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man:getdents64

GETDENTS(2) Linux Programmer's Manual GETDENTS(2)

NAME

     getdents, getdents64 - get directory entries

SYNOPSIS

     int getdents(unsigned int fd, struct linux_dirent *dirp,
                  unsigned int count);
     int getdents64(unsigned int fd, struct linux_dirent64 *dirp,
                  unsigned int count);
     Note: There are no glibc wrappers for these system calls; see NOTES.

DESCRIPTION

     These are not the interfaces you are interested in.  Look at readdir(3)
     for the POSIX-conforming C library interface.  This page documents  the
     bare kernel system call interfaces.
 getdents()
     The  system  call getdents() reads several linux_dirent structures from
     the directory referred to by the  open  file  descriptor  fd  into  the
     buffer  pointed  to  by dirp.  The argument count specifies the size of
     that buffer.
     The linux_dirent structure is declared as follows:
         struct linux_dirent {
             unsigned long  d_ino;     /* Inode number */
             unsigned long  d_off;     /* Offset to next linux_dirent */
             unsigned short d_reclen;  /* Length of this linux_dirent */
             char           d_name[];  /* Filename (null-terminated) */
                               /* length is actually (d_reclen - 2 -
                                  offsetof(struct linux_dirent, d_name)) */
             /*
             char           pad;       // Zero padding byte
             char           d_type;    // File type (only since Linux
                                       // 2.6.4); offset is (d_reclen - 1)
             */ }
     d_ino is an inode number.  d_off is the distance from the start of  the
     directory  to the start of the next linux_dirent.  d_reclen is the size
     of this entire linux_dirent.  d_name is a null-terminated filename.
     d_type is a byte at the end of the structure that  indicates  the  file
     type.  It contains one of the following values (defined in <dirent.h>):
     DT_BLK      This is a block device.
     DT_CHR      This is a character device.
     DT_DIR      This is a directory.
     DT_FIFO     This is a named pipe (FIFO).
     DT_LNK      This is a symbolic link.
     DT_REG      This is a regular file.
     DT_SOCK     This is a UNIX domain socket.
     DT_UNKNOWN  The file type is unknown.
     The d_type field is implemented since Linux 2.6.4.  It occupies a space
     that  was  previously  a  zero-filled  padding byte in the linux_dirent
     structure.  Thus, on kernels up to and including 2.6.3,  attempting  to
     access this field always provides the value 0 (DT_UNKNOWN).
     Currently,  only  some  filesystems (among them: Btrfs, ext2, ext3, and
     ext4) have full support for returning the file  type  in  d_type.   All
     applications must properly handle a return of DT_UNKNOWN.
 getdents64()
     The original Linux getdents() system call did not handle large filesys-
     tems and large  file  offsets.   Consequently,  Linux  2.4  added  get-
     dents64(),  with  wider types for the d_ino and d_off fields.  In addi-
     tion, getdents64() supports an explicit d_type field.
     The getdents64() system call is like getdents(), except that its second
     argument  is a pointer to a buffer containing structures of the follow-
     ing type:
         struct linux_dirent64 {
             ino64_t        d_ino;    /* 64-bit inode number */
             off64_t        d_off;    /* 64-bit offset to next structure */
             unsigned short d_reclen; /* Size of this dirent */
             unsigned char  d_type;   /* File type */
             char           d_name[]; /* Filename (null-terminated) */ };

RETURN VALUE

     On success, the number of bytes read is returned.  On end of directory,
     0  is  returned.   On error, -1 is returned, and errno is set appropri-
     ately.

ERRORS

     EBADF  Invalid file descriptor fd.
     EFAULT Argument points outside the calling process's address space.
     EINVAL Result buffer is too small.
     ENOENT No such directory.
     ENOTDIR
            File descriptor does not refer to a directory.

CONFORMING TO

     SVr4.

NOTES

     Glibc does not provide a wrapper for  these  system  calls;  call  them
     using  syscall(2).   You  will  need  to  define  the  linux_dirent  or
     linux_dirent64 structure yourself.  However, you probably want  to  use
     readdir(3) instead.
     These calls supersede readdir(2).

EXAMPLE

     The  program  below  demonstrates the use of getdents().  The following
     output shows an example of what we see when running this program on  an
     ext2 directory:
         $ ./a.out /testfs/ --------------- nread=120 --------------- inode#
         file type  d_reclen  d_off   d_name
                2  directory    16         12  .
                2  directory    16         24  ..
               11  directory    24         44  lost+found
               12  regular      16         56  a
           228929  directory    16         68  sub
            16353  directory    16         80  sub2
           130817  directory    16       4096  sub3
 Program source
      #define _GNU_SOURCE #include <dirent.h>     /* Defines DT_*  constants
     */  #include  <fcntl.h> #include <stdio.h> #include <unistd.h> #include
     <stdlib.h> #include <sys/stat.h> #include <sys/syscall.h>
     #define handle_error(msg) \
             do { perror(msg); exit(EXIT_FAILURE); } while (0)
     struct linux_dirent {
         long           d_ino;
         off_t          d_off;
         unsigned short d_reclen;
         char           d_name[]; };
     #define BUF_SIZE 1024
     int main(int argc, char *argv[]) {
         int fd, nread;
         char buf[BUF_SIZE];
         struct linux_dirent *d;
         int bpos;
         char d_type;
         fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
         if (fd == -1)
             handle_error("open");
         for ( ; ; ) {
             nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
             if (nread == -1)
                 handle_error("getdents");
             if (nread == 0)
                 break;
             printf("--------------- nread=%d ---------------\n", nread);
             printf("inode#    file type  d_reclen  d_off   d_name\n");
             for (bpos = 0; bpos < nread;) {
                 d = (struct linux_dirent *) (buf + bpos);
                 printf("%8ld  ", d->d_ino);
                 d_type = *(buf + bpos + d->d_reclen - 1);
                 printf("%-10s ", (d_type == DT_REG) ?  "regular" :
                                  (d_type == DT_DIR) ?  "directory" :
                                  (d_type == DT_FIFO) ? "FIFO" :
                                  (d_type == DT_SOCK) ? "socket" :
                                  (d_type == DT_LNK) ?  "symlink" :
                                  (d_type == DT_BLK) ?  "block dev" :
                                  (d_type == DT_CHR) ?  "char dev" : "???");
                 printf("%4d %10lld  %s\n", d->d_reclen,
                         (long long) d->d_off, d->d_name);
                 bpos += d->d_reclen;
             }
         }
         exit(EXIT_SUCCESS); }

SEE ALSO

     readdir(2), readdir(3), inode(7)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 GETDENTS(2)

/data/webs/external/dokuwiki/data/pages/man/getdents64.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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