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| #define _GNU_SOURCE // 启用 GNU 扩展
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h> // 包含 stat, lstat
#include <fcntl.h> // 包含 open, O_* flags
#include <string.h>
#include <errno.h>
#include <libgen.h> // 包含 dirname, basename
// 辅助函数：打印文件信息
void print_file_info(const char *path, const char* description) {
struct stat sb;
printf("--- %s: %s ---\n", description, path);
// 使用 lstat 获取符号链接本身的信息
if (lstat(path, &sb) == -1) {
perror("lstat");
return;
}
printf(" Inode: %ld\n", (long) sb.st_ino);
printf(" Links: %ld\n", (long) sb.st_nlink);
printf(" Size: %ld bytes\n", (long) sb.st_size);
if (S_ISLNK(sb.st_mode)) {
printf(" Type: Symbolic Link\n");
// 读取符号链接指向的目标
char target_path[1024];
ssize_t len = readlink(path, target_path, sizeof(target_path) - 1);
if (len != -1) {
target_path[len] = '\0';
printf(" -> Points to: %s\n", target_path);
} else {
perror("readlink");
}
} else if (S_ISREG(sb.st_mode)) {
printf(" Type: Regular File\n");
} else if (S_ISDIR(sb.st_mode)) {
printf(" Type: Directory\n");
} else {
printf(" Type: Other (Mode: %o)\n", sb.st_mode & S_IFMT);
}
printf("\n");
}
int main() {
const char *original_file = "original_file.txt";
const char *hard_link = "hard_link_to_original.txt";
const char *sym_link_absolute = "symlink_absolute_to_original.txt";
const char *sym_link_relative = "symlink_relative_to_original.txt";
const char *sym_link_dangling = "symlink_to_nonexistent.txt";
const char *test_dir = "test_symlink_dir";
const char *sym_link_in_dir = "symlink_in_dir.txt";
int dirfd;
printf("--- Demonstrating symlink and symlinkat ---\n");
// 1. 创建一个原始文件
FILE *f = fopen(original_file, "w");
if (!f) {
perror("fopen original_file.txt");
exit(EXIT_FAILURE);
}
fprintf(f, "This is the content of the original file.\n");
fclose(f);
printf("Created original file: %s\n", original_file);
// 2. 创建硬链接
if (link(original_file, hard_link) == -1) {
perror("link");
// 清理并退出
unlink(original_file);
exit(EXIT_FAILURE);
}
printf("Created hard link: %s -> %s\n", hard_link, original_file);
// 3. 创建符号链接 (绝对路径)
// 注意：target 是字符串，不一定需要存在
if (symlink("/full/path/to/somewhere", sym_link_absolute) == -1) {
perror("symlink absolute");
// 清理并退出
unlink(original_file);
unlink(hard_link);
exit(EXIT_FAILURE);
}
printf("Created symbolic link (absolute target): %s -> /full/path/to/somewhere\n", sym_link_absolute);
// 4. 创建符号链接 (相对路径)
if (symlink(original_file, sym_link_relative) == -1) {
perror("symlink relative");
// 清理并退出
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
exit(EXIT_FAILURE);
}
printf("Created symbolic link (relative target): %s -> %s\n", sym_link_relative, original_file);
// 5. 创建一个指向不存在文件的符号链接 (悬空链接)
if (symlink("this_file_does_not_exist.txt", sym_link_dangling) == -1) {
perror("symlink dangling");
// 清理并退出
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
unlink(sym_link_relative);
exit(EXIT_FAILURE);
}
printf("Created dangling symbolic link: %s -> this_file_does_not_exist.txt\n", sym_link_dangling);
// 6. 演示 symlinkat
// 首先创建一个目录
if (mkdir(test_dir, 0755) == -1 && errno != EEXIST) {
perror("mkdir test_symlink_dir");
// 清理并退出
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
unlink(sym_link_relative);
unlink(sym_link_dangling);
exit(EXIT_FAILURE);
}
printf("\nCreated directory: %s\n", test_dir);
// 打开目录以获取文件描述符
dirfd = open(test_dir, O_RDONLY | O_DIRECTORY);
if (dirfd == -1) {
perror("open test_symlink_dir");
// 清理并退出
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
unlink(sym_link_relative);
unlink(sym_link_dangling);
rmdir(test_dir);
exit(EXIT_FAILURE);
}
// 使用 symlinkat 在目录中创建符号链接
// linkpath 是相对路径 "symlink_in_dir.txt"，它相对于 dirfd (test_symlink_dir) 解析
// target 是 "../original_file.txt"，这是符号链接存储的内容
if (symlinkat("../original_file.txt", dirfd, sym_link_in_dir) == -1) {
perror("symlinkat");
close(dirfd);
// 清理并退出
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
unlink(sym_link_relative);
unlink(sym_link_dangling);
rmdir(test_dir);
exit(EXIT_FAILURE);
}
printf("Created symbolic link using symlinkat: %s/%s -> ../original_file.txt\n", test_dir, sym_link_in_dir);
close(dirfd);
// 7. 检查所有创建的文件/链接的信息
printf("\n--- File Information ---\n");
print_file_info(original_file, "Original File");
print_file_info(hard_link, "Hard Link");
print_file_info(sym_link_absolute, "Symbolic Link (Absolute)");
print_file_info(sym_link_relative, "Symbolic Link (Relative)");
print_file_info(sym_link_dangling, "Dangling Symbolic Link");
char full_sym_link_in_dir[512];
snprintf(full_sym_link_in_dir, sizeof(full_sym_link_in_dir), "%s/%s", test_dir, sym_link_in_dir);
print_file_info(full_sym_link_in_dir, "Symbolic Link (Created with symlinkat)");
// 8. 演示访问符号链接
printf("--- Accessing Files ---\n");
// 访问原始文件
if (access(original_file, F_OK) == 0) {
printf("Can access original file '%s'.\n", original_file);
}
// 访问硬链接 (效果同原始文件)
if (access(hard_link, F_OK) == 0) {
printf("Can access hard link '%s'.\n", hard_link);
}
// 访问相对符号链接 (应该成功，指向存在的文件)
if (access(sym_link_relative, F_OK) == 0) {
printf("Can access symbolic link '%s' (follows to existing target).\n", sym_link_relative);
} else {
printf("Cannot access symbolic link '%s': %s\n", sym_link_relative, strerror(errno));
}
// 访问悬空符号链接 (会失败)
if (access(sym_link_dangling, F_OK) == 0) {
printf("Can access dangling symbolic link '%s' (unexpected).\n", sym_link_dangling);
} else {
printf("Cannot access dangling symbolic link '%s': %s (expected)\n", sym_link_dangling, strerror(errno));
}
// 9. 清理创建的文件和目录
printf("\n--- Cleaning Up ---\n");
unlink(original_file);
unlink(hard_link);
unlink(sym_link_absolute);
unlink(sym_link_relative);
unlink(sym_link_dangling);
unlink(full_sym_link_in_dir);
rmdir(test_dir);
printf("All files and directory cleaned up.\n");
printf("\n--- Summary ---\n");
printf("1. Hard links (link) point to the same inode as the original file.\n");
printf("2. Symbolic links (symlink) are separate files containing a path string.\n");
printf("3. Symbolic links can point to non-existent targets (dangling).\n");
printf("4. symlinkat allows creating symlinks relative to a directory file descriptor.\n");
return 0;
}
|