Unix Prog: Files -- System Calls

1. System calls for getting the file information
stat, fstat, and lstat are used to get the specific file detailed information, and save in buffer "struct stat".
The difference between fstat and lstat:
for lstat, if the given file is one symbolic link, it will just return the symbolic link file's information instead of the file it points to.

Above three system calls are defined at /usr/include/x86_64-linux-gnu/sys/stat.h in current system, and the struct stat is defined at /usr/include/x86_64-linux-gnu/bits/stat.h

 ubuntu@ip-172-31-23-227:~$ less /usr/include/x86_64-linux-gnu/sys/stat.h  
 ......  
 /* Get file attributes for FILE and put them in BUF. */  
 extern int stat (const char *__restrict __file,  
          struct stat *__restrict __buf) __THROW __nonnull ((1, 2));  
   
 /* Get file attributes for the file, device, pipe, or socket  
   that file descriptor FD is open on and put them in BUF. */  
 extern int fstat (int __fd, struct stat *__buf) __THROW __nonnull ((2));  
 ......  
 /* Get file attributes about FILE and put them in BUF.  
   If FILE is a symbolic link, do not follow it. */  
 extern int lstat (const char *__restrict __file,  
          struct stat *__restrict __buf) __THROW __nonnull ((1, 2));  
 ......  
ubuntu@ip-172-31-23-227:~$ less /usr/include/x86_64-linux-gnu/bits/stat.h
......
struct stat
  {
    __dev_t st_dev;             /* Device.  */
#ifndef __x86_64__
    unsigned short int __pad1;
#endif
#if defined __x86_64__ || !defined __USE_FILE_OFFSET64
    __ino_t st_ino;             /* File serial number.  */
#else
    __ino_t __st_ino;                   /* 32bit file serial number.    */
#endif
#ifndef __x86_64__
    __mode_t st_mode;                   /* File mode.  */
    __nlink_t st_nlink;                 /* Link count.  */
#else
    __nlink_t st_nlink;         /* Link count.  */
    __mode_t st_mode;           /* File mode.  */
#endif
    __uid_t st_uid;             /* User ID of the file's owner. */
    __gid_t st_gid;             /* Group ID of the file's group.*/
#ifdef __x86_64__
    int __pad0;
#endif
    __dev_t st_rdev;            /* Device number, if device.  */
#ifndef __x86_64__
    unsigned short int __pad2;
#endif
#if defined __x86_64__ || !defined __USE_FILE_OFFSET64
    __off_t st_size;                    /* Size of file, in bytes.  */
#else
    __off64_t st_size;                  /* Size of file, in bytes.  */
#endif
    __blksize_t st_blksize;     /* Optimal block size for I/O.  */
#if defined __x86_64__  || !defined __USE_FILE_OFFSET64
    __blkcnt_t st_blocks;               /* Number 512-byte blocks allocated. */
#else
    __blkcnt64_t st_blocks;             /* Number 512-byte blocks allocated. */
#endif
#if defined __USE_MISC || defined __USE_XOPEN2K8
    /* Nanosecond resolution timestamps are stored in a format
       equivalent to 'struct timespec'.  This is the type used
       whenever possible but the Unix namespace rules do not allow the
       identifier 'timespec' to appear in the  header.
       Therefore we have to handle the use of this header in strictly
       standard-compliant sources special.  */
    struct timespec st_atim;            /* Time of last access.  */
    struct timespec st_mtim;            /* Time of last modification.  */
    struct timespec st_ctim;            /* Time of last status change.  */
# define st_atime st_atim.tv_sec        /* Backward compatibility.  */
# define st_mtime st_mtim.tv_sec
# define st_ctime st_ctim.tv_sec
#else
    __time_t st_atime;                  /* Time of last access.  */
    __syscall_ulong_t st_atimensec;     /* Nscecs of last access.  */
    __time_t st_mtime;                  /* Time of last modification.  */
    __syscall_ulong_t st_mtimensec;     /* Nsecs of last modification.  */
    __time_t st_ctime;                  /* Time of last status change.  */
    __syscall_ulong_t st_ctimensec;     /* Nsecs of last status change.  */
#endif
#ifdef __x86_64__
    __syscall_slong_t __glibc_reserved[3];
#else
# ifndef __USE_FILE_OFFSET64
    unsigned long int __glibc_reserved4;
    unsigned long int __glibc_reserved5;
# else
    __ino64_t st_ino;                   /* File serial number.  */
# endif
#endif
  };
......