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

User · Accepted Answer

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);

User · Answer

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

[c] Create a file if one doesn't exist - C

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