Why is d_reclen variable

Question

I can get the size of struct dirent precisely using sizeof operator。In my PC it printed 280.Its definition in dirent.h is as follows.

struct dirent
  {
#ifndef __USE_FILE_OFFSET64
    __ino_t d_ino;
    __off_t d_off;
#else
    __ino64_t d_ino;
    __off64_t d_off;
#endif
    unsigned short int d_reclen;
    unsigned char d_type;
    char d_name[256];       /* We must not include limits.h! */
  };

It's correct.But I also noted that d_reclen is a member that shows the length of the record,it prints numbers like 24, 32, 40.etc. So in memeory, which one is the real size of the struct?

Answer 1

The fact of sizeof() showing 280, even if that reflects the real room for the struct in memory, is a don't care. Albeit d_name is [256] in this particular definition, POSIX defines it as d_name[], a character array of unspecified size. Depending on directory name size (shorter / longer), one indeed should be able to observe different values in d_reclen, such as 24 or 32 in the above example.

Please see "The d_name field" section under "NOTES" in the man page.

The short of it, doing sizeof() on d_name is incorrect. Applications should either use d_reclen or perform strlen() on d_name to know the name length.