Why is d_reclen variable-CodePudding

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?

CodePudding user response：

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.