getdents

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



NAME
       getdents, getdents64 - get directory entries

SYNOPSIS
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       long syscall(SYS_getdents, unsigned int fd, struct linux_dirent *dirp,
                    unsigned int count);

       #define _GNU_SOURCE           /* See feature_test_macros(7) */
       #include <dirent.h>

       ssize_t getdents64(int fd, void *dirp, size_t count);

       Note: glibc provides no wrapper for getdents(), necessitating the use of
       syscall(2).

       Note: There is no definition of struct linux_dirent in glibc; 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
       filesystems and large file offsets.  Consequently, Linux 2.4 added
       getdents64(), with wider types for the d_ino and d_off fields.  In
       addition, 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 following
       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 to indicate the
       error.

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
       Library support for getdents64() was added in glibc 2.30; Glibc does not
       provide a wrapper for getdents(); call getdents() (or getdents64() on
       earlier glibc versions) using syscall(2).  In that case you will need to
       define the linux_dirent or linux_dirent64 structure yourself.

       Probably, you want to use readdir(3) instead of these system calls.

       These calls supersede readdir(2).

EXAMPLES
       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 <stdint.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 {
           unsigned 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;
           long nread;
           char buf[BUF_SIZE];
           struct linux_dirent *d;
           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 (long 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 %10jd  %s\n", d->d_reclen,
                           (intmax_t) 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 5.13 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                              2021-03-22                        GETDENTS(2)