Is there a reason fopen() wouldn't work after several hundred opens?

Question

Hey, for this piece of code, the person who wrote the system communicates data between processes using textfiles. I have a loops that looks (for all intents and purposes) like this:

while (true)
{
 //get the most up-to-date info from the other processes
 pFile = fopen(paramsFileName, "r");

 // Do a bunch of stuff with pFile

 Sleep(100);
}

This will work for several hundred times, but for whatever reason it will return NULL after a while, even though it has opened that same file path several hundred times already! I have double checked that the file exists and has data in it when the fopen returns NULL, and have tried to put a delay/retry in there to no effect.

What can you think of that would cause this?

Answer 1

You're hitting the open file / file descriptor limit for your OS. It should run forever if you do fclose(pFile) in your loop.

Answer 2

Try doing a count of how many times the file has been opened before it fails.

As well, it's possible the OS is opening the file for a scan, which blocks the fopen function, returning null, since it was unsuccessful in opening the file.

Answer 3

Why are you doing it that way? Two ways to deal with this

while (true)
{
 //get the most up-to-date info from the other processes
 pFile = fopen(paramsFileName, "r");

 // Do a bunch of stuff with pFile

 fclose(pFile);

 //
 Sleep(100);
}

or Move the fopen call to outside of the loop

//get the most up-to-date info from the other processes
    pFile = fopen(paramsFileName, "r");
    while (true)
    {
     // Do a bunch of stuff with pFile

     Sleep(100);
    }
    fclose(pFile);

Not surprising that you hit the OS's limit on the number of files open by constantly calling fopen in your case...

Answer 4

You really want to check your return codes. I suspect perror/strerror with the right errno would report that you've exausted your file descriptor limit.

Try something like this and see if you get a good error message.

FILE* f = fopen(filename);
if (NULL == f) {
    fprintf(stderr, 
            "Could not open: %s. %s\n", 
            filename, 
            strerror(errno);
}