How to simulate memory allocation errors

Question

My C application uses 3rd libraries, which do their own memory management. In order to be robust, my application has code to deal with failures of library functions due to lack of free memory.

I would like to test this code, and for this, I need to simulate failures due to lack of memory.

What tool/s are recommended for this? My environment is Linux/gcc.

Answer 1

Create your own malloc wrapper which will randomly return null instead of a valid pointer. Well, or which fails consistently if you want to unit test.

Answer 2

You can write your own mock library with the same interface as your 3rd party library instead of it. You can also use LD_PRELOAD to override selected functions of the 3rd party library.

/Allan

Answer 3

I can give a Linux (maybe POSIX) specific version: __malloc_hook, __realloc_hook, __free_hook. These are declared in malloc.h.

EDIT: A little elaboration: these are function pointers (see malloc.h and their man-page for the exact declaration), but beware: these are not exactly standards, just GNU extensions. So if portability is an issue, don't use this.

A little less platform-dependent solution might be that you declare a malloc macro. If you're testing, this calls a hook and the real malloc.

memhook.h:

#define malloc(s)    (my_malloc(s))

memhook.c:

#include "memhook.h"
#undef malloc
#include <stdlib.h>

etc.

You can use this to detect leaks, randomly fail the allocation, etc.

Answer 4

You can use ulimit to limit the amount of resources a user can use, including memory. So you create a test user, limit their memory use to something just enough to launch your program, and watch it die :)

Example:

ulimit -m 64

Sets a memory limit of 64kb.

Answer 5

You want the ulimit command in bash. Try

help ulimit

at a bash shell prompt.

Answer 6

In addition, you should use Valgrind to test it all and get real useful reports about memory behavior of your program

Answer 7

(As a complement to some of the previous answers)

Checkout "Electric Fence" for an example of a malloc-intercepting library that you can use with your executable (using the LD_PRELOAD trick, for instance).

Once you've intercepted malloc, you can use whatever you want to trigger failures. A randomly triggered failure would be a good stress test for the various parts of the system. You could also modify the failure probability based on the amount of memory requested.

Yours is an interesting idea, by the way, clearly something I'd like to do on some of my code...

Answer 8

On operating systems that overcommit memory (for example, Linux or Windows), it is simply not possible to handle out-of-memory errors. malloc may return a valid pointer and later, when you try to dereference it, your operating system may determine that you are out of memory and kill the process.

http://www.reddit.com/comments/60vys/how_not_to_write_a_shared_library/ is a good write-up on this.

Answer 9

You might want to look at some of the recovery oriented computing sites, such as the Berkeley/Stanford ROC group. I've heard some of these folks talk before, and they use code to randomly inject errors in the C runtime. There's a link to their FIT tool at the bottom of their page.

Answer 10

Have a look at the way sqlite3 does this. They perform extensive unit testing, including out of memory testing.

You may also want to look at their page on malloc, particularly Section 4.0.