race-condition - 检查文件是否存在然后创建文件时如何避免竞争条件？

Question

我在考虑我的代码中的极端情况，我不知道如何在检查文件是否存在时避免问题，如果不存在，则创建一个具有该文件名的文件。代码大致如下所示:

// 1
status = stat(filename);
if (!status) {
  // 2
  create_file(filename);
}

在调用 1 和 2 之间，另一个进程可以创建文件名。如何避免这个问题，是否有针对此类问题的通用解决方案？它们经常发生在系统编程中。

Answer 1

这就是 O_EXCL | open() 的 O_CREAT 标志设计用于:

If O_CREAT and O_EXCL are set, open() shall fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executing open() naming the same filename in the same directory with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT are set, and path names a symbolic link, open() shall fail and set errno to [EEXIST], regardless of the contents of the symbolic link. If O_EXCL is set and O_CREAT is not set, the result is undefined.

所以:

fd = open(FILENAME, O_EXCL | O_CREAT | O_RDWR);
if (fd <0) { /* file exists or there were problems like permissions */
    fprintf(stderr, "open() failed: \"%s\"\n", strerror(errno));
    abort();
}
 /* file was newly created */