Create a file if one doesn't exist - C - 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);

Answer 2

If fptr is NULL, then you don't have an open file. Therefore, you can't freopen it, you should just fopen it.
FILE *fptr;
fptr = fopen("scores.dat", "rb+");
if(fptr == NULL) //if file does not exist, create it
{
fptr = fopen("scores.dat", "wb");
}
note: Since the behavior of your program varies depending on whether the file is opened in read or write modes, you most probably also need to keep a variable indicating which is the case.
A complete example
int main()
{
FILE *fptr;
char there_was_error = 0;
char opened_in_read = 1;
fptr = fopen("scores.dat", "rb+");
if(fptr == NULL) //if file does not exist, create it
{
opened_in_read = 0;
fptr = fopen("scores.dat", "wb");
if (fptr == NULL)
there_was_error = 1;
}
if (there_was_error)
{
printf("Disc full or no permission\n");
return EXIT_FAILURE;
}
if (opened_in_read)
printf("The file is opened in read mode."
" Let's read some cached data\n");
else
printf("The file is opened in write mode."
" Let's do some processing and cache the results\n");
return EXIT_SUCCESS;
}