Question

I have to create program, which is able to generate a GML file.

For this purpose I implemented a function GetEdges, this functions should returns three arrays (call by reference).

The signature of the function looks like:

bool GetEdges(DG_NODE_ID **sourceIds, DG_NODE_ID **destIds, int **weights, int *count)

Within the function I want to malloc space:

*sourceIds = (DG_NODE_ID *) malloc(cntEdges * sizeof (DG_NODE_ID));

As soon as I use 4 nodes i get following output:

graph: malloc.c:2451: sYSMALLOc: Assertion `(old_top == (((mbinptr)
(((char *) &((av)->bins[((1) – 1) * 2])) – __builtin_offsetof (struct
malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size)

= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) – 1)) & ~((2 * (sizeof(size_t)))
– 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)’ failed.

I tried everything and I do not have any clue why this fails after I use 4 nodes.

I uploaded the whole source code: Download – Source code

Answer 1

It most probably means you are abusing the memory that is allocated by:

1. Writing before the start of what you were allocated (uncommon).
2. Writing after the end of what you were allocated (common).
3. Freeing something that wasn’t previously allocated (uncommon).
4. Writing to previously allocated space after it was freed (common).

Do you have valgrind? If so, use it. If not, get it and use it if at all possible. (It’s available for many Unix-like systems; it is not available for Windows AFAIK.)