Question

I wrote a UNIX daemon (targeting Debian, but it shouldn’t matter) and I wanted to provide some way of creating a “.pid” file, (a file which contains the process identifier of the daemon).

I searched for a way of opening a file only if it doesn’t exist, but couldn’t find one.

Basically, I could do something like:

if (fileexists())
{
  //fail...
}
else
{
  //create it with fopen() or similar
}

But as it stands, this code does not perform the task in a atomic fashion, and doing so would be dangerous, because another process might create the file during my test, and the file creation.

Do you guys have any idea on how to do that?

Thank you.

P.S: Bonus point for a solution which only involves std::streams.

Answer 1

man 2 open:

O_EXCL Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open()
will fail. The behavior of O_EXCL is undefined if O_CREAT is not specified.

so, you could call fd = open(name, O_CREAT | O_EXCL, 0644); /* Open() is atomic. (for a reason) */

UPDATE: and you should of course OR one of the O_RDONLY, O_WRONLY, or O_RDWR flags into the flags argument.