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| #define _GNU_SOURCE // 启用 GNU 扩展
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h> // 包含 open, O_* flags, AT_FDCWD, AT_SYMLINK_NOFOLLOW
#include <sys/stat.h> // 包含 utimensat, futimens, timespec, stat
#include <string.h>
#include <errno.h>
#include <time.h> // 包含 time, localtime, strftime
// 辅助函数：打印文件的时间信息
void print_file_times(const char *filename) {
struct stat sb;
if (stat(filename, &sb) == -1) {
perror("stat");
return;
}
printf("File: %s\n", filename);
printf(" Last Status Change (ctime): %s", ctime(&sb.st_ctime)); // ctime 包含换行符
printf(" Last Modification (mtime): %s", ctime(&sb.st_mtime));
printf(" Last Access (atime): %s", ctime(&sb.st_atime));
printf("\n");
}
// 辅助函数：创建一个测试文件
void create_test_file(const char *filename) {
FILE *f = fopen(filename, "w");
if (!f) {
perror("fopen");
exit(EXIT_FAILURE);
}
fprintf(f, "This is a test file for utimensat/futimens example.\n");
fclose(f);
printf("Created test file: %s\n\n", filename);
}
int main() {
const char *test_file = "utimensat_test_file.txt";
const char *test_symlink = "utimensat_test_symlink.txt";
struct timespec new_times[2];
time_t fixed_time_sec;
struct tm tm_tmp;
printf("--- Demonstrating utimensat and futimens ---\n");
// 1. 创建一个测试文件
create_test_file(test_file);
// 2. 创建一个指向测试文件的符号链接
if (symlink(test_file, test_symlink) == -1) {
perror("symlink");
unlink(test_file);
exit(EXIT_FAILURE);
}
printf("Created symlink: %s -> %s\n\n", test_symlink, test_file);
// 3. 显示初始时间
printf("1. Initial timestamps:\n");
print_file_times(test_file);
print_file_times(test_symlink); // 符号链接的时间通常和目标文件一样（除非文件系统特殊支持）
// 4. 准备一个固定的时间
printf("2. Preparing fixed time...\n");
memset(&tm_tmp, 0, sizeof(tm_tmp));
tm_tmp.tm_year = 2023 - 1900; // tm_year is years since 1900
tm_tmp.tm_mon = 10 - 1; // tm_mon is 0-11
tm_tmp.tm_mday = 27;
tm_tmp.tm_hour = 10;
tm_tmp.tm_min = 0;
tm_tmp.tm_sec = 0;
tm_tmp.tm_isdst = 0;
fixed_time_sec = timegm(&tm_tmp);
if (fixed_time_sec == -1) {
perror("timegm");
unlink(test_file);
unlink(test_symlink);
exit(EXIT_FAILURE);
}
// 5. 使用 futimens 设置时间
printf("3. --- Using futimens() ---\n");
int fd = open(test_file, O_RDONLY);
if (fd == -1) {
perror("open test_file");
unlink(test_file);
unlink(test_symlink);
exit(EXIT_FAILURE);
}
new_times[0].tv_sec = fixed_time_sec; // atime
new_times[0].tv_nsec = 123456789; // atime 纳秒
new_times[1].tv_sec = fixed_time_sec; // mtime
new_times[1].tv_nsec = 987654321; // mtime 纳秒
printf("Setting timestamps using futimens()...\n");
if (futimens(fd, new_times) == -1) {
perror("futimens");
} else {
printf("futimens() succeeded.\n");
}
close(fd);
printf("Timestamps after futimens:\n");
print_file_times(test_file);
// 6. 使用 utimensat 设置时间 (相对路径)
printf("4. --- Using utimensat() with relative path ---\n");
// 等待几秒，让时间不同
sleep(2);
// 将 atime 设置为当前时间，mtime 保持不变
new_times[0].tv_sec = 0; // 无关紧要
new_times[0].tv_nsec = UTIME_NOW; // 设置为当前时间
new_times[1].tv_sec = 0; // 无关紧要
new_times[1].tv_nsec = UTIME_OMIT; // 保持 mtime 不变
printf("Setting atime to NOW and mtime to OMIT using utimensat(AT_FDCWD, ...)...\n");
// AT_FDCWD 表示相对于当前工作目录解析路径
if (utimensat(AT_FDCWD, test_file, new_times, 0) == -1) {
perror("utimensat");
} else {
printf("utimensat() succeeded.\n");
}
printf("Timestamps after utimensat (atime updated, mtime unchanged):\n");
print_file_times(test_file);
// 7. 使用 utimensat 处理符号链接
printf("5. --- Using utimensat() with symlinks ---\n");
// 准备新的时间
new_times[0].tv_sec = fixed_time_sec + 3600; // atime + 1 小时
new_times[0].tv_nsec = 111111111;
new_times[1].tv_sec = fixed_time_sec + 7200; // mtime + 2 小时
new_times[1].tv_nsec = 222222222;
// 默认情况下，utimensat 会跟随符号链接，修改目标文件
printf("Calling utimensat() on symlink WITHOUT AT_SYMLINK_NOFOLLOW...\n");
printf(" This will modify the TARGET file's timestamps.\n");
if (utimensat(AT_FDCWD, test_symlink, new_times, 0) == -1) {
perror("utimensat (follow symlink)");
} else {
printf("utimensat() succeeded (followed symlink).\n");
}
printf("Target file timestamps after utimensat (followed symlink):\n");
print_file_times(test_file);
printf("Symlink file timestamps (should be unchanged or linked):\n");
print_file_times(test_symlink);
// 现在使用 AT_SYMLINK_NOFOLLOW 来修改符号链接本身
new_times[0].tv_sec = fixed_time_sec + 10800; // atime + 3 小时
new_times[0].tv_nsec = 333333333;
new_times[1].tv_sec = fixed_time_sec + 14400; // mtime + 4 小时
new_times[1].tv_nsec = 444444444;
printf("\nCalling utimensat() on symlink WITH AT_SYMLINK_NOFOLLOW...\n");
printf(" This will modify the SYMLINK's timestamps (if filesystem supports it).\n");
if (utimensat(AT_FDCWD, test_symlink, new_times, AT_SYMLINK_NOFOLLOW) == -1) {
if (errno == EOPNOTSUPP) {
printf("utimensat with AT_SYMLINK_NOFOLLOW failed: %s\n", strerror(errno));
printf(" This is expected on many filesystems (e.g., ext4).\n");
} else {
perror("utimensat (nofollow symlink)");
}
} else {
printf("utimensat() succeeded (modified symlink itself).\n");
print_file_times(test_symlink);
}
// 8. 清理
printf("\n6. --- Cleaning up ---\n");
if (unlink(test_file) == 0) {
printf("Deleted test file '%s'.\n", test_file);
} else {
perror("unlink test_file");
}
if (unlink(test_symlink) == 0) {
printf("Deleted symlink '%s'.\n", test_symlink);
} else {
perror("unlink test_symlink");
}
printf("\n--- Summary ---\n");
printf("1. futimens(fd, times[2]): Sets atime/mtime for a file via its file descriptor.\n");
printf("2. utimensat(dirfd, pathname, times[2], flags): Sets atime/mtime via pathname, with more options.\n");
printf("3. Both use struct timespec, providing nanosecond precision.\n");
printf("4. Special timespec values:\n");
printf(" - tv_nsec = UTIME_NOW: Set timestamp to current time.\n");
printf(" - tv_nsec = UTIME_OMIT: Leave timestamp unchanged.\n");
printf("5. utimensat flags:\n");
printf(" - 0 (default): Follow symlinks.\n");
printf(" - AT_SYMLINK_NOFOLLOW: Modify symlink itself (filesystem support varies).\n");
printf(" - dirfd allows relative path resolution (like openat).\n");
printf("6. These are the modern, preferred functions for changing file timestamps.\n");
return 0;
}
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