examining and modifying memory addresses in C - c

I want to essentially do the following (which is probably dangerous and what not) just for the heck of it:
int main() {
int x = 0x00ff00ff;
printf("Value at addr x: %x\n",*x);
return 0;
}
Basically take a look at the contents of a certain address in my machine. Maybe write to it. I'm guessing I'm not allowed to do the latter.
The error I get is error: invalid type argument of 'unary *'.
Is there any way to do this?

You need a pointer:
int *x = (int*)0x00ff00ff;
And you're right, it's probably not a good idea, unless you know that 0x00ff00ff is a valid address of some sort. It's not actually undefined behaviour since the standard says you can't dereference illegal addresses but then states that "illegal" includes things like:
addresses of freed heap objects.
NULL pointers.
wrong alignment.
but doesn't explicitly list arbitrary pointer values, since that would make memory-mapped I/O in embedded systems problematic.
For example, you may control a UART (universal asynchronous receiver/transmitter, basically a serial port device) in an embedded system by reading or writing known memory-mapped I/O addresses:
#define UART_READ_READY ((char*)0xff00)
#define UART_READ_CLEAR ((char*)0xff01)
#define UART_DATA ((char*)0xff02)
char getUartCharWithWait (unsigned int tries) {
char retChar;
unsigned int limit;
// Keep looping until character available, at least for a while.
limit = tries;
while (*UART_READ_READY == 0)
if (limit-- == 0)
return '\0';
// Get character, tell UART to clear it, then return it.
retChar = *UART_DATA;
*UART_CLEAR = 1;
return retChar;
}
In this example, you have code like:
retChar = *UART_DATA;
which will read a byte (C char) from "memory" address 0xff02, which will actually be from a device monitoring the address bus and intercepting specific addresses.

You are getting the previously mentioned error because there is no way to dereference an int, making x a pointer-to-int will yield the "correct" result (ie. it will be able to compile).
int * x = (int*)0x00ff00ff;
"It works, IT WORKS! Or er.. I mean, it compiles. Now, what's a segfault?"

Related

How test if a value is (or is not) a valid pointer? [duplicate]

Is there any way to determine (programatically, of course) if a given pointer is "valid"? Checking for NULL is easy, but what about things like 0x00001234? When trying to dereference this kind of pointer an exception/crash occurs.
A cross-platform method is preferred, but platform-specific (for Windows and Linux) is also ok.
Update for clarification:
The problem is not with stale/freed/uninitialized pointers; instead, I'm implementing an API that takes pointers from the caller (like a pointer to a string, a file handle, etc.). The caller can send (in purpose or by mistake) an invalid value as the pointer. How do I prevent a crash?
Update for clarification: The problem is not with stale, freed or uninitialized pointers; instead, I'm implementing an API that takes pointers from the caller (like a pointer to a string, a file handle, etc.). The caller can send (in purpose or by mistake) an invalid value as the pointer. How do I prevent a crash?
You can't make that check. There is simply no way you can check whether a pointer is "valid". You have to trust that when people use a function that takes a pointer, those people know what they are doing. If they pass you 0x4211 as a pointer value, then you have to trust it points to address 0x4211. And if they "accidentally" hit an object, then even if you would use some scary operation system function (IsValidPtr or whatever), you would still slip into a bug and not fail fast.
Start using null pointers for signaling this kind of thing and tell the user of your library that they should not use pointers if they tend to accidentally pass invalid pointers, seriously :)
Here are three easy ways for a C program under Linux to get introspective about the status of the memory in which it is running, and why the question has appropriate sophisticated answers in some contexts.
After calling getpagesize() and rounding the pointer to a page
boundary, you can call mincore() to find out if a page is valid and
if it happens to be part of the process working set. Note that this requires
some kernel resources, so you should benchmark it and determine if
calling this function is really appropriate in your api. If your api
is going to be handling interrupts, or reading from serial ports
into memory, it is appropriate to call this to avoid unpredictable
behaviors.
After calling stat() to determine if there is a /proc/self directory available, you can fopen and read through /proc/self/maps
to find information about the region in which a pointer resides.
Study the man page for proc, the process information pseudo-file
system. Obviously this is relatively expensive, but you might be
able to get away with caching the result of the parse into an array
you can efficiently lookup using a binary search. Also consider the
/proc/self/smaps. If your api is for high-performance computing then
the program will want to know about the /proc/self/numa which is
documented under the man page for numa, the non-uniform memory
architecture.
The get_mempolicy(MPOL_F_ADDR) call is appropriate for high performance computing api work where there are multiple threads of
execution and you are managing your work to have affinity for non-uniform memory
as it relates to the cpu cores and socket resources. Such an api
will of course also tell you if a pointer is valid.
Under Microsoft Windows there is the function QueryWorkingSetEx that is documented under the Process Status API (also in the NUMA API).
As a corollary to sophisticated NUMA API programming this function will also let you do simple "testing pointers for validity (C/C++)" work, as such it is unlikely to be deprecated for at least 15 years.
Preventing a crash caused by the caller sending in an invalid pointer is a good way to make silent bugs that are hard to find.
Isn't it better for the programmer using your API to get a clear message that his code is bogus by crashing it rather than hiding it?
On Win32/64 there is a way to do this. Attempt to read the pointer and catch the resulting SEH exeception that will be thrown on failure. If it doesn't throw, then it's a valid pointer.
The problem with this method though is that it just returns whether or not you can read data from the pointer. It makes no guarantee about type safety or any number of other invariants. In general this method is good for little else other than to say "yes, I can read that particular place in memory at a time that has now passed".
In short, Don't do this ;)
Raymond Chen has a blog post on this subject: http://blogs.msdn.com/oldnewthing/archive/2007/06/25/3507294.aspx
AFAIK there is no way. You should try to avoid this situation by always setting pointers to NULL after freeing memory.
On Unix you should be able to utilize a kernel syscall that does pointer checking and returns EFAULT, such as:
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdbool.h>
bool isPointerBad( void * p )
{
int fh = open( p, 0, 0 );
int e = errno;
if ( -1 == fh && e == EFAULT )
{
printf( "bad pointer: %p\n", p );
return true;
}
else if ( fh != -1 )
{
close( fh );
}
printf( "good pointer: %p\n", p );
return false;
}
int main()
{
int good = 4;
isPointerBad( (void *)3 );
isPointerBad( &good );
isPointerBad( "/tmp/blah" );
return 0;
}
returning:
bad pointer: 0x3
good pointer: 0x7fff375fd49c
good pointer: 0x400793
There's probably a better syscall to use than open() [perhaps access], since there's a chance that this could lead to actual file creation codepath, and a subsequent close requirement.
Regarding the answer a bit up in this thread:
IsBadReadPtr(), IsBadWritePtr(), IsBadCodePtr(), IsBadStringPtr() for Windows.
My advice is to stay away from them, someone has already posted this one:
http://blogs.msdn.com/oldnewthing/archive/2007/06/25/3507294.aspx
Another post on the same topic and by the same author (I think) is this one:
http://blogs.msdn.com/oldnewthing/archive/2006/09/27/773741.aspx ("IsBadXxxPtr should really be called CrashProgramRandomly").
If the users of your API sends in bad data, let it crash. If the problem is that the data passed isn't used until later (and that makes it harder to find the cause), add a debug mode where the strings etc. are logged at entry. If they are bad it will be obvious (and probably crash). If it is happening way to often, it might be worth moving your API out of process and let them crash the API process instead of the main process.
Firstly, I don't see any point in trying to protect yourself from the caller deliberately trying to cause a crash. They could easily do this by trying to access through an invalid pointer themselves. There are many other ways - they could just overwrite your memory or the stack. If you need to protect against this sort of thing then you need to be running in a separate process using sockets or some other IPC for communication.
We write quite a lot of software that allows partners/customers/users to extend functionality. Inevitably any bug gets reported to us first so it is useful to be able to easily show that the problem is in the plug-in code. Additionally there are security concerns and some users are more trusted than others.
We use a number of different methods depending on performance/throughput requirements and trustworthyness. From most preferred:
separate processes using sockets (often passing data as text).
separate processes using shared memory (if large amounts of data to pass).
same process separate threads via message queue (if frequent short messages).
same process separate threads all passed data allocated from a memory pool.
same process via direct procedure call - all passed data allocated from a memory pool.
We try never to resort to what you are trying to do when dealing with third party software - especially when we are given the plug-ins/library as binary rather than source code.
Use of a memory pool is quite easy in most circumstances and needn't be inefficient. If YOU allocate the data in the first place then it is trivial to check the pointers against the values you allocated. You could also store the length allocated and add "magic" values before and after the data to check for valid data type and data overruns.
I've got a lot of sympathy with your question, as I'm in an almost identical position myself. I appreciate what a lot of the replies are saying, and they are correct - the routine supplying the pointer should be providing a valid pointer. In my case, it is almost inconceivable that they could have corrupted the pointer - but if they had managed, it would be MY software that crashes, and ME that would get the blame :-(
My requirement isn't that I continue after a segmentation fault - that would be dangerous - I just want to report what happened to the customer before terminating so that they can fix their code rather than blaming me!
This is how I've found to do it (on Windows): http://www.cplusplus.com/reference/clibrary/csignal/signal/
To give a synopsis:
#include <signal.h>
using namespace std;
void terminate(int param)
/// Function executed if a segmentation fault is encountered during the cast to an instance.
{
cerr << "\nThe function received a corrupted reference - please check the user-supplied dll.\n";
cerr << "Terminating program...\n";
exit(1);
}
...
void MyFunction()
{
void (*previous_sigsegv_function)(int);
previous_sigsegv_function = signal(SIGSEGV, terminate);
<-- insert risky stuff here -->
signal(SIGSEGV, previous_sigsegv_function);
}
Now this appears to behave as I would hope (it prints the error message, then terminates the program) - but if someone can spot a flaw, please let me know!
There are no provisions in C++ to test for the validity of a pointer as a general case. One can obviously assume that NULL (0x00000000) is bad, and various compilers and libraries like to use "special values" here and there to make debugging easier (For example, if I ever see a pointer show up as 0xCECECECE in visual studio I know I did something wrong) but the truth is that since a pointer is just an index into memory it's near impossible to tell just by looking at the pointer if it's the "right" index.
There are various tricks that you can do with dynamic_cast and RTTI such to ensure that the object pointed to is of the type that you want, but they all require that you are pointing to something valid in the first place.
If you want to ensure that you program can detect "invalid" pointers then my advice is this: Set every pointer you declare either to NULL or a valid address immediately upon creation and set it to NULL immediately after freeing the memory that it points to. If you are diligent about this practice, then checking for NULL is all you ever need.
Setting the pointer to NULL before and after using is a good technique. This is easy to do in C++ if you manage pointers within a class for example (a string):
class SomeClass
{
public:
SomeClass();
~SomeClass();
void SetText( const char *text);
char *GetText() const { return MyText; }
void Clear();
private:
char * MyText;
};
SomeClass::SomeClass()
{
MyText = NULL;
}
SomeClass::~SomeClass()
{
Clear();
}
void SomeClass::Clear()
{
if (MyText)
free( MyText);
MyText = NULL;
}
void SomeClass::Settext( const char *text)
{
Clear();
MyText = malloc( strlen(text));
if (MyText)
strcpy( MyText, text);
}
Indeed, something could be done under specific occasion: for example if you want to check whether a string pointer string is valid, using write(fd, buf, szie) syscall can help you do the magic: let fd be a file descriptor of temporary file you create for test, and buf pointing to the string you are tesing, if the pointer is invalid write() would return -1 and errno set to EFAULT which indicating that buf is outside your accessible address space.
Peeter Joos answer is pretty good. Here is an "official" way to do it:
#include <sys/mman.h>
#include <stdbool.h>
#include <unistd.h>
bool is_pointer_valid(void *p) {
/* get the page size */
size_t page_size = sysconf(_SC_PAGESIZE);
/* find the address of the page that contains p */
void *base = (void *)((((size_t)p) / page_size) * page_size);
/* call msync, if it returns non-zero, return false */
int ret = msync(base, page_size, MS_ASYNC) != -1;
return ret ? ret : errno != ENOMEM;
}
There isn't any portable way of doing this, and doing it for specific platforms can be anywhere between hard and impossible. In any case, you should never write code that depends on such a check - don't let the pointers take on invalid values in the first place.
As others have said, you can't reliably detect an invalid pointer. Consider some of the forms an invalid pointer might take:
You could have a null pointer. That's one you could easily check for and do something about.
You could have a pointer to somewhere outside of valid memory. What constitutes valid memory varies depending on how the run-time environment of your system sets up the address space. On Unix systems, it is usually a virtual address space starting at 0 and going to some large number of megabytes. On embedded systems, it could be quite small. It might not start at 0, in any case. If your app happens to be running in supervisor mode or the equivalent, then your pointer might reference a real address, which may or may not be backed up with real memory.
You could have a pointer to somewhere inside your valid memory, even inside your data segment, bss, stack or heap, but not pointing at a valid object. A variant of this is a pointer that used to point to a valid object, before something bad happened to the object. Bad things in this context include deallocation, memory corruption, or pointer corruption.
You could have a flat-out illegal pointer, such as a pointer with illegal alignment for the thing being referenced.
The problem gets even worse when you consider segment/offset based architectures and other odd pointer implementations. This sort of thing is normally hidden from the developer by good compilers and judicious use of types, but if you want to pierce the veil and try to outsmart the operating system and compiler developers, well, you can, but there is not one generic way to do it that will handle all of the issues you might run into.
The best thing you can do is allow the crash and put out some good diagnostic information.
In general, it's impossible to do. Here's one particularly nasty case:
struct Point2d {
int x;
int y;
};
struct Point3d {
int x;
int y;
int z;
};
void dump(Point3 *p)
{
printf("[%d %d %d]\n", p->x, p->y, p->z);
}
Point2d points[2] = { {0, 1}, {2, 3} };
Point3d *p3 = reinterpret_cast<Point3d *>(&points[0]);
dump(p3);
On many platforms, this will print out:
[0 1 2]
You're forcing the runtime system to incorrectly interpret bits of memory, but in this case it's not going to crash, because the bits all make sense. This is part of the design of the language (look at C-style polymorphism with struct inaddr, inaddr_in, inaddr_in6), so you can't reliably protect against it on any platform.
It's unbelievable how much misleading information you can read in articles above...
And even in microsoft msdn documentation IsBadPtr is claimed to be banned. Oh well - I prefer working application rather than crashing. Even if term working might be working incorrectly (as long as end-user can continue with application).
By googling I haven't found any useful example for windows - found a solution for 32-bit apps,
http://www.codeproject.com/script/Content/ViewAssociatedFile.aspx?rzp=%2FKB%2Fsystem%2Fdetect-driver%2F%2FDetectDriverSrc.zip&zep=DetectDriverSrc%2FDetectDriver%2Fsrc%2FdrvCppLib%2Frtti.cpp&obid=58895&obtid=2&ovid=2
but I need also to support 64-bit apps, so this solution did not work for me.
But I've harvested wine's source codes, and managed to cook similar kind of code which would work for 64-bit apps as well - attaching code here:
#include <typeinfo.h>
typedef void (*v_table_ptr)();
typedef struct _cpp_object
{
v_table_ptr* vtable;
} cpp_object;
#ifndef _WIN64
typedef struct _rtti_object_locator
{
unsigned int signature;
int base_class_offset;
unsigned int flags;
const type_info *type_descriptor;
//const rtti_object_hierarchy *type_hierarchy;
} rtti_object_locator;
#else
typedef struct
{
unsigned int signature;
int base_class_offset;
unsigned int flags;
unsigned int type_descriptor;
unsigned int type_hierarchy;
unsigned int object_locator;
} rtti_object_locator;
#endif
/* Get type info from an object (internal) */
static const rtti_object_locator* RTTI_GetObjectLocator(void* inptr)
{
cpp_object* cppobj = (cpp_object*) inptr;
const rtti_object_locator* obj_locator = 0;
if (!IsBadReadPtr(cppobj, sizeof(void*)) &&
!IsBadReadPtr(cppobj->vtable - 1, sizeof(void*)) &&
!IsBadReadPtr((void*)cppobj->vtable[-1], sizeof(rtti_object_locator)))
{
obj_locator = (rtti_object_locator*) cppobj->vtable[-1];
}
return obj_locator;
}
And following code can detect whether pointer is valid or not, you need probably to add some NULL checking:
CTest* t = new CTest();
//t = (CTest*) 0;
//t = (CTest*) 0x12345678;
const rtti_object_locator* ptr = RTTI_GetObjectLocator(t);
#ifdef _WIN64
char *base = ptr->signature == 0 ? (char*)RtlPcToFileHeader((void*)ptr, (void**)&base) : (char*)ptr - ptr->object_locator;
const type_info *td = (const type_info*)(base + ptr->type_descriptor);
#else
const type_info *td = ptr->type_descriptor;
#endif
const char* n =td->name();
This gets class name from pointer - I think it should be enough for your needs.
One thing which I'm still afraid is performance of pointer checking - in code snipet above there is already 3-4 API calls being made - might be overkill for time critical applications.
It would be good if someone could measure overhead of pointer checking compared for example to C#/managed c++ calls.
It is not a very good policy to accept arbitrary pointers as input parameters in a public API. It's better to have "plain data" types like an integer, a string or a struct (I mean a classical struct with plain data inside, of course; officially anything can be a struct).
Why? Well because as others say there is no standard way to know whether you've been given a valid pointer or one that points to junk.
But sometimes you don't have the choice - your API must accept a pointer.
In these cases, it is the duty of the caller to pass a good pointer. NULL may be accepted as a value, but not a pointer to junk.
Can you double-check in any way? Well, what I did in a case like that was to define an invariant for the type the pointer points to, and call it when you get it (in debug mode). At least if the invariant fails (or crashes) you know that you were passed a bad value.
// API that does not allow NULL
void PublicApiFunction1(Person* in_person)
{
assert(in_person != NULL);
assert(in_person->Invariant());
// Actual code...
}
// API that allows NULL
void PublicApiFunction2(Person* in_person)
{
assert(in_person == NULL || in_person->Invariant());
// Actual code (must keep in mind that in_person may be NULL)
}
Following does work in Windows (somebody suggested it before):
static void copy(void * target, const void* source, int size)
{
__try
{
CopyMemory(target, source, size);
}
__except(EXCEPTION_EXECUTE_HANDLER)
{
doSomething(--whatever--);
}
}
The function has to be static, standalone or static method of some class.
To test on read-only, copy data in the local buffer.
To test on write without modifying contents, write them over.
You can test first/last addresses only.
If pointer is invalid, control will be passed to 'doSomething',
and then outside the brackets.
Just do not use anything requiring destructors, like CString.
On Windows I use this code:
void * G_pPointer = NULL;
const char * G_szPointerName = NULL;
void CheckPointerIternal()
{
char cTest = *((char *)G_pPointer);
}
bool CheckPointerIternalExt()
{
bool bRet = false;
__try
{
CheckPointerIternal();
bRet = true;
}
__except (EXCEPTION_EXECUTE_HANDLER)
{
}
return bRet;
}
void CheckPointer(void * A_pPointer, const char * A_szPointerName)
{
G_pPointer = A_pPointer;
G_szPointerName = A_szPointerName;
if (!CheckPointerIternalExt())
throw std::runtime_error("Invalid pointer " + std::string(G_szPointerName) + "!");
}
Usage:
unsigned long * pTest = (unsigned long *) 0x12345;
CheckPointer(pTest, "pTest"); //throws exception
On macOS, you can do this with mach_vm_region, which as well as telling you if a pointer is valid, also lets you validate what access you have to the memory to which the pointer points (read/write/execute). I provided sample code to do this in my answer to another question:
#include <mach/mach.h>
#include <mach/mach_vm.h>
#include <stdio.h>
#include <stdbool.h>
bool ptr_is_valid(void *ptr, vm_prot_t needs_access) {
vm_map_t task = mach_task_self();
mach_vm_address_t address = (mach_vm_address_t)ptr;
mach_vm_size_t size = 0;
vm_region_basic_info_data_64_t info;
mach_msg_type_number_t count = VM_REGION_BASIC_INFO_COUNT_64;
mach_port_t object_name;
kern_return_t ret = mach_vm_region(task, &address, &size, VM_REGION_BASIC_INFO_64, (vm_region_info_t)&info, &count, &object_name);
if (ret != KERN_SUCCESS) return false;
return ((mach_vm_address_t)ptr) >= address && ((info.protection & needs_access) == needs_access);
}
#define TEST(ptr,acc) printf("ptr_is_valid(%p,access=%d)=%d\n", (void*)(ptr), (acc), ptr_is_valid((void*)(ptr),(acc)))
int main(int argc, char**argv) {
TEST(0,0);
TEST(0,VM_PROT_READ);
TEST(123456789,VM_PROT_READ);
TEST(main,0);
TEST(main,VM_PROT_READ);
TEST(main,VM_PROT_READ|VM_PROT_EXECUTE);
TEST(main,VM_PROT_EXECUTE);
TEST(main,VM_PROT_WRITE);
TEST((void*)(-1),0);
return 0;
}
The SEI CERT C Coding Standard recommendation MEM10-C. Define and use a pointer validation function says it is possible to do a check to some degree, especially under Linux OS.
The method described in the link is to keep track of the highest memory address returned by malloc and add a function that tests if someone tries to use a pointer greater than that value. It is probably of limited use.
IsBadReadPtr(), IsBadWritePtr(), IsBadCodePtr(), IsBadStringPtr() for Windows.
These take time proportional to the length of the block, so for sanity check I just check the starting address.
I have seen various libraries use some method to check for unreferenced memory and such. I believe they simply "override" the memory allocation and deallocation methods (malloc/free), which has some logic that keeps track of the pointers. I suppose this is overkill for your use case, but it would be one way to do it.
Technically you can override operator new (and delete) and collect information about all allocated memory, so you can have a method to check if heap memory is valid.
but:
you still need a way to check if pointer is allocated on stack ()
you will need to define what is 'valid' pointer:
a) memory on that address is
allocated
b) memory at that address
is start address of object (e.g.
address not in the middle of huge
array)
c) memory at that address
is start address of object of expected type
Bottom line: approach in question is not C++ way, you need to define some rules which ensure that function receives valid pointers.
There is no way to make that check in C++. What should you do if other code passes you an invalid pointer? You should crash. Why? Check out this link: http://blogs.msdn.com/oldnewthing/archive/2006/09/27/773741.aspx
Addendum to the accpeted answer(s):
Assume that your pointer could hold only three values -- 0, 1 and -1 where 1 signifies a valid pointer, -1 an invalid one and 0 another invalid one. What is the probability that your pointer is NULL, all values being equally likely? 1/3. Now, take the valid case out, so for every invalid case, you have a 50:50 ratio to catch all errors. Looks good right? Scale this for a 4-byte pointer. There are 2^32 or 4294967294 possible values. Of these, only ONE value is correct, one is NULL, and you are still left with 4294967292 other invalid cases. Recalculate: you have a test for 1 out of (4294967292+ 1) invalid cases. A probability of 2.xe-10 or 0 for most practical purposes. Such is the futility of the NULL check.
You know, a new driver (at least on Linux) that is capable of this probably wouldn't be that hard to write.
On the other hand, it would be folly to build your programs like this. Unless you have some really specific and single use for such a thing, I wouldn't recommend it. If you built a large application loaded with constant pointer validity checks it would likely be horrendously slow.
you should avoid these methods because they do not work. blogs.msdn.com/oldnewthing/archive/2006/09/27/773741.aspx – JaredPar Feb 15 '09 at 16:02
If they don't work - next windows update will fix it ?
If they don't work on concept level - function will be probably removed from windows api completely.
MSDN documentation claim that they are banned, and reason for this is probably flaw of further design of application (e.g. generally you should not eat invalid pointers silently - if you're in charge of design of whole application of course), and performance/time of pointer checking.
But you should not claim that they does not work because of some blog.
In my test application I've verified that they do work.
these links may be helpful
_CrtIsValidPointer
Verifies that a specified memory range is valid for reading and writing (debug version only).
http://msdn.microsoft.com/en-us/library/0w1ekd5e.aspx
_CrtCheckMemory
Confirms the integrity of the memory blocks allocated in the debug heap (debug version only).
http://msdn.microsoft.com/en-us/library/e73x0s4b.aspx

Pointer array of pointers with C?

I want an array of pointers and I want to set byte values in the memory addresses where the pointers (of the array) are pointing.
Would this work:
unsigned int *pointer[4] = {(unsigned int *) 0xFF200020, (unsigned int *) 0xFF20001C, (unsigned int *) 0xFF200018, (unsigned int *) 0xFF200014};
*pointer[0] = 0b0111111; // the value is correct for the address
Or is the syntax somehow different?
EDIT:
I'm coding for an SOC board and these are memory addresses that contain the case of some UI elements.
unsigned int *element1 = (unsigned int *) 0xFF200020;
*element1 = 0b0111111;
works so I'm just interested about the C syntax of this.
EDIT2: There was one 0 too much in ... = 0b0...
Short answer:
Everything you've written is fine.
Thoughts:
I'm a big fan of using the types from stdint.h. This would let you write uint32_t which is more clearly a 32 bit unsigned number than unsigned long.
You'll often see people write macros to refer to these registers:
#define REG_IRQ (*(volatile uint32_t *)(0xFF200020))
REG_IRQ = 0x42;
It's possible that you actually want these pointers to be to volatile integers. You want it to be volatile if the value can change outside of the execution of your program. That is, if that memory position doesn't act strictly like a piece of memory. (For example, it's a register that stores the interrupt flags).
With most compilers I've used on embedded platforms, you'll have problems from ignoring volatile once optimizations have been enabled.
0b00111111 is, sadly, non-standard. You can use octal, decimal, or hexadecimal.
Sure, this should work, providing you can find addresses in your own segment.
Most probably, you'll have a segmentation fault when running this code, because 0xFF200020 have really few chances to be in your program segment.
This will not throw any error and will work fine but hard-coding memory address the pointer is pointing to is not a good idea. De-referencing some unknown/non-existing memory location will cause segmentation fault but if you are sure about the memory location and hard-coding values to them as done here is totally fine.

Can reading garbage memory break program flow?

In my application, I have a nested pair of loops which follow similarly-nested linked lists in order to parse the data. I made a stupid blunder and cast one struct as the child struct, EG:
if (((ENTITY *) OuterEntityLoop->data)->visible == true) {
instead of:
if (((ENTITY_RECORD *) OuterEntityLoop->data)->entity->visible == true) {
This caused a problem where about 70% of runs would result in the application halting completely - not crashing, just sitting and spinning. Diagnostic printfs in program flow would fire in odd order or not at all, and though it spontaneously recovered a couple of times for the most part it broke the app.
So here's the thing. Even after paring down the logic inside to be absolutely it wasn't infinite looping based on a logic bug, to the point where the loop only contained my printf, it was still broken.
Thing two: when the struct was identified incorrectly, it still complained if I tried to access a nonexistent property even though it didn't have the extant property.
My questions are:
Why did this corrupt memory? Can simply reading garbage memory trash the program's control structures? If not, does this mean I still have a leak somewhere even though Electric Fence doesn't complain anymore?
I assume that the reason it complained about a nonexistent property is because it goes by the type definition given, not what's actually there. This is less questionable in my mind now that I've typed it out, but I'd like confirmation that I'm not off base here.
There's really no telling what will happen when a program accesses invalid memory, even for reading. On some systems, any memory read operation will either be valid or cause an immediate program crash, but on other systems it's possible that an erroneous read could be misinterpreted as a signal to do something. You didn't specify whether you're using a PC or an embedded system, but on embedded systems there are often many addresses by design which trigger various actions when they are read [e.g. dequeueing received data from a serial port, or acknowledging an interrupt]; an erroneous read of such an address might cause serial data to be lost, or might cause the interrupt controller to think an interrupt had been processed when it actually hadn't.
Further, in some embedded systems, an attempt to read an invalid address may have other even worse effects that aren't really by design, but rather by happenstance. On one system I designed, for example, I had to interface a memory device which was a little slow to get off the bus following a read cycle. Provided that the next memory read was performed from a memory area which had at least one wait sate or was on a different bus, there would be no problem. If code which was running in the fast external memory partition tried to read that area, however, the failure of the memory device to get off the bus quickly would corrupt some bits of the next fetched instruction. The net effect of all this was that accessing the slow device from code located in some places was no problem, but accessing it--intentionally or not--from code located in the fast partition would cause weird and non-reproduceable failures.
welcome to C, where the power of casting, allows you to make any piece of memory look like any object you want, but at your own risk. If the thing you cast is not really an object of that type, and that type contains a pointer to something else, you run the risk of crashing. Since even attempting to read random memory that has not been actually mapped into a processes virtual memory address space can cause a core or reading from the certain areas of memory that do not have read permission will also cause a core, like the NULL pointer.
example:
#include <stdio.h>
#include <stdlib.h>
struct foo
{
int x;
int y;
int z;
};
struct bar
{
int x;
int y;
struct foo *p;
};
void evil_cast(void *p)
{
/* hmm... maybe this is a bar pointer */
struct bar *q = (struct bar *)p;
if (q != NULL) /* q is some valid pointer */
{
/* as long as q points readable memory q->x will return some value, */
/* this has a fairly high probability of success */
printf("random pointer to a bar, x value x(%d)\n", q->x);
/* hmm... lets use the foo pointer from my random bar */
if (q->p != NULL)
{
/* very high probabilty of coring, since the likely hood that a */
/* random piece of memory contains a valid address is much lower */
printf("random value of x from a random foo pointer, from a random bar pointer x value x(%d)\n", q->p->x);
}
}
}
int main(int argc, char *argv[])
{
int *random_heap_data = (int *)malloc(1024); /* just random head memory */
/* setup the first 5 locations to be some integers */
random_heap_data[0] = 1;
random_heap_data[1] = 2;
random_heap_data[2] = 3;
random_heap_data[3] = 4;
random_heap_data[4] = 5;
evil_cast(random_heap_data);
return 0;
}

Why my memory check code didn't work well

I'm trying to Implement simple OS and now have to implement memory management.
At first, we typed simple code code to check memory size as below.
What the problem i met is that the result of this function depends on increment size.
If I set increment to 1024, this function return 640Kb.
However, If I set increment to 1024*1024, this functinon return 120Mb.
(my system(bochs)'s memory set to 120MB.)
I checked the optimization option and A20 gate.
Anyone who knows why my function didn't work well?
unsigned int memtest_sub(unsigned int start, unsigned int end)
{
unsigned int i;
unsigned int* ptr;
unsigned int orgValue;
const unsigned int testValue = 0xbfbfbfbf;
for (i = start; i <= end; i += 1024*1024) {
ptr = (unsigned int*) i;
orgValue = *ptr;
*ptr = testValue;
if (*ptr != testValue) {
break;
}
*ptr = orgValue;
}
return i;
}
You can't do probes like that.
First the memory isn't necessarily contiguous as you've already discovered. It almost never is. The hole at 640k is for legacy reasons, but even further in the memory is usually split up. You have to ask your firmware for the memory layout.
Second some memory banks might be double mapped into the physical space and you'll end up in real trouble if you start using them. This isn't very common, but it's a real pain to deal with it.
Third, and probably most important, there are devices mapped into that space. By writing to random addresses you're potentially writing to registers of important hardware. Writing back whatever you read won't do you good because some hardware registers have side effects as soon as you write them. As a matter of fact, some hardware registers have side effects when you read them. Some of that hardware isn't necessarily protected and you might do permanent damage. I've bricked ethernet hardware in the past by having pointer errors in a 1:1 mapped kernel because the EEPROM/flash was unprotected. Other places you write to might actually change the layout of the memory itself.
Since you're most likely on i386 read this: http://wiki.osdev.org/Detecting_Memory_(x86)
Also, consider using a boot loader that detects memory for you and communicates that and other important information you need to know in a well defined API. The boot loader is better debugged with respect to all weird variants of hardware.
Following assignments are buggy:
ptr = (unsigned int*) i;
orgValue = *ptr;
*ptr = testValue;
ptr not pointing any valid memory, you can't treat i's value as address where you can perform some read-write operation - Undefined behaviour

Why does this function cast an int argument to a volatile pointer and immediately dereferences it?

I just want to know what below function is doing
static int myfunc(int val)
{
return *(volatile int *)val;
}
If val is a pointer when you pass it to this function, it makes sure that the value pointed by this pointer is read and returned to the caller.
I suspect this might be a trick for embedded devices, where sometimes the operation of reading a value at an address has some effect on the hardware.
For instance, reading from an hardware FIFO will pop the read value from the FIFO.
Marking here the pointer as volatile make the compiler not optimize the read if it detects that the value is not used.
Example:
#define FIFO_ADDRESS 0x800050
static int myfunc(int val)
{
return *(volatile int *)val; // the address *will* be read
}
static int bad( int val )
{
return *(int*)val; // might be optimized to nop()
// by the compiler if the value
// is not used by the caller
}
int main(){
bad( FIFO_ADDRESS ); // could be NOP since return value is not used
myfunc( FIFO_ADDRESS ); // *WILL* perform a READ operation on the FIFO,
// even though the result is not used, because
// of the volatile keyword
}
Note that I would do it differently, probably with a smartly named macro:
#define FORCE_INT_PTR_READ( address ) *(volatile int *)address
Could you give us an example of usage in your case?
It appears to be trying to confuse the optimiser (and possibly the user). It is taking an integer, treating it as a pointer to an integer, and dereferencing it. The 'volatile' ensures that the optimiser will generate code for that dereference at that point and won't let the optimiser omit the fetch from memory. This is usually used for accessing memory mapped hardware registers,
It does the following:
Passes a signed integer to a function.
Converts the contents of the signed integer to a pointer. This doesn't make sense, since pointers cannot be signed. Formally, this is also not well-defined behavior and anything can happen.
The volatile keyword suggests that the contents of val should equal to a physical hardware address. When reading from such addresses, the volatile keyword is necessary, since the hardware register's contents may change at any time.
It takes the contents of that address and treats it as int, which is then returned.
static means that the function has local scope.
Please note that if the target OS uses virtual addressing (such as a PC), the OS will just give you a slap on the fingers should you attempt to run this code.
All in all, this is poorly written code, which is neither safe, well-defined nor portable. You won't gain any valuable knowledge by staring at it.
It casts val to a pointer and returns it's value. Calls to myfunc() will work only if thy look like
int i, j;
i = 10;
j = myfunc( (int)&i )`
That should set j to 10
BUT it won't work on 64bit systems where sizeof(int *) is 8 and thus larger then sizeof(int) or on some old systems with sizeof(int) == 2

Resources