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Closed 13 years ago.
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What is the best solution to replace a new memory allocator in an existing code?
I'm writing a library in C. I'd like to know if there is a way to divert every malloc() call my library makes to a different "augmented" testmalloc() function that I provide without (significantly) modifying my library. This question is inspired from p158 of Kernighan and Pike's "The Practice of Programming", where they say
Write a version of your storage allocator that intentionally fails early, to test your code for recovering from out-of-memory errors.
I am in a position where I could provide a wrapper mymalloc() and use that exclusively in my library. I suspect it will be necessary to use this freedom to avoid multiple symbol definitions during linking.
yeah. you should include the library at last, and use #define malloc mymalloc
example:
library.h:
void * myalloc(int);
#define malloc myalloc
source.c:
#include <stdlib.h>
int* i = malloc(4);
-> uses myalloc
I guess writing your own malloc:
char* malloc(size_t sz)
{
return (char*)0;
}
and then linking it in doesn't work here?
(Background note: You can usually replace a function in a library with another by linking it in first in the link step. This doesn't replace the calls in the library, so the library still uses its own function, but everything that needed a link to malloc from your own code when the linker gets to your version will use your version.)
If you cannot modify the code you can consider using __malloc_hook.
See (http://www.gnu.org/s/libc/manual/html_node/Hooks-for-Malloc.html)
in addition to Yossarian's answer, you can use malloc hooks, defined at least for the GNU C library.
It is even possible to write a malloc() implementation that can succeed or fail depending on a global. Unix linkers won't look for the real malloc function as it finds one in the object file(s). I do not know how this would behave on Windows.
void *malloc(size_t aSize)
{
if (gNextMallocShallFail)
{
gNextMallocShallFail = 0; //--- next call will succeed
return NULL;
}
else
{
return realloc(NULL, aSize);
}
}
Related
I want to mock functions of the c lib such as malloc without altering too much the original source file (the one that uses malloc).
I tried including a header file "mock.h" like
#ifndef MOCK_H_
# define MOCK_H_
# ifdef MOCK_MODE
# include <sys/types.h>
extern void *my_mock_malloc(size_t n);
void *malloc(size_t n) __attribute__((weak, alias ("my_mock_malloc")));
# endif /* MOCK_MODE */
#endif /* !MOCK_H_ */
but it gives me an error
in file included from ...:
/usr/include/stdlib.h:466:14: error: ‘malloc’ aliased to undefined symbol ‘my_mock_malloc’
extern void *malloc (size_t __size) __THROW __attribute_malloc__ __wur;
GCC alias to function outside of translation unit -AKA- is this even the right tool for the job? gives a partial solution: using the linker I create an alias on a symbol. I can now compile with -Xlinker --defsym "malloc"="my_mock_malloc". The problem is that all my .o files are linked using this option and thus the unit testing framework I use (check) is affected by the mocking (and thus it receives is SIGSEGV when I make my mock function return NULL).
Is there a way to perform such symbol aliasing locally, so I can make my tesing framework use the real malloc? Os is there a better solution than this one?
Most implementations of the C language specify all symbols of the libc as weak, that is, you can override them as needed. Try it out! You can write your own function named malloc and it automatically replaces the library supplied malloc. Have a look at your platforms documentation as there are a couple more functions (like free, realloc, calloc, etc) that you need to implement for a malloc replacement to be complete.
I found a solution which is to include a header file mock.h like
#ifndef MOCK_H_
# define MOCK_H_
# ifdef MOCK_MODE
# include <sys/types.h>
extern void *my_mock_malloc(size_t n);
# define malloc(x) (my_mock_malloc(x))
# endif /* MOCK_MODE */
#endif /* !MOCK_H_ */
but I am still curious about another solution.
You could do it like this:
#include <stdio.h>
#include <stdlib.h>
#define malloc my_malloc
void *my_malloc(size_t size) {
printf ("Called my_malloc\n");
return NULL;
}
int main(void) {
char * array = malloc(100);
return 0;
}
Program output:
Called my_malloc
Although not a full answer to your question, you will find that CppUTest is very useful for testing C code as well, and includes most of the malloc/free library in its mock capabilities, allowing you to control malloc failure and so on. It's also very good for debugging memory allocation issues since it uses a debug allocator.
I found that the framework documentation was however a little lacking in detail and examples.
When I used that framework "in anger" a couple of years ago, I found it necessary to implement my own mocked strdup() function, which I did within the CppUTest source.
https://cpputest.github.io/mocking_manual.html
I added the DeathHandler facility as well, to help capture and diagnose segfaults during test.
I have some C code that I want to do some tests on. It uses malloc, calloc, and free through out the code. I want to change those functions to a custom function that internally calls the original function. For example:
emxArray->size = (int *)malloc((unsigned int)(sizeof(int) * numDimensions));
would become:
emxArray->size = (int *)myMalloc((unsigned int)(sizeof(int) * numDimensions));
where myMalloc is:
void* myMalloc(unsigned size)
{
if (size < 8)
{
//printf("*** Bumped from %d....\n", size);
size = 8;
}
allocated += size;
return malloc(size);
}
As you can see, myMalloc internally calls malloc. It just does some extra stuff. I wanted to replace the usage of malloc through out the code with myMalloc. I have done this successfully by going through all the code and replacing malloc with myMalloc manually, but this is far from ideal. I will be replacing this code on a test only basis, thus the production code should contain only malloc calls. I realize I could also do this with a script, but wanted to just use a define statement in the Makefile:
-Dmalloc=myMalloc
But that also replaces malloc in the myMalloc function, which causes an infinite recursive situation. I tried changing the malloc call in the myMalloc function to malloc_d, and added a second define to the Makefile.
-Dmalloc=myMalloc -Dmalloc_d=malloc
I was thinking that the first define would not replace the malloc_d (which it didn't) and that the second define would only change the malloc_d (which it didn't). I got the same recursive situation. Is there anyway to do this with Makefile defines? Or are multipass pre-compile situations going to always mess this up?
UPDATE:
Ok, so I have started looking at the LD_PRELOAD option that has been pointed out. I thought I had a workable solution, however, I am still having trouble! Here is what I did...
I moved myMalloc() and myFree() out of the main file and into its own file. I then compiled it into a shared library using:
gcc -shared -o libMyMalloc.so -fPIC myMalloc.c
I then added the following 'dummy functions' to the main file:
void* myMalloc(unsigned size)
{
void* ptr;
return ptr;
}
void myFree(void* ptr)
{
}
As you can see, they do nothing.
I added the following defines to the make file:
-Dmalloc=myMalloc \
-Dfree=myFree
I compiled the code and ran it against the libMyMalloc.so library I created:
LD_PRELOAD=/home/rad/Desktop/myMalloc/libMyMalloc.so ./testRegress
However, I am still not getting it to run with the myMalloc functions that are defined in the libMyMalloc.so file.
The simplest solution is to not call malloc directly in your code: If you choose a different name (say MALLOC), it's trivial to switch to a custom allocator.
Example code:
#ifndef MALLOC
#define MALLOC malloc
#endif
For test builds, you'd do -DMALLOC=myMalloc.
It gets more complicated if for some reason you want keep the calls to malloc. Then you'd have to add something like the following after all standard library headers have been included:
#ifdef USE_MY_MALLOC
#undef malloc
#define malloc(SIZE) myMalloc(SIZE)
#endif
You can call the standard library function by using parens, ie
void* myMalloc(unsigned size)
{
...
return (malloc)(size);
}
and enable it via -DUSE_MY_MALLOC.
Considering the additional requirements given in the comments, two approaches come to mind:
pre-process the generated source, textually replacing calls to malloc
intercept inclusion of stdlib.h (assuming that's where MATLAB gets its malloc declaration from)
Your own version of stdlib.h would look something like this:
#ifndef MY_STDLIB_H_
#define MY_STDLIB_H_
#include "/usr/include/stdlib.h"
#undef malloc
#define malloc(SIZE) myMalloc(SIZE)
#endif
Then, you can conditionally add the directory where you've placed that file to the include path. Also note that this is not a particularly robust solution, but it might work for you anyway.
You can use a pointer to a function. In the normal case, make it point to malloc. In debugging mode, let it point to you function.
In some h file:
extern void *(*myMalloc)(size_t size);
In one of you c files:
#ifdef DEBUG
void *(*myMalloc)(size_t size) = dMalloc;
#else
void *(*myMalloc)(size_t size) = malloc; // derived from libc
#endif
I found my solution and wanted to share. Thank you to everyone that contributed and pointed me in the right direction.
I ended up creating my custom library code and compiling it into a shared library:
gcc -shared -o libtmalloc.so -fPIC tmalloc.c
I then modified the makefile to use the shared library and globally define 'malloc' to my custom function name (which internally calls malloc()) to malloc_t, as well as calloc() and free():
gcc -L/home/path/to/mallocTrace -Wall -o test test.c lib/coder/*.c -lm -ltmalloc \
-Dmalloc=malloc_t \
-Dcalloc=calloc_t \
-Dfree=free_t
The defines changed all the function calls for me which were linked to the implementation in the shared library. Because I am using a shared library (which is already compiled), I didn't have to worry about my makefile defines causing a recursive call situation in my custom functions. With this usage, I can take any pre-generated C code from my other tools and observe the memory usage with these simple makefile changes and using my custom malloc trace library.
I am writing a memory profiler for C and for that am intercepting calls to the malloc, realloc and free functions via malloc_hooks. Unfortunately, these are deprecated because of their poor behavior in multi threaded environments. I could not find a document describing the alternative best practice solution to achieve the same thing, can someone enlighten me?
I've read that a simple #define malloc(s) malloc_hook(s) would do the trick, but that does not work with the system setup I have in mind, because it is too intrusive to the original code base to be suitable for use in a profiling / tracing tool. Having to manually change the original application code is a killer for any decent profiler. Optimally, the solution I am looking for should be enabled or disabled just by linking to an optional shared library. For example, my current setup uses a function declared with __attribute__ ((constructor)) to install the intercepting malloc hooks.
Thanks
After trying some things, I finally managed to figure out how to do this.
First of all, in glibc, malloc is defined as a weak symbol, which means that it can be overwritten by the application or a shared library. Hence, LD_PRELOAD is not necessarily needed. Instead, I implemented the following function in a shared library:
void*
malloc (size_t size)
{
[ ... ]
}
Which gets called by the application instead of glibcs malloc.
Now, to be equivalent to the __malloc_hooks functionality, a couple of things are still missing.
1.) the caller address
In addition to the original parameters to malloc, glibcs __malloc_hooks also provide the address of the calling function, which is actually the return address of where malloc would return to. To achieve the same thing, we can use the __builtin_return_address function that is available in gcc. I have not looked into other compilers, because I am limited to gcc anyway, but if you happen to know how to do such a thing portably, please drop me a comment :)
Our malloc function now looks like this:
void*
malloc (size_t size)
{
void *caller = __builtin_return_address(0);
[ ... ]
}
2.) accessing glibcs malloc from within your hook
As I am limited to glibc in my application, I chose to use __libc_malloc to access the original malloc implementation. Alternatively, dlsym(RTLD_NEXT, "malloc") can be used, but at the possible pitfall that this function uses calloc on its first call, possibly resulting in an infinite loop leading to a segfault.
complete malloc hook
My complete hooking function now looks like this:
extern void *__libc_malloc(size_t size);
int malloc_hook_active = 0;
void*
malloc (size_t size)
{
void *caller = __builtin_return_address(0);
if (malloc_hook_active)
return my_malloc_hook(size, caller);
return __libc_malloc(size);
}
where my_malloc_hook looks like this:
void*
my_malloc_hook (size_t size, void *caller)
{
void *result;
// deactivate hooks for logging
malloc_hook_active = 0;
result = malloc(size);
// do logging
[ ... ]
// reactivate hooks
malloc_hook_active = 1;
return result;
}
Of course, the hooks for calloc, realloc and free work similarly.
dynamic and static linking
With these functions, dynamic linking works out of the box. Linking the .so file containing the malloc hook implementation will result of all calls to malloc from the application and also all library calls to be routed through my hook. Static linking is problematic though. I have not yet wrapped my head around it completely, but in static linking malloc is not a weak symbol, resulting in a multiple definition error at link time.
If you need static linking for whatever reason, for example translating function addresses in 3rd party libraries to code lines via debug symbols, then you can link these 3rd party libs statically while still linking the malloc hooks dynamically, avoiding the multiple definition problem. I have not yet found a better workaround for this, if you know one,feel free to leave me a comment.
Here is a short example:
gcc -o test test.c -lmalloc_hook_library -Wl,-Bstatic -l3rdparty -Wl,-Bdynamic
3rdparty will be linked statically, while malloc_hook_library will be linked dynamically, resulting in the expected behaviour, and addresses of functions in 3rdparty to be translatable via debug symbols in test. Pretty neat, huh?
Conlusion
the techniques above describe a non-deprecated, pretty much equivalent approach to __malloc_hooks, but with a couple of mean limitations:
__builtin_caller_address only works with gcc
__libc_malloc only works with glibc
dlsym(RTLD_NEXT, [...]) is a GNU extension in glibc
the linker flags -Wl,-Bstatic and -Wl,-Bdynamic are specific to the GNU binutils.
In other words, this solution is utterly non-portable and alternative solutions would have to be added if the hooks library were to be ported to a non-GNU operating system.
You can use LD_PRELOAD & dlsym
See "Tips for malloc and free" at http://www.slideshare.net/tetsu.koba/presentations
Just managed to NDK build code containing __malloc_hook.
Looks like it's been re-instated in Android API v28, according to https://android.googlesource.com/platform/bionic/+/master/libc/include/malloc.h, esp:
extern void* (*volatile __malloc_hook)(size_t __byte_count, const void* __caller) __INTRODUCED_IN(28);
I'm creating a cross platform library using C. I have a piece of code like the following, in which I'm using the libc memory management functions directly:
myObject* myObjectCreate(void)
{
...
myObject *pObject = (myObject*)malloc(sizeof(*pObject));
...
}
void myObjectDestroy(myObject *pObject)
{
...
free(pObject);
...
}
I understand these memory management functions are not always available, especially on embedded systems based on low-end microcontrollers. Unfortunately my library needs to be compilable on these systems.
To work around this problem, I suppose I'd have to make these functions customisable by my library client.
So, what are the recommended ways to achieve this?
There are many approaches.
I use #if, combined with compiler provided defines, to have per platform behaviour.
Should a given functionality (such as malloc) be found, #define MYLIB_MALLOC can be defined.
Then, later, you can check for #ifdef MYLIB_MALLOC and if not present, provide a dummy malloc function, which will allow your code to compile.
Use function pointers.
Define the following pointers in the library:
void* (*CustomMalloc)(size_t) = NULL;
void (*CustomFree)(void*) = NULL;
And prior to using of the library functions initialize these pointers to point to custom implementations of malloc() and free(). Or initialize them to point to the real malloc() and free().
Inside of the library replace malloc(size) with CustomMalloc(size) and free(pointer) with CustomFree(pointer).
Use conditional compile, i.e. define some macro's like LIBC_AVAIL, LIBC_NOT_AVAIL and include different code when compiling.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
How can one grab a stack trace in C?
Hi,
I would like to know how to print the contents of the current program stack (using C language).
say for eg.
call_some_function()
{
...
...
print_stack_till_now();
return something;
}
call_some_other_function()
{
...
...
print_stack_till_now();
return something;
}
main()
{
print_stack_till_now();
call_some_function();
print_stack_till_now();
call_some_other_function();
print_stack_till_now();
return 0;
}
In the prev example (may be not an example exactly :)) when I call the print_stack_till_now() function I should be able to print the current stack built till that point (including the newer function call entries, return location, their arguments etc.)
Is such a function possible in C language (even inlined assembly). Please point me to the theory (or existing code would be even better) needed to write such a function.
In gdb we can use backtrace to look at the current stack, I'm looking for something similar. Why do I need this ?... just wanted to learn.
Thank you.
There's no portable way to do this for the simple reason that C by itself doesn't define the concept of a stack data structure. It's completely open to the implementation how it does automatic storage and returns from function calls.
That being said most implementations provide some kind of stack unwinding mechanism. For example GCC/glibc provides the runtime specific function backtrace
http://www.gnu.org/s/libc/manual/html_node/Backtraces.html
There are similar stack unwinders for other plattforms. And of course you can implement your own backtracing mechanisms through a global, thread local storage array (it can be static and must provide only enough entries for how many function calls are supported by the stack size), where at each function call the calling module (using C preprocessor __FILE__), the line (C preprocessor __LINE__) and the called function (some additional preprocessor magic) are placed.