Related
In a C program, having an array that is meant to work as a buffer FooBuffer[] for storing the contents of a data member of a struct like this:
struct Foo {
uint64_t data;
};
I was told that this line might cause unaligned access:
uint8_t FooBuffer[10] = {0U};
I have some knowledge that unaligned access depends on the alignment offset of the processor and in general, it consumes more read/write cycles. Under what circumstances would this cause unaligned memory access and how could I prevent it?
Edit:
A variable of type struct Foo would be stored in the buffer. Particularly, its member data would be split up into eight bytes that would be stored in the array FooBuffer. See attached code with some options for this.
#include <stdio.h>
#include <string.h>
typedef unsigned long uint64;
typedef unsigned char uint8;
struct Foo
{
uint64 data;
};
int main()
{
struct Foo foo1 = {0x0123456789001122};
uint8 FooBuffer[10] = {0U};
FooBuffer[0] = (uint8)(foo1.data);
FooBuffer[1] = (uint8)(foo1.data >> 8);
FooBuffer[2] = (uint8)(foo1.data >> 16);
FooBuffer[3] = (uint8)(foo1.data >> 24);
FooBuffer[4] = (uint8)(foo1.data >> 32);
FooBuffer[5] = (uint8)(foo1.data >> 40);
FooBuffer[6] = (uint8)(foo1.data >> 48);
FooBuffer[7] = (uint8)(foo1.data >> 56);
struct Foo foo2 = {0x9876543210112233};
uint8 FooBuffer2[10] = {0U};
memcpy(FooBuffer2, &foo2, sizeof(foo2));
return 0;
}
However, it is not clear how this process is done since a piece of privative software performs the operation. What would be the scenarios that could result in unaligned memory access after the "conversion"?
Defining either a structure such as struct Foo { uint64_t data; } or an array such as uint8_t FooBuffer[10]; and using them in normal ways will not cause an unaligned access. (Why did you use 10 for FooBuffer? Only 8 bytes are needed for this example?)
A method that novices sometimes attempt that can cause unaligned accesses is attempting to reinterpret an array of bytes as a data structure. For example, consider:
// Get raw bytes from network or somewhere.
uint8_t FooBuffer[10];
CallRoutineToReadBytes(FooBuffer,...);
// Reinterpret bytes as original type.
struct Foo x = * (struct Foo *) FooBuffer; // Never do this!
The problem here is that struct Foo has some alignment requirement, but FooBuffer does not. So FooBuffer could be at any address, but the cast to struct Foo * attempts to force it to an address for a struct Foo. If the alignment is not correct, the behavior is not defined by the C standard. Even if the system allows it and the program “works,” it may be accessing a struct Foo at an improperly aligned address and suffering performance problems.
To avoid this, a proper way to reinterpret bytes is to copy them into a new object:
struct Foo x;
memcpy(&x, FooBuffer, sizeof x);
Often a compiler will recognize what is happening here and, especially if struct Foo is not large, implement the memcpy in an efficient way, perhaps as two load-four-byte instructions or one load-eight-byte instruction.
Something you can do to help that along is ask the compiler to align FooBuffer by declaring it with the _Alignas keyword:
uint8_t _Alignas(Struct Foo) FooBuffer[10];
Note that that might not help if you need to take bytes from the middle of a buffer, such as from a network message that includes preceding protocol bytes and other data. And, even if it does give the desired alignment, never use the * (struct Foo *) FooBuffer shown above. It has more problems than just alignment, one of which is that the C standard does not guarantee the behavior of reinterpreting data like this. (A supported way to do it in C is through unions, but memcpy is a fine solution.)
In the code you show, bytes are copied from foo1.data to FooBuffer using bit shifts. This also will not cause alignment problems; expressions that manipulate data like this work just fine. But there are two issues with it. One is that it nominally manipulates individual bytes one by one. That is perfectly legal in C, but it can be slow. A compiler might optimize it, and there might be built-ins or library functions to assist with it, depending on your platform.
The other issue is that it puts the bytes in an order according to their position values: The low-position-value bytes are put into the buffer first. In contrast, the memcpy method copies the bytes in the order they are stored in memory. Which method you want to use depends on the problem you are trying to solve. To store data on one system and read it back later on the same system, the memcpy method is fine. To send data between two systems using the same byte ordering, the memcpy method is fine. However, if you want to send data from one system on the Internet to another, and the two systems do not use the same byte order in memory, you need to agree on an order to use in the network packages. In this case, it is common to use the arrange-bytes-by-position-value method. Again, your platform may have builtins or library routines to assist with this. For example, the htonl and ntohl routines are BSD routines that take a normal 32-bit unsigned integer and return it with its bytes arranged for network order or vice-versa.
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
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
Basicly i have a custom structure that contains different kind of data. For example:
typedef struct example_structure{
uint8_t* example_1[4];
int example_2[4];
int example_3;
} example_structure;
What i need to do is to copy context of this structure to a const char* buffer so i can send that copied data (buffer) using winsock2's send(SOCKET s, const char* buffer, int len, int flags) function. I tried using memcpy() but wouldn't i just copy address of pointers and not the data?
Yes, if you copied or sent that structure through a socket you would end up copying/sending pointers, which would obviously be meaningless to the recipient, however, if the recipient is running on different hardware (e.g. not the same endian), all of the data may be meaningless anyway. On top of that, differences in the amount of padding between structure members may also become a problem.
For non-trivial situations it is best to use an existing protocol (such as protobuf), or roll your own protocol, keeping in mind the potential differences in hardware representation of your data.
You need to design a protocol before you can encode the data in accord with that protocol. Decide exactly how the data will be encoded at the byte level. Then write code to encode and decode to that format that you decided on.
Do not skip the step of actually documenting the wire protocol at the byte level. It will save you pain later, I promise.
See this answer for a bit more detail.
const char* buffer
This buffer has a constant value so u cant copy anything to it. You probably don't need to copy anything. Just use send function in such a way:
send(s, (char*)&example_structure, sizeof(structure), flags)
But here is the problem with pointers in your structure (uint8_t* example_1[4];).
Sending pointers between different applications / machine does not make sense.
Hmm, your struct contains uint8_t * fields, what looks like C strings... It does not make sense copying or sending a pointer which is just a mere memory address in sending process user space.
If your struct has been (note, no pointers):
typedef struct example_structure{
uint8_t example_1[4];
int example_2[4];
int example_3;
} example_structure;
and provided you transfer it on exactly same architecture (same hardware, same compiler, same compiler options), you could do simply:
example_structure ex_struc;
// initialize the struct
...
send(s, &ex_struc, sizeof(ex_struc), flags);
And even in that case, I would strongly advise you to define and use a protocol - as already said by #DavidSchwartz, it could save you time and headaches later...
But as you have pointers, you cannot do that and must define a protocol.
it could be (but you are free to prefere little endian order, or 2 or 8 bytes for each int depending on your actual data):
one byte (or two) for length of first uint8_t array, followed by the array
above repeated 3 more times
four bytes in big endian order for first int of example_2
repeated 3 times
four bytes in big endian order for int of example_3
This clearly defines the format of a message.
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