getdents64 - Linux

Overview

getdents64 is a Linux system call used to retrieve directory entries from a file system. It is designed to efficiently handle large directories with a 64-bit data structure, enabling it to retrieve more entries per system call.

Syntax

#include <dirent.h>

int getdents64(int fd, struct dirent64 *buf, size_t nbytes);

Options/Flags

None

Parameters

  • fd: File descriptor of the directory to read entries from.
  • buf: Pointer to a buffer where the directory entries will be stored.
  • nbytes: Size of the buffer in bytes.

Return Value

  • On success, getdents64 returns the number of bytes written to the buffer.
  • On error, it returns -1 and sets errno to indicate the error.

Examples

Simple Usage:

Retrieve directory entries from the current working directory:

#include <dirent.h>
#include <stdio.h>

int main() {
  DIR *dir = opendir(".");
  struct dirent64 *entry;

  while ((entry = readdir64(dir))) {
    printf("%s\n", entry->d_name);
  }

  closedir(dir);
  return 0;
}

Retrieving Specific Information:

Obtain file type and size from directory entries:

#include <dirent.h>
#include <stdio.h>
#include <sys/stat.h>

int main() {
  DIR *dir = opendir(".");
  struct dirent64 *entry;

  while ((entry = readdir64(dir))) {
    struct stat st;
    stat(entry->d_name, &st);
    printf("%s: %ld bytes, type: ", entry->d_name, st.st_size);
    if (S_ISREG(st.st_mode))
      printf("regular file\n");
    else if (S_ISDIR(st.st_mode))
      printf("directory\n");
    else
      printf("other\n");
  }

  closedir(dir);
  return 0;
}

Common Issues

  • Insufficient Buffer Size: If the provided buffer (buf) is too small to hold all the directory entries, getdents64 will return ENOENT.
  • Invalid File Descriptor: Using an invalid file descriptor for fd will result in EBADF.

Integration

getdents64 can be used in conjunction with other Linux commands and tools, such as:

  • ls: Retrieve a list of files and directories in a given directory.
  • find: Search for files and directories based on specific criteria.
  • cp and mv: Copy or move files and directories by leveraging directory entries to identify the source and destination.

Related Commands

  • opendir, readdir, closedir: Other system calls for directory operations.
  • lsof: Lists open files and sockets, which can include file descriptors for directories.