OK, there’s a fair deal of confusion explaining exactly what order the necessary free() calls have to be in, so I’ll try to clarify what people are trying to get at and why. Starting with the basics, to free up memory which has been allocated using malloc(), you simply call free() with exactly the pointer … Read more
The C99 standard (actually WG14/N1124. Committee Draft — May 6, 2005. ISO/IEC 9899:TC2) says about malloc(): The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation defined: … Read more
You can’t. The way to track this would be to assign the pointer to 0 or NULL after freeing it. However as Fred Larson mentioned, this does nothing to other pointers pointing to the same location. int* ptr = (int*)malloc(sizeof(int)); free(ptr); ptr = NULL;
OK, so I’ve managed to overcome this issue. First of all – A practical cause to “corrupted size vs. prev_size” is quite simple – memory chunk control structure fields in the adjacent following chunk are being overwritten due to out-of-bounds access by the code. if you allocate x bytes for pointer p but wind up … Read more
Besides Klatchko’s correct point that the standard does not provide for it, real malloc/free implementations often allocate more space then you ask for. E.g. if you ask for 12 bytes it may provide 16 (see A Memory Allocator, which notes that 16 is a common size). So it doesn’t need to know you asked for … Read more
The second one is way more important: re-using a freed pointer can be a subtle error. Your code keeps right on working, and then crashes for no clear reason because some seemingly unrelated code wrote in the memory that the re-used pointer happens to be pointing at. I once had to work on a really … Read more
Do we really need to assign the pointer to a temporary one? Does it help in terms of concurrency and shared memory? It has nothing to do concurrency or shared memory. It’s pointless. Is it really a good idea to set the whole block to 0 to force the program to crash or at least … Read more
The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system: mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, this indicates the minimum amount of … Read more
Whenever you pass a parameter to a function, a copy is made, and the function works on that copy. So in your case, you are trying to free a copy of the original object, which doesn’t make any sense. You should modify your function to take a pointer, and then you can have it call … Read more
When you malloc a block, it actually allocates a bit more memory than you asked for. This extra memory is used to store information such as the size of the allocated block, and a link to the next free/used block in a chain of blocks, and sometimes some “guard data” that helps the system to … Read more