CODEWITHSUNDEEP
cfile-io
czchlong
czchlong

Reputation: 2574

How to overwrite a file in C?

I am trying to overwrite the contents of a FILE in C. Currently I have:

FILE* file  = fopen("filename.txt",  "r+");
fprintf(file, "%d", 1); // regardless of what's in the file, i want to clear it and put 1 in there
...
// legacy code somewhere else in the code base. can't change.
rewind(file);
fprintf(file, "%d", 2);
fflush(file);

However, this will not work properly. The result will be:

1, 21

Each subsequent number will be written to the beginning of the 1. For example:

1, 21, 31, 41, ...

I would like to know if there is a way to always overwrite what's in the file so the following is produced:

1, 2, 3, 4, ...

Any assistance would be appreciated.

Thank you.

EDIT:

I have changed the code to:

FILE* file  = fopen("filename.txt",  "w+");

The problem still persists.

Upvotes: 8

Views: 72212

Answers (4)

angry_nerd
angry_nerd

Reputation: 49

I was going through the same problem. I wanted to overwrite the same file from what I was taking the input. So after trying different methods, I decided to create another pointer that points to the same file. So I have one pointer which reads the input and then I close that using fclose() and I have another pointer for overwriting the same file and again I close that using fclose(). That worked for me.

Upvotes: -1

Bo Persson
Bo Persson

Reputation: 92211

You decide that in fopen. Just use "w" or "w+" instead of "r+".

Upvotes: 20

Servalun
Servalun

Reputation: 21

use following command before fprintf:

freopen(NULL,"w+",file);

Upvotes: 2

Daniel Landau
Daniel Landau

Reputation: 2314

As far as I can tell, you can't overwrite anything with fprintf. You have to use fwrite, e.g. something like

rewind(file);
char buf[10];
sprintf(buf, "%d", 2);
fwrite(buf, strlen(buf), 1, file);

From your code and question I don't quite follow what it is you actually try to do, but hope this helps (half a year after you asked).

Upvotes: 4

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