Stupid question that I'm sure is some bit of syntax that's not right. How do I get dlsym to work with a function that returns a value? I'm getting the error 'invalid conversion of void* to LSError (*)()' in the following code - trying to get the compile the linux lightscribe sample program hoping that I can link it against the OSX dylib (why the hell won't HP release an actual Cocoa SDK? LS has only been around for what? 6 or 7 years now?):
void* LSHandle = dlopen("liblightscribe.1.dylib", RTLD_LOCAL|RTLD_LAZY);
if (LSHandle) {
LSError (*LS_DiscPrinter_ReleaseExclusiveUse)() = dlsym(LSHandle, "LS_DiscPrinter_ReleaseExclusiveUse");
..
lsError = LS_DiscPrinter_ReleaseExclusiveUse( pDiscPrinter);
The C standard does not actually define behaviour for converting to and from function pointers. Explanations vary as to why; the most common being that not all architectures implement function pointers as simple pointers to data. On some architectures, functions may reside in an entirely different segment of memory that is unaddressable using a pointer to void.
The “recommended” way to use dlsym is:
LSError (*LS_DiscPrinter_ReleaseExclusiveUse)(LS_DiscPrinterHandle);
*(void **)&LS_DiscPrinter_ReleaseExclusiveUse = dlsym("LS_DiscPrinter_ReleaseExclusiveUse");
Read the rationale and example on the POSIX page for dlsym for more information.
Related
So I tried using code from another post around here to see if I could use it, it was a code meant to utilize a potentiometer to move a servo motor, but when I attempted to compile it is gave the error above saying No operator "=" matches these operands in "Servo_Project.cpp". How do I go about fixing this error?
Just in case ill say this, the boards I was trying to compile the code were a NUCLEO-L476RG, the board from the post I mentioned utilized Nucleo L496ZG board and a Tower Pro Micro Servo 9G.
#include "mbed.h"
#include "Servo.h"
Servo myservo(D6);
AnalogOut MyPot(A0);
int main() {
float PotReading;
PotReading = MyPot.read();
while(1) {
for(int i=0; i<100; i++) {
myservo = (i/100);
wait(0.01);
}
}
}
This line:
myservo = (i/100);
Is wrong in a couple of ways. First, i/100 will always be zero - integer division truncates in C++. Second, there's not an = operator that allows an integer value to be assigned to a Servo object. YOu need to invoke some kind of Servo method instead, likely write().
myservo.write(SOMETHING);
The SOMETHING should be the position or speed of the servo you're trying to get working. See the Servo class reference for an explanation. Your code tries to use fractions from 0-1 and thatvisn't going to work - the Servo wants a position/speed between 0 and 180.
You should look in the Servo.h header to see what member functions and operators are implemented.
Assuming what you are using is this, it does have:
Servo& operator= (float percent);
Although note that the parameter is float and you are passing an int (the parameter is also in the range 0.0 to 1.0 - so not "percent" as its name suggests - so be wary, both the documentation and the naming are poor). You should have:
myservo = i/100.0f;
However, even though i / 100 would produce zero for all i in the loop, that does not explain the error, since an implicit cast should be possible - even if clearly undesirable. You should look in the actual header you are using to see if the operator= is declared - possibly you have the wrong file or a different version or just an entirely different implementation that happens to use teh same name.
I also notice that if you look in the header, there is no documentation mark-up for this function and the Servo& operator= (Servo& rhs); member is not documented at all - hence the confusing automatically generated "Shorthand for the write and read functions." on the Servo doc page when the function shown is only one of those things. It is possible it has been removed from your version.
Given that the documentation is incomplete and that the operator= looks like an after thought, the simplest solution is to use the read() / write() members directly in any case. Or implement your own Servo class - it appears to be only a thin wrapper/facade of the PwmOut class in any case. Since that is actually part of mbed rather than user contributed code of unknown quality, you may be on firmer ground.
I'm currently trying to do some tests with the buffer overflow vulnerability.
Here is the vulnerable code
void win()
{
printf("code flow successfully changed\n");
}
int main(int argc, char **argv)
{
volatile int (*fp)();
char buffer[64];
fp = 0;
gets(buffer);
if(fp) {
printf("calling function pointer, jumping to 0x%08x\n", fp);
fp();
}
}
The exploit is quite sample and very basic: all what I need here is to overflow the buffer and override the fp value to make it hold the address of win() function.
While trying to debug the program, I figured out that fb is placed below the buffer (i.e with a lower address in memory), and thus I am not able to modify its value.
I thought that once we declare a local variable x before y, x will be higher in memory (i.e at the bottom of the stack) so x can override y if it exceeds its boundaries which is not the case here.
I'm compiling the program with gcc gcc version 5.2.1, no special flags (only tested -O0)
Any clue?
The order of local variable on the stack is unspecified.
It may change between different compilers, different versions or different optimization options. It may even depend on the names of the variables or other seemingly unrelated things.
The order of local variables on the stack is not defined until compile/link (build) time. I'm no expert certainly, but I think you'd have to do some sort of a "hex dump", or perhaps run the code in a debugger environment to find out where it's allocated. And I'd also guess that this would be a relative address, not an absolute one.
And as #RalfFriedl has indicated in his answer, the location could change under any number of compiler options invoked for different platforms, etc.
But we do know it's possible to do buffer overflows. Although you do hear less about them now due to defensive measures such as address space layout randomization (ASLR), they're still around and paying the bills for someone I'd guess. There are of course many many online articles on the subject; here's one that seems fairly current(https://www.synopsys.com/blogs/software-security/detect-prevent-and-mitigate-buffer-overflow-attacks/).
Good luck (should you even say that to someone practicing buffer overflow attacks?). At any rate, I hope you learn some things, and use it for good :)
I have been programming Objective-C for only a few weeks. My experience in programming languages such as basic, visual basic, C++ and PHP is much more extensive starting back in 1987 and continuing forward to today. Although, for the last 5 years, I have exclusively coded PHP.
Today, I find myself confused by what I perceive to be bit conversion errors within the Objective-C language. I first noticed this the other day when trying to divide an integer (84) converted to a float by a float (10.0). This produced 8.399999, instead of the 8.400 I was hoping for. I coded a way around the issue and moved on.
Today, I am extracting an (int) 0 from an NSMutableDictionary. I store it first in an NSInteger and second in an int variable. The values should be 0 for both cases, but for both cases, I get the integer value 151229568. (See screenshot)
I remember from my early programming years that we had to worry about the size of the container, because pointing to block of memory with a 32-bit pointer to access a 4-bit value resulted in capturing all the data associated with other values and thus resulted in what appeared to be the wrong number being captured. With implicit memory management and type-conversions becoming the norm, I have not had to worry about this kind of issue for years, and now that I am confronted with it again, I need advice and clarification from programmers who are more familiar with this topic in todays programming environments.
Questions:
Is this a case of incorrect pointer sizing or something else?
What is happening on the back-end to produce this conversion from 0 to another number?
What can I do to get better precision and accuracy from my Objective-C calculations and variable assignments?
Code:
NSInteger hsibs = [keyData objectForKey:#"half_sibs"];
int hsibsi = [keyData objectForKey:#"half_sibs"];
//breakpoint and screen capture of variables in stack
I don't know Objective C all that well, but it looks like the method you use to obtain your data is returning a data type of id (see this reference), and not int
Looks like you either need to cast it or get the integer value in such a manner:
NSInteger hsibs = [[keyData objectForKey:#"half_sibs"] integerValue];
int hsibsi = [[keyData objectForKey:#"half_sibs"] intValue];
and then see if you get the expected results.
i'm just about to learn inline assembly.the GCC inline assembly cookbook http://www.ethernut.de/en/documents/arm-inline-asm.html
says:
A strict rule is: Never ever write to an input operand.
can someone tell me whether - and if so why - this rule is true?
let's say i get the value of an input operand through some register. Am I not allowed to reuse this register within the same assembly block if i don't inted to declare it also as output operand?
example:
asm volatile("add %[value], %[value], %[value] \n\t"
"mov %[result], %[value] \n\t"
: [result]"=r" (y)
: [value]"r" (x)
: //no clober
);
I know the example doesn't make much sense - but is it invalid?
I ask because i'm writing some assembly function that takes many input operands, each taking a general purpose register. since there are only 12 GPR's available on my architecture, with each input operand i get less "free" registers to work with. so do I really have to declare the input registers also as output in order to use them to "work" with them inside the function (even though i don't need theyr value outside the inline-assembly body? If so - can someone explain why?
hope the question is clear
thanks!
The compiler doesn't know x is clobbered (and I think there is no way to clobber an input register in a valid way). So it might reuse the register holding x later in the code assuming it still holds an unaltered value which isn't true since you changed it.
Ok I finally found the problem. It was inside the C function(CarbonTuner2) not the objC method. I was creating inside the function an array of the same size as the file size so if the filesize was big it created a really big array and my guess is that when I called another function from there, the local variables were put on the stack which created the EXC_BAD_ACCESS. What I did then is instead of using a variable to declare to size of the array I put the number directly. Then the code didnt even compile. it knew. The error wassomething like: Array size too big. I guess working 20+hours in a row isnt good XD But I am definitly gonna look into tools other than step by step debuggin to figure these ones out. Thanks for your help. Here is the code. If you divide gFileByteCount by 2 you dont get the error anymore:
// ConverterController.h
# import <Cocoa/Cocoa.h>
# import "Converter.h"
#interface ConverterController : NSObject {
UInt64 gFileByteCount ;
}
-(IBAction)ProcessFile:(id)sender;
void CarbonTuner2(long numSampsToProcess, long fftFrameSize, long osamp);
#end
// ConverterController.m
# include "ConverterController.h"
#implementation ConverterController
-(IBAction)ProcessFile:(id)sender{
UInt32 packets = gTotalPacketCount;//alloc a buffer of memory to hold the data read from disk.
gFileByteCount=250000;
long LENGTH=(long)gFileByteCount;
CarbonTuner2(LENGTH,(long)8192/2, (long)4*2);
}
#end
void CarbonTuner2(long numSampsToProcess, long fftFrameSize, long osamp)
{
long numFrames = numSampsToProcess / fftFrameSize * osamp;
float g2DFFTworksp[numFrames+2][2 * fftFrameSize];
double hello=sin(2.345);
}
Your crash has nothing to do with incompatibilities between C and ObjC.
And as previous posters said, you don't need to include math.h.
Run your code under gdb, and see where the crash happens by using backtrace.
Are you sure you're not sending bad arguments to the math functions?
E.g. this causes BAD_ACCESS:
double t = cos(*(double *)NULL);
Objective C is built directly on C, and the C underpinnings can and do work.
For an example of using math.h and parts of standard library from within an Objective C module, see:
http://en.wikibooks.org/wiki/Objective-C_Programming/syntax
There are other examples around.
Some care is needed around passing the variables around; use the C variables for the C and standard library calls; don't mix the C data types and Objective C data types incautiously. You'll usually want a conversion here.
If that is not the case, then please consider posting the code involved, and the error(s) you are receiving.
And with all respect due to Mr Hellman's response, I've hit errors when I don't have the header files included; I prefer to include the headers. But then, I tend to dial the compiler diagnostics up a couple of notches, too.
For what it's worth, I don't include math.h in my Cocoa app but have no problem using math functions (in C).
For example, I use atan() and don't get compiler errors, or run time errors.
Can you try this without including math.h at all?
First, you should add your code to your question, rather than posting it as an answer, so people can see what you're asking about. Second, you've got all sorts of weird problems with your memory management here - gFileByteCount is used to size a bunch of buffers, but it's set to zero, and doesn't appear to get re-set anywhere.
err = AudioFileReadPackets (fileID,
false, &bytesReturned, NULL,0,
&packets,(Byte *)rawAudio);
So, at this point, you pass a zero-sized buffer to AudioFileReadPackets, which prompty overruns the heap, corrupting the value of who knows what other variables...
fRawAudio =
malloc(gFileByteCount/(BITS/8)*sizeof(fRawAudio));
Here's another, minor error - you want sizeof(*fRawAudio) here, since you're trying to allocate an array of floats, not an array of float pointers. Fortunately, those entities are the same size, so it doesn't matter.
You should probably start with some example code that you know works (SpeakHere?), and modify it. I suspect there are other similar problems in the code yoou posted, but I don't have time to find them right now. At least get the rawAudio buffer appropriately-sized and use the values returned from AudioFileReadPackets appropriately.