malloc问题和内存堆

Question

i've been running into a weird problem with mallocing/freeing memory. i can't show the whole code itself so please make do with a very bare partial:

#include <stdio.h>

int main(void)
{
     void *ptr;

     ptr = malloc(sizeof(node));

     /* making sure what the value is when it was initialized */
     printf("head_node: %p\n", ptr);


     /* i do a lot of things here, such as appending the node, sometimes truncating them */


     /* --------------------------------------------------------------------------------*/

     /* making sure what the value is still the same: */
     printf("head_node: %p\n", ptr);
     free(ptr);

     printf("done");
}

where node is declared somewhere else.

at the point where i free the ptr and it's initial value is the same right before freeing it and it's not NULL, i get the error dialog box in windows.

now here's is the odd part. when i compile it under MinGW and run it within MSYS (MinGW console shell) or outside of it, it doesn't run into any error. when i build it under MS Visual Express 2012 and debug it under it, it doesn't run into any error. but when i run the same program built by MS Visual Express outside MS Visual, i always get an error.

what's even weirder is that it first prints out "done", which is at the VERY LAST point of the program before the error pops up. not very helpful when tracking the source of error...

if there was any error that's being caught in the normal windows environment, then why not in MS Visual??

here's the exception message being displayed:

File: f:\dd\vctools\crt_bld\self_x86\crt\src\dbgheap.c
Line: 1322
Expression: _CrtIsValidHeapPointer(pUserData)

Answer 1

Your symptoms point to you corrupting the heap somewhere in your missing code. You are probably either writing to an invalid pointer or writing too much to a valid one somewhere. The reason you only get the crash when you exit is the heap corruption is only noticed when the program is cleaning up, and different compilers do different amounts of checking in this phase.

Is this your only malloc in the whole program? If so, that is likely part of your problem -- your 'node' (aka head_node) implies you are doing something with a list. If you are accessing node->next (or whatever you are calling your linking pointer) without doing a malloc for that, then there is your error. without more of your code, it is impossible to say more.

Answer 2

One other subtle thing that might be an issue, especially since the issue disappears with different build environment is you are not including stdlib.h. It might be that you just left it out of your "minimal" example, but not having a prototype in scope for malloc() can cause some very strange issues.

Answer 3

It may be a memory leak. Your pointer may be pointing to an invalid location.Its called a dangling pointer. So check whether your pointer is pointing to a location which is valid(ur pointer may also point to an invalid junk location)