Question

I am trying to create a free list to implement a heap allocator. Here is how I defined the struct so far:

typedef struct headerT headerT;

struct headerT{
   size_t payloadSize;
   struct headerT* next; // unused field to make struct 8 bytes, simplifies alignment                                      
};

headerT* freeList;

and here is how I initialize my heap:

int myinit()
{
  freeList = InitHeapSegment(0); // InitHeapSegment resets heap segment to empty, no pages                                                  
                              //allocated                                                                             
  freeList->payloadSize = 0;
  freeList->next=NULL;
  return 0;
}

I get a segfault immediately when the function tries to change the payload size to zero. What am I missing here?

Answer 1

The problem is here:

 freeList = InitHeapSegment(0); // InitHeapSegment resets heap segment to empty, no pages                                                  
                              //allocated     

You say that InitHeapSegment “resets” heap segment to empty, but no pages are allocated. Probably its not malloc()ing. Hence freeList could point to a junk value which you do not have the permission to access, giving you a SIGSEGV or SEGMENTATION VIOLATION error. In Windows, this error is of type Cx00000005