Question

I want my program to open a file if it exists, or else create the file. I’m trying the following code but I’m getting a debug assertion at freopen.c. Would I be better off using fclose and then fopen immediately afterward?

FILE *fptr;
    fptr = fopen("scores.dat", "rb+");
    if(fptr == NULL) //if file does not exist, create it
    {
        freopen("scores.dat", "wb", fptr);
    } 

Answer 1

You typically have to do this in a single syscall, or else you will get a race condition.

This will open for reading and writing, creating the file if necessary.

FILE *fp = fopen("scores.dat", "ab+");

If you want to read it and then write a new version from scratch, then do it as two steps.

FILE *fp = fopen("scores.dat", "rb");
if (fp) {
    read_scores(fp);
}

// Later...

// truncates the file
FILE *fp = fopen("scores.dat", "wb");
if (!fp)
    error();
write_scores(fp);