open with O_CREAT - was it opened or created?

Question

I have 10 processes which try open the same file more or less at the same time using open(O_CREAT) call, then delete it. Is there any robust way to find out which process actually did create the file and which did open already create file, for instance, if I want to accurately count how many times that file was opened in such scenario.

I guess I could put a global mutex on file open operation, and do a sequence of open() calls using O_CREAT and O_EXCL flags, but that doesn't fit my definition of "robust".

Answer 1

Use O_EXCL flag with O_CREAT. This will fail if the file exists and errno will be set to EEXIST. If it does fail then attempt open again without O_CREAT and without O_EXCL modes.

e.g.

int fd = open(path, O_WRONLY | O_CREAT | O_EXCL, 0644);
if ((fd == -1) && (EEXIST == errno))
{
    /* open the existing file with write flag */
    fd = open(path, O_WRONLY);
}