Thread: dynamic allocation of file* pointers

I have written the following:

Code:

 FILE **fHandle;fHandle = (FILE**) calloc (4,sizeof (FILE*));

/*-----open and write and close files (code skipped)-----*/

free(fHandle);

My program breaks at free(fHandle);

What am I missing?

Maybe the problem lies in code that you did not show, e.g., you forgot to #include <stdlib.h> or there's undefined behaviour somewhere.

You're probably missing some other code that is molesting a pointer.

Your code, as shown (and assuming it is all in the one function and <stdlib.h> has been #include'd) does nothing wrong.

However, a crash on free() is a common symptom of some other code having molested a pointer, with the effect of overwriting data structures used internally by malloc/calloc/realloc/free. Whatever the offending code is, you haven't shown it. But it can be any code executed before the call of free(), including before the definition of fHandle.

mostly crash on free indicates corrupted heap

corrupted heap mostly caused by 2 main errors - memory overrun on the allocated array or double free

So problem could relate to any other dynamically allocated memory and not to this pointer exactly

I'm just curious. why do you need to dynamically allocate a pointer?

Originally Posted by uz_mz

Because I want to open and write and close files dynamically.

you can do that with a normal pointer on the stack.

Code:

FILE* file = fopen(...);

is effectively the same as

Code:

FILE** pFile = malloc(sizeof(FILE*));
*pFile = fopen(...);

the latter is harder to read, harder to maintain, risks additional memory leaks, and may be slower on some platforms.

Originally Posted by uz_mz

skipped code:

Code:

char cfileName[20];
for (int i=0; i<4; i++)
      {
         sprintf(cfileName, "Signal_%d.pcm",i);
         fHandle[i] = fopen(cfileName, "ab");
         fclose(fHandle);
      }

I ran through the code by setting breakpoints. It goes thru smoothly.
It breaks only where I free the file pointer. If I comment out that line, then it does not break anywhere else.
(implying the mistake is in the mem dealloc).
Even after I include fWrite, it runs well and the output file is written properly. Only when I exit the program, it breaks in the destructor where memory is freed for file pointer..
I dont know whats the reason! :-(

You're calling fclose(fHandle) on every iteration of the loop. That is certainly a mistake: fclose should not even be called with fHandle to begin with.

Originally Posted by uz_mz

sorry it should be:

fclose([fHandle[i]);

Is this right?

You have a typo error: fclose(fHandle[i]);

sorry..my mistake..was a typo.
Also, since I am allocating memory for 1st dimension only,

Code:

fHandle = (FILE**) calloc (4,sizeof (FILE*));

and free it by:

Code:

free(fHandle);

I do not need to loop thru the fHandle dimensions as in:

Code:

for(i=0;i<4;i++)
{ 
   free(fHandle[i]);
}

?? please suggest..