02-18-2006
Quote:
Originally Posted by blowtorch
Just a question, it it a good practice to assign NULL to any pointer that you free()? Or does it not really matter?
After you use free(), you should not dereference a pointer pointing to the area you just freed. The area that was freed is not returned to the OS, it remains available to be reused by the next malloc(). Some people will malloc and then free immediately in a function. Then they go ahead a use the area malloc'ed and figure that they don't need to free() at each return point. I feel this is poor technique.
So I believe that you should never dereference a pointer to an area that has been freed. But what purpose is served by setting the pointer to NULL? You should never dereference a NULL pointer either. I guess it might be argued that, like superfluous parentheses in complex expressions, it makes things a bit clearer to the reader. I rarely will set a pointer to NULL when I'm done with it, but I often initialize a pointer to NULL. I think it is mostly a matter of style.
But it is critical to realize that correct code will absolutely never dereference a NULL pointer.
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LEARN ABOUT OPENDARWIN
bsdmalloc
bsdmalloc(3MALLOC) bsdmalloc(3MALLOC)
NAME
bsdmalloc - memory allocator
SYNOPSIS
cc [ flag ... ] file ... -lbsdmalloc [ library ... ]
char *malloc(size);
unsigned size;
int free( ptr);
char *ptr;
char *realloc( ptr, size);
char *ptr;
unsigned size;
These routines provide a general-purpose memory allocation package. They maintain a table of free blocks for efficient allocation and coa-
lescing of free storage. When there is no suitable space already free, the allocation routines call sbrk(2) to get more memory from the
system. Each of the allocation routines returns a pointer to space suitably aligned for storage of any type of object. Each returns a
null pointer if the request cannot be completed.
The malloc() function returns a pointer to a block of at least size bytes, which is appropriately aligned.
The free() function releases a previously allocated block. Its argument is a pointer to a block previously allocated by malloc() or real-
loc(). The free() function does not set errno.
The realloc() function changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block.
The contents will be unchanged up to the lesser of the new and old sizes. If the new size of the block requires movement of the block, the
space for the previous instantiation of the block is freed. If the new size is larger, the contents of the newly allocated portion of the
block are unspecified. If ptr is NULL, realloc() behaves like malloc() for the specified size. If size is 0 and ptr is not a null pointer,
the space pointed to is freed.
The malloc() and realloc() functions return a null pointer if there is not enough available memory. They return a non-null pointer if size
is 0. These pointers should not be dereferenced. When realloc() returns NULL, the block pointed to by ptr is left intact. Always cast the
value returned by malloc() and realloc().
If malloc() or realloc() returns unsuccessfully, errno will be set to indicate the following:
ENOMEM size bytes of memory cannot be allocated because it exceeds the physical limits of the system.
EAGAIN There is not enough memory available at this point in time to allocate size bytes of memory; but the application could try
again later.
Using realloc() with a block freed before the most recent call to malloc() or realloc() results in an error.
Comparative features of the various allocation libraries can be found in the umem_alloc(3MALLOC) manual page.
brk(2), malloc(3C), malloc(3MALLOC), mapmalloc(3MALLOC), umem_alloc(3MALLOC)
WARNINGS
Use of libbsdmalloc renders an application non-SCD compliant.
The libbsdmalloc routines are incompatible with the memory allocation routines in the standard C-library (libc): malloc(3C), alloca(3C),
calloc(3C), free(3C), memalign(3C), realloc(3C), and valloc(3C).
21 Mar 2005 bsdmalloc(3MALLOC)