Is FILE * reassignment without fclose() okay?

user_13863346 0
Viewed 227

Suppose I have the following code :

FILE *x = fopen("story.txt",r);
if(x!=NULL)
{
    x = fopen("story.txt",wb); /* <- Does this waste/leak memory ?? */
    /* ... do something .... */
    /* if(fclose(x)==EOF)...else... */
}
else
{
    printf("story.txt does not exist\n");
}

Here, I'm reassigning the pointer to where a buffer for story.txtwas created in memory , without fclose(x) , assuming that since it is a pointer, the memory area will simply be overwritten to with the ' new ' buffer. Is this assumption accurate ? Am I leaking or wasting memory , or is the approach okay ?

fclose file-pointer fopen file-io c
2 Answers

You just leaked a file handle. If you do it enough, you'll run out of open file handles to allocate, and all attempts to open a file will fail. Don't do that. fclose your FILE*s.

There is no reason to think the "memory area" would be overwritten here; you're overwriting the pointer, not what it points to, and what it points to is the file related structures (stdio buffers, descriptor of the underlying kernel file handle, etc.). It no more overwrites the memory being pointed to than assigning a second malloc to the same pointer overwrites what the first malloc pointed to; you're leaking the first resource acquisition in both cases by losing the original pointer in favor of the new one.

user_3646960

The major problem is that many operating systems have a limit on a number of open files each process can have, which is because there is literally an array of all files in the kernel data structures for each process which must have enough space for all files in the process.

If you overwrite the pointer, there is (usually = never if you did not do it by yourself) no garbage collection in C, so the system would not know that the FILE structure pointed to by the pointer should be free - and this will result in a memory leak. But the worse leak is the leak of limited operating system resources.

The situation will naturally fix itself after you exit your program - all unclosed files will be closed and flushed then. But is it really that hard to put the fclose there:

FILE *x = fopen("story.txt", r);
if (x != NULL)
{
    fclose(x);
    x = fopen("story.txt", wb); // <- this certainly does not leak any more
user_9189590
Related