I'm a new programmer,my program is about getting LED on,maybe the simplest in C language,but it is always having _'Delay10ms':function contains unnamed parameter,hoping someone help me to solve it.
here are my words:
#include<reg51.h>
#include<intrins.h>
#define GPIO_LED P2
void Delay10ms(unsigned char time)
{
unsigned char i,j;
for(i=1;i<110*time;i++)
for(j=1;j<110;j++);
}
void main()
{
unsigned char n,i,j;
GPIO_LED=0X01;
while(1)
{
for(n=0;n<7;n++)
{
GPIO_LED=_crol_(GPIO_LED,1);
Delay10ms(50);
}
for(n=0;n<7;n++)
{
GPIO_LED=_cror_(GPIO_LED,1);
Delay10ms(50);
}
}
}
Either <reg51.h> or <intrins.h> probably have standard library includes. One of these library includes contains the standard header function time() that returns the current system time. Because your function parameter's name is time the IDE probably can't figure out what you meant with it and thus it gives you that error/warning. Try renaming your variable.
Also, the return type of main() has never been void (C++), use int.
It has been a Long time gone since I played c, but your code Looks o.k., maybe there is a name conflict ("time")
But there are some more questions:
- Why do you use char (unsigned char = 0..255) instead of classic int? This would be make code more readable.
- What do you expect how relyable your timer could be on a multitasking operating System?
I would implement such a function (if it not exists) as
void MyDelay (unsigned int delay_time)
{
start = MyOs.GiveMeTheTime()
while (MyOs.GiveMeTheTime() - start < delay_time)
{};
}
MyOs and GiveMeTheTime are names chosen by me. You must find out, which functions are available on your System, I am sure, they are.
I suppose, you try to program anything like a raspberry pi. Maybe this link could help:
Example Which Provides Accurate uS Timing
Related
I have been working on this simply hobbyist OS, and I have decided to add some C++ support. Here is the simple script I wrote. When I compile it, I get this message:
cp.o: In function `caller':
test.cpp:(.text+0x3a): undefined reference to `__stack_chk_fail'
Here is the script:
class CPP {
public:
int a;
void test(void);
};
void CPP::test(void) {
// Code here
}
int caller() {
CPP caller;
caller.test();
return CPP.a;
}
Try it like this.
class CPP {
public:
int a;
void test(void);
};
void CPP::test(void) {
CPP::a = 4;
}
int caller() {
CPP caller;
caller.test();
return caller.a;
}
int main(){
int called = caller();
std::cout << called << std::endl;
return 0;
}
It seems to me that the linker you are using can't find the library containing a security function crashing the program upon detecting stack smashing. (It may be that the compiler doesn't include the function declaration for some reason? I am not familiar who actually defies this specific function.) Try compiling with -fno-stack-protector or equivalent.
What is the compiler used? A workaround might be defining the function as something like exit(1); or similar. That would produce the intended effect yet fix the problem for now.
I created a test program to show how this actually plays out. Test program:
int main(){
int a[50]; // To have the compiler manage the stack
return 0;
}
With only -O0 as the flag ghidra decompiles this to:
undefined8 main(void){
long in_FS_OFFSET;
if (*(long *)(in_FS_OFFSET + 0x28) != *(long *)(in_FS_OFFSET + 0x28)) {
/* WARNING: Subroutine does not return */
__stack_chk_fail();
}
return 0;
}
With -fno-stack-protector:
undefined8 main(void){
return 0;
}
The array was thrown out by ghidra in decompilation, but we see that the stack protection is missing if you use the flag. There are also some messed up parts of this in ghidra (e.g. int->undefined8), but this is standard in decompilation.
Consequences of using the flag
Compiling without stack protection is not good per se, but it shouldn't affect you in much. If you write some code (that the compiler shouts you about) you can create a buffer overflowable program, which should not be that big of an issue in my optinion.
Alternative
Alternatively have a look at this. They are talking about embedded systems, but the topic seems appropriate.
Why is the code there
Look up stack smashing, but to my knowledge I will try to explain. When the program enters a function (main in this case) it stores the location of the next instruction in the stack.
If you write an OS you probably know what the stack is, but for completeness: The stack is just some memory onto which you can push and off which you can pop data. You always pop the last pushed thing off the stack. C++ and other languages also use the stack as a way to store local variables. The stack is at the end of memory and when you push something, the new thing will be further forward rather than back, it fills up 'backwards'.
You can initialise buffers as a local variable e.g. char[20]. If you filled the buffer without checking the length you might overfill this, and overwrite things in the stack other than the buffer. The return address of the next instruction is in the stack as well. So if we have a program like this:
int test(){
int a;
char buffer[20];
int c;
// someCode;
}
Then the stack will look something like this at someCode:
[ Unused space, c, buffer[0], buffer[1] ..., buffer[19], a, Return Address, variables of calling function ]
Now if I filled the buffer without checking the length I can overwrite a (which is a problem as I can modify how the program runs) or even the return address (which is a major flaw as I might be able to execute malicious shellcode, by injecting it into the buffer). To avoid this compilers insert a 'stack cookie' between a and the return address. If that variable is changed then the program should terminate before calling return, and that is what __stack_chk_fail() is for. It seems that it is defined in some library as well so you might not be able use this, despite technically the compiler being the one that uses this.
Consider the following code:
file_1.hpp:
typedef void (*func_ptr)(void);
func_ptr file1_get_function(void);
file1.cpp:
// file_1.cpp
#include "file_1.hpp"
static void some_func(void)
{
do_stuff();
}
func_ptr file1_get_function(void)
{
return some_func;
}
file2.cpp
#include "file1.hpp"
void file2_func(void)
{
func_ptr function_pointer_to_file1 = file1_get_function();
function_pointer_to_file1();
}
While I believe the above example is technically possible - to call a function with internal linkage only via a function pointer, is it bad practice to do so? Could there be some funky compiler optimizations that take place (auto inline, for instance) that would make this situation problematic?
There's no problem, this is fine. In fact , IMHO, it is a good practice which lets your function be called without polluting the space of externally visible symbols.
It would also be appropriate to use this technique in the context of a function lookup table, e.g. a calculator which passes in a string representing an operator name, and expects back a function pointer to the function for doing that operation.
The compiler/linker isn't allowed to make optimizations which break correct code and this is correct code.
Historical note: back in C89, externally visible symbols had to be unique on the first 6 characters; this was relaxed in C99 and also commonly by compiler extension.
In order for this to work, you have to expose some portion of it as external and that's the clue most compilers will need.
Is there a chance that there's a broken compiler out there that will make mincemeat of this strange practice because they didn't foresee someone doing it? I can't answer that.
I can only think of false reasons to want to do this though: Finger print hiding, which fails because you have to expose it in the function pointer decl, unless you are planning to cast your way around things, in which case the question is "how badly is this going to hurt".
The other reason would be facading callbacks - you have some super-sensitive static local function in module m and you now want to expose the functionality in another module for callback purposes, but you want to audit that so you want a facade:
static void voodoo_function() {
}
fnptr get_voodoo_function(const char* file, int line) {
// you tagged the question as C++, so C++ io it is.
std::cout << "requested voodoo function from " << file << ":" << line << "\n";
return voodoo_function;
}
...
// question tagged as c++, so I'm using c++ syntax
auto* fn = get_voodoo_function(__FILE__, __LINE__);
but that's not really helping much, you really want a wrapper around execution of the function.
At the end of the day, there is a much simpler way to expose a function pointer. Provide an accessor function.
static void voodoo_function() {}
void do_voodoo_function() {
// provide external access to voodoo
voodoo_function();
}
Because here you provide the compiler with an optimization opportunity - when you link, if you specify whole program optimization, it can detect that this is a facade that it can eliminate, because you let it worry about function pointers.
But is there a really compelling reason not just to remove the static from infront of voodoo_function other than not exposing the internal name for it? And if so, why is the internal name so precious that you would go to these lengths to hide that?
static void ban_account_if_user_is_ugly() {
...;
}
fnptr do_that_thing() {
ban_account_if_user_is_ugly();
}
vs
void do_that_thing() { // ban account if user is ugly
...
}
--- EDIT ---
Conversion. Your function pointer is int(*)(int) but your static function is unsigned int(*)(unsigned int) and you don't want to have to cast it.
Again: Just providing a facade function would solve the problem, and it will transform into a function pointer later. Converting it to a function pointer by hand can only be a stumbling block for the compiler's whole program optimization.
But if you're casting, lets consider this:
// v1
fnptr get_fn_ptr() {
// brute force cast because otherwise it's 'hassle'
return (fnptr)(static_fn);
}
int facade_fn(int i) {
auto ui = static_cast<unsigned int>(i);
auto result = static_fn(ui);
return static_cast<int>(result);
}
Ok unsigned to signed, not a big deal. And then someone comes along and changes what fnptr needs to be to void(int, float);. One of the above becomes a weird runtime crash and one becomes a compile error.
Suppose I have a class Utility in a file utility.h:
class Utility {
public:
static double longDescriptiveName(double x) { return x + 42; }
};
And then I find that I use the function longDescriptiveName(...) a LOT. So like an irresponsible C++ programmer that I am when I've had too much coffee, I create a new file utilitymacros.h and add the following there:
#define ldn Utility::longDescriptiveName
Now I include "utilitymacros.h" in any *.cpp where I use ldn(...) and my heart is filled with joy over how much more convinient it is to type 3 letters vs 28.
Question: Is there a safer (more proper) way of doing this than with #define?
I've noticed that I have to include "utilitymacros.h" after including boost headers, which I obviously don't like because it's a sign of clashes (though the Boost errors I get are not very clear as to what the clash is).
Clarification 1: On Code Readability
In case you might say that this negatively affects code readability, I assure you it does not, because it's a small set of functions that are used A LOT. An example that is widely know is stoi for stringToInteger. Another is pdf for probabilityDensityFunction, etc. So if I want to do the following, stoi is more readable in my opinion:
int x = stoi(a) + stoi(b) + stoi(c) + stoi(d);
Than:
int x = Utility::stringToInteger(a) + Utility::stringToInteger(b)
+ Utility::stringToInteger(c) + Utility::stringToInteger(d);
Or:
int x = Utility::stringToInteger(a);
x += Utility::stringToInteger(b);
x += Utility::stringToInteger(c);
x += Utility::stringToInteger(d);
Clarification 2: Editor Macro
I use Emacs as my IDE of choice and a Kinesis keyboard so you KNOW I use a ton of keyboard macros, custom keyboard shortcuts, as well as actually modifying what I see in the editor vs what's actually stored in the h/cpp file. But still, I feel like the simplicity and visual readability (as argued above) of using a function abbreviation in a few select cases really is the result I'm looking for (this is certainly subject to a degree).
Instead of macro, you could write inline function that forwards the call to the actual function:
inline double ldn(double x)
{
return Utility::longDescriptiveName(x);
}
That is certainly safer than macro.
You could use a function reference:
double (&ldn)(double) = Utility::longDescriptiveName;
How about configuring a snippit/macro/similar thing in your text editor? This way you only have to type ldn or something like that and the code doesn't have to run through the preprocessor risking difficult to find bugs later.
I don't know if this helps, but I think part of the problem may be the use of overly general namespaces (or class names, in this case), such as Utility.
If instead of Utility::stringToInteger, we had
namespace utility {
namespace type_conversion {
namespace string {
int to_int(const std::string &s);
}
}
}
Then the function could locally be used like this:
void local_function()
{
using namespace utility::type_conversion::string;
int sum = to_int(a) + to_int(b) + to_int(c) + to_int(d);
}
Analogously, if classes/structs and static functions are used (and there can be good reasons for this), we have something like
strut utility {
struct type_conversion {
struct string {
static int to_int(const std::string &s);
};
};
};
and the local function would look something like this:
void local_function()
{
typedef utility::type_conversion::string str;
int sum = str::to_int(a) + str::to_int(b)
+ str::to_int(c) + str::to_int(d);
}
I realize I am not telling you anything about syntax you didn't know already; it's more a reminder of the fact that the organization and structure of namespaces and classes itself plays an important role in making code more readable (and writable).
One alternative is to rename your function and put it in a namespace instead of a class, since it is static anyway. utility.h becomes
namespace Utility {
// long descriptive comment
inline double ldn(double x) { return x + 42; }
}
Then you can put using namespace Utility; in your client code.
I know there are lots of style guides out there saying short names are a bad thing, but I don't see the point of obeying some style and then circumventing it.
You can use alias template (since C++11).
using shortName = my::complicate::function::name;
this is my first question after long time checking on this marvelous webpage.
Probably my question is a little silly but I want to know others opinion about this. What is better, to create several specific methods or, on the other hand, only one generic method? Here is an example...
unsigned char *Method1(CommandTypeEnum command, ParamsCommand1Struct *params)
{
if(params == NULL) return NULL;
// Construct a string (command) with those specific params (params->element1, ...)
return buffer; // buffer is a member of the class
}
unsigned char *Method2(CommandTypeEnum command, ParamsCommand2Struct *params)
{
...
}
unsigned char *Method3(CommandTypeEnum command, ParamsCommand3Struct *params)
{
...
}
unsigned char *Method4(CommandTypeEnum command, ParamsCommand4Struct *params)
{
...
}
or
unsigned char *Method(CommandTypeEnum command, void *params)
{
switch(command)
{
case CMD_1:
{
if(params == NULL) return NULL;
ParamsCommand1Struct *value = (ParamsCommand1Struct *) params;
// Construct a string (command) with those specific params (params->element1, ...)
return buffer;
}
break;
// ...
default:
break;
}
}
The main thing I do not really like of the latter option is this,
ParamsCommand1Struct *value = (ParamsCommand1Struct *) params;
because "params" could not be a pointer to "ParamsCommand1Struct" but a pointer to "ParamsCommand2Struct" or someone else.
I really appreciate your opinions!
General Answer
In Writing Solid Code, Steve Macguire's advice is to prefer distinct functions (methods) for specific situations. The reason is that you can assert conditions that are relevant to the specific case, and you can more easily debug because you have more context.
An interesting example is the standard C run-time's functions for dynamic memory allocation. Most of it is redundant, as realloc can actually do (almost) everything you need. If you have realloc, you don't need malloc or free. But when you have such a general function, used for several different types of operations, it's hard to add useful assertions and it's harder to write unit tests, and it's harder to see what's happening when debugging. Macquire takes it a step farther and suggests that, not only should realloc just do _re_allocation, but it should probably be two distinct functions: one for growing a block and one for shrinking a block.
While I generally agree with his logic, sometimes there are practical advantages to having one general purpose method (often when operations is highly data-driven). So I usually decide on a case by case basis, with a bias toward creating very specific methods rather than overly general purpose ones.
Specific Answer
In your case, I think you need to find a way to factor out the common code from the specifics. The switch is often a signal that you should be using a small class hierarchy with virtual functions.
If you like the single method approach, then it probably should be just a dispatcher to the more specific methods. In other words, each of those cases in the switch statement simply call the appropriate Method1, Method2, etc. If you want the user to see only the general purpose method, then you can make the specific implementations private methods.
Generally, it's better to offer separate functions, because they by their prototype names and arguments communicate directly and visibly to the user that which is available; this also leads to more straightforward documentation.
The one time I use a multi-purpose function is for something like a query() function, where a number of minor query functions, rather than leading to a proliferation of functions, are bundled into one, with a generic input and output void pointer.
In general, think about what you're trying to communicate to the API user by the API prototypes themselves; a clear sense of what the API can do. He doesn't need excessive minutae; he does need to know the core functions which are the entire point of having the API in the first place.
First off, you need to decide which language you are using. Tagging the question with both C and C++ here makes no sense. I am assuming C++.
If you can create a generic function then of course that is preferable (why would you prefer multiple, redundant functions?) The question is; can you? However, you seem to be unaware of templates. We need to see what you have omitted here to tell if you if templates are suitable however:
// Construct a string (command) with those specific params (params->element1, ...)
In the general case, assuming templates are appropriate, all of that turns into:
template <typename T>
unsigned char *Method(CommandTypeEnum command, T *params) {
// more here
}
On a side note, how is buffer declared? Are you returning a pointer to dynamically allocated memory? Prefer RAII type objects and avoid dynamically allocating memory like that if so.
If you are using C++ then I would avoid using void* as you don't really need to. There is nothing wrong with having multiple methods. Note that you don't actually have to rename the function in your first set of examples - you can just overload a function using different parameters so that there is a separate function signature for each type. Ultimately, this kind of question is very subjective and there are a number of ways of doing things. Looking at your functions of the first type, you would perhaps be well served by looking into the use of templated functions
You could create a struct. That's what I use to handle console commands.
typedef int (* pFunPrintf)(const char*,...);
typedef void (CommandClass::*pKeyFunc)(char *,pFunPrintf);
struct KeyCommand
{
const char * cmd;
unsigned char cmdLen;
pKeyFunc pfun;
const char * Note;
long ID;
};
#define CMD_FORMAT(a) a,(sizeof(a)-1)
static KeyCommand Commands[]=
{
{CMD_FORMAT("one"), &CommandClass::CommandOne, "String Parameter",0},
{CMD_FORMAT("two"), &CommandClass::CommandTwo, "String Parameter",1},
{CMD_FORMAT("three"), &CommandClass::CommandThree, "String Parameter",2},
{CMD_FORMAT("four"), &CommandClass::CommandFour, "String Parameter",3},
};
#define AllCommands sizeof(Commands)/sizeof(KeyCommand)
And the Parser function
void CommandClass::ParseCmd( char* Argcommand )
{
unsigned int x;
for ( x=0;x<AllCommands;x++)
{
if(!memcmp(Commands[x].cmd,Argcommand,Commands[x].cmdLen ))
{
(this->*Commands[x].pfun)(&Argcommand[Commands[x].cmdLen],&::printf);
break;
}
}
if(x==AllCommands)
{
// Unknown command
}
}
I use a thread safe printf pPrintf, so ignore it.
I don't really know what you want to do, but in C++ you probably should derive multiple classes from a Formatter Base class like this:
class Formatter
{
virtual void Format(unsigned char* buffer, Command command) const = 0;
};
class YourClass
{
public:
void Method(Command command, const Formatter& formatter)
{
formatter.Format(buffer, command);
}
private:
unsigned char* buffer_;
};
int main()
{
//
Params1Formatter formatter(/*...*/);
YourClass yourObject;
yourObject.Method(CommandA, formatter);
// ...
}
This removes the resposibility to handle all that params stuff from your class and makes it closed for changes. If there will be new commands or parameters during further development you don't have to modifiy (and eventually break) existing code but add new classes that implement the new stuff.
While not full answer this should guide you in correct direction: ONE FUNCTION ONE RESPONSIBILITY. Prefer the code where it is responsible for one thing only and does it well. The code whith huge switch statement (which is not bad by itself) where you need cast void * to some other type is a smell.
By the way I hope you do realise that according to standard you can only cast from void * to <type> * only when the original cast was exactly from <type> * to void *.
I need help making an c++ program with a function that uses int Disc(int A, int B, int C) and calculates returns B*B-4*A*C and use the function Disc in the program..... i have this so far.
void main(){
cout << Disc(a,b,c);
}
What does the book you are using say about functions?
Assuming you can't get a book, take a look that this tutorial on functions (See the edit below)
You already have the function definition. The name, what parameters it takes and what it returns so if you take some time looking at the above tutorial you should be able to put something together (All you need to do is to write the body of the function).
The one thing that may cause you an issue (compiler error) is if you don't put it above the main function as either the function definition or the function itself must come before it is used. For simplicity at this point I reccomend you just put the function itself above the main function as shown in first example in the tutorial I linked to.
EDIT about linked tutorial
It suggests you use return with brackets. Example:
return (5);
Where as it should be used without them. Example:
return 5;
Ok, so you define the function Disc then:
int Disc(int A, int B, int C)
{
/* tricky part goes here... */
}