Segmentation Fault using C macros

Question

I have a curious issue while working with a slighty modified MobileC build.

I get a segmentation fault in a macro but when i put printfs in the code (its not possible to debug otherwise since it's interpeted C) I can't get the exact point where it happens.

This is the code:

...
printf("just to check there is no problem accessing "node" %d\n", node);\
printf("this will be printed\n"); \
node_type##_Destroy(node); \
printf("this will not be printed\n"); \
...

And the code of Destroy is the following:

int name##_Destroy( name##_p name ) \
{ \
printf("this will not be printed\n");    \
...

I have not modified this part of the code (except for the printfs ) so I guess it's ok. Do you have any idea what can be happening here?

Answer 1

There are two red flags in this code, but you haven't really shown us enough.

printf("just to check there is no problem accessing "node" %d\n", node);\
printf("this will be printed\n"); \
node_type##_Destroy(node); \
printf("this will not be printed\n"); \

This is clearly a macro, but it's a multi-statement macro. This is the easiest way to screw up with macros, because it can be called like this:

#define MACRO() puts("A"); puts("B")
// prints "B"
if (0) MACRO();

Secondly, that string constant is very suspicious:

"just to check there is no problem accessing "node" %d\n", node

Notice how node is purportedly an int , but it appears between two string constants without a compilation error. An int shouldn't be able to do that, so node is probably not quite an int . It might be an entire expression, who knows?

Answer 2

Segfault on delete almost always points to a corrupted heap. Something has trashed the heap where your pointer lives (or where it is keeping track of prev/next pointers) and causing a jump to hyperspace. Managed code makes this less likely, but it is still where I would start.

Answer 3

Thank you for your answers. Now I know what happened.

Dietrich Epp, indeed node wasn't an int (I just wanted to know whether there was a problem accessing it since I was screwed up by the error and didn't have a clue about what was happening). Thank you for warning me about C Macros. I've read a bit about and finally I've figured out what was happening.

node_type type was agent . Therefore the macro expanded to agent_Destroy(node). The issue was that in Mobile C code there was a function than was named agent_Destroy(agent_p agent). What happened was that both functions executed and the first one was the one defined directly in the code where there was a segmentation fault.