessarily be at most 255 (which is indeed its value on my OS X machine).