There are many questions on this website regarding freeing pointers after use and, further, setting them to NULL. Arguments are fierce and the topic is seemingly divided equally. For example: This question.
I am confused about freeing pointers in general.
Imagine you have a pointer to some memory space. After using the space, you free the pointer but do not set it to NULL. Later, you have another pointer that calls
char *ptr = malloc(sizeof(*ptr)); //first allocation
free(ptr); //release memory
ptr = NULL;
ptr = malloc(sizeof(*ptr)); //reallocate
Given a freed pointer, what is keeping you from reassiging that pointer to a new memory location?
Technically, nothing. You do not even need to set
ptr = NULL in between of freeing and re-assigning the pointer. When freeing and re-assigning are separated by other lines of code, however, setting the pointer to
NULL may improve readability slightly.
Why would writing to a memory block that was previously freed, that still has the original pointer to it, cause the program to crash?
Simply holding a pointer to a block of memory accessible through another pointer is absolutely OK, as long as your program does not try dereferencing that pointer. Unfortunately, even if you dereference the freed pointer, it would not necessarily cause your program to crash: more often than not, such behavior would go unnoticed. It remains an undefined behavior, though. Another part of your program may have written data incompatible with what you expect, in which case you will see bugs that are extremely hard to find or explain.