为什么将数据写入使用 O_APPEND 标志打开的文件，始终写入最后，即使使用 `lseek` 也是如此？

Question

我被分配了一个编程任务：

编写一个程序，使用 O_APPEND 标志打开现有文件进行写入，然后在写入一些数据之前查找文件的开头。数据出现在文件中的什么位置？为什么？

我想出的是：

main() {
    int fd = open("test.txt", O_WRONLY | O_APPEND);
    lseek(fd, 0, SEEK_SET);
    write(fd, "abc", 3);
    close(fd);
}

通过上面的尝试，我发现数据总是写在文件的末尾。这是为什么？是不是因为我注明了O_APPEND？

Answer 1

当您使用O_APPEND 打开文件时，所有数据都会写入到末尾，无论当前文件指针是来自最近对lseek(2) 的调用还是最近的读/写操作。来自open(2) documentation：

O_APPEND
该文件以附加模式打开。在每个write(2)之前，文件偏移量位于文件末尾，就像lseek(2)一样。

如果你想在文件的末尾写入数据，然后在文件的开头，打开它没有O_APPEND，使用fstat(2)获取文件大小（st_size成员在struct stat中），然后寻找该偏移量以写入结尾。

Answer 2

实际上，O_APPEND 只影响write 的行为，而不影响read 的行为。无论lseek如何改变文件的当前位置，write将始终append-only。

当您open 使用O_RDWR | O_APPEND 的文件时，read 仍将从文件的开头开始。

在open(man 2 open)的手册中，

O_APPEND 该文件以附加模式打开。在每次write(2)之前，文件偏移量位于文件末尾。

在write(man 2 write)的手册中，

如果设置了文件状态标志的 O_APPEND 标志，则文件偏移量 应在每次写入之前设置到文件末尾。

在Linux内核fs/ext4 syscall write -> vfs_write -> ext4_file_write_iter中， ext4_file_write_iter 将调用 ext4_write_checks

然后拨打generic_write_checks

你会找到设置pos = file.size的地方

/* FIXME: this is for backwards compatibility with 2.4 */
if (iocb->ki_flags & IOCB_APPEND)
    iocb->ki_pos = i_size_read(inode);
pos = iocb->ki_pos;

下面的demo可以验证一下。

cat open_append.cc
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

#include <string>
#include <iostream>

int main(int argc, char *argv[]) {
  std::string path = "./test.txt";
  std::string content = "hello_world";
  std::string read_buf(content.size(), 0x0);
  struct stat st_buf;
  ssize_t bytes_read = -1;
  ssize_t bytes_write = -1;
  int ret = -1;
  off_t cur_off = -1;
  int fd = ::open(path.c_str(), O_CREAT | O_RDWR | O_TRUNC, 0644);
  if (fd < 0) {
    std::cerr << "open err path " << path
              << " errno " << errno << std::endl;
    return -1;
  }
  std::cout << "open ok path " << path
            << " fd " << fd << std::endl;

  // Step 1 write some data into an empty file
  bytes_write = ::write(fd, content.data(), content.size());
  if (bytes_write < 0) {
    std::cerr << "write err fd " << fd
              << " errno " << errno << std::endl;
    goto out;
  }
  std::cout << "write ok fd " << fd
            << " data " << content
            << " nbytes " << bytes_write << std::endl;
  ::close(fd);

  // Step 2 open the file again with O_APPEND
  fd = -1;
  fd = ::open(path.c_str(), O_CREAT | O_RDWR | O_APPEND, 0644);
  if (fd < 0) {
    std::cerr << "open again err path " << path
              << " errno " << errno << std::endl;
    return -1;
  }
  std::cout << "open again ok path " << path
            << " fd " << fd << std::endl;

  // Step 3 the current position of the file NOT affected by O_APPEND
  cur_off = ::lseek(fd, 0, SEEK_CUR);
  if (cur_off < 0) {
    std::cerr << "lseek err SEEK_CUR fd " << fd
              << " errno " << errno << std::endl;
    goto out;
  }
  // cur_off expected to be 0
  std::cout << "lseek ok SEEK_CUR fd " << fd
            << " cur_off " << cur_off << std::endl;

  // Step 4  the read will start from the beginning of the file
  bytes_read = read(fd, (char*)read_buf.data(), content.size());
  if (bytes_read < 0) {
    std::cerr << "read err fd " << fd
              << " errno " << errno << std::endl;
    goto out;
  }
  std::cout << "read ok fd " << fd
            << " data " << read_buf
            << " nbytes " << bytes_read << std::endl;

  // Step 5 change the position to the half of the file size
  cur_off = ::lseek(fd, content.size() / 2, SEEK_SET);
  if (cur_off < 0) {
    std::cerr << "lseek err SEEK_SET fd " << fd
              << " errno " << errno << std::endl;
    goto out;
  }
  // cur_off expected to be content.size() / 2
  std::cout << "lseek ok SEEK_SET fd " << fd
            << " cur_off " << cur_off << std::endl;

  // Step 6 write will append data from the end of the file
  // the current position is ignored
  bytes_write = ::write(fd, content.data(), content.size());
  if (bytes_write < 0) {
    std::cerr << "append write err fd " << fd
              << " errno " << errno << std::endl;
    goto out;
  }
  std::cout << "append write ok fd " << fd
            << " append data " << content
            << " append nbytes " << bytes_write << std::endl;

  // Step 7 the file size is double content.size()
  memset((void*)&st_buf, 0x0, sizeof(struct stat));
  ret = lstat(path.c_str(), &st_buf);
  if (ret < 0) {
    std::cerr << "lstat err path " << path
              << " errno " << errno << std::endl;
    goto out;
  }
  std::cout << "lstat ok path " << path
            << " st_size " << st_buf.st_size << std::endl;
  ret = 0;

out:
  if (fd >= 0) {
    close(fd);
  }
  return ret;
}

输出结果

open ok path ./test.txt fd 3
write ok fd 3 data hello_world nbytes 11
open again ok path ./test.txt fd 3
lseek ok SEEK_CUR fd 3 cur_off 0
read ok fd 3 data hello_world nbytes 11
lseek ok SEEK_SET fd 3 cur_off 5
append write ok fd 3 append data hello_world append nbytes 11
lstat ok path ./test.txt st_size 22

Answer 3

O_APPEND 标志强制文件指针仅指向文件末尾。因此，如果您从文件开头执行 lseek，它会将更新后的文件指针位置作为文件的开头，即旧文件的结束位置。