Question

When using malloc, if it produces a core dump with the error:

malloc(): memory corruption: ....... ***

Does this mean that malloc tried to allocate memory that was not free to allocate? IF so what are the causes of this?

Answer 1

It completely depends on your malloc implementation, but usually what this is means is that at some point prior to that malloc something wrote more data to a malloced buffer than its size.

A lot of malloc implementations store some of their data inline with their memory, in other words:

+--------------------------------+
|14 bytes -> Padding             | 
+--------------------------------+
|2 bytes -> Internal malloc info |
+--------------------------------+
|6 bytes -> Your data            |
+--------------------------------+
|8 bytes -> Padding              |
+--------------------------------+
|2 bytes -> Internal malloc info |
+--------------------------------+

So if some code of yours or a library wrote 16 bytes to that 6 byte buffer it would overwrite the padding and the 2 bytes of internal malloc info. The next time you call malloc it will try to walk through its data to find space, hit the overwritten space, and it will be nonsensical since you overwrote it, corrupting the heap.

Depending on the implementation such an error could also be caused by making a double free.