Create a file if one doesn't exist - C

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);