C function call question.
-
I read a program, a function definition like:
void A( int channel)
{
}While the caller like:
A(); //no parameter here.
I know it's OK. can't find a explanation.
why do you say it's OK ? it's not.
void A( int channel)
{
}int _tmain(int argc, _TCHAR* argv[])
{
A();
return 0;
}1>c:\temp\fargs.cpp(11) : error C2660: 'A' : function does not take 0 arguments
-
why do you say it's OK ? it's not.
void A( int channel)
{
}int _tmain(int argc, _TCHAR* argv[])
{
A();
return 0;
}1>c:\temp\fargs.cpp(11) : error C2660: 'A' : function does not take 0 arguments
-
But in the program, our system. It works. I tested it. just the caller.h: extern void A(void); I debugged the code, it stepped into the A(i); in fact, only one A() function in the program.
what compiler are you using? at best, i would think the behavior is undefined. the function is going to try to find a parameter, somewhere. maybe it pulls some random garbage off the stack where it expects a parameter to be, or maybe your compiler puts variables into certain registers (and so the function will just grab random garbage out of a register).
-
But in the program, our system. It works. I tested it. just the caller.h: extern void A(void); I debugged the code, it stepped into the A(i); in fact, only one A() function in the program.
-
what compiler are you using? at best, i would think the behavior is undefined. the function is going to try to find a parameter, somewhere. maybe it pulls some random garbage off the stack where it expects a parameter to be, or maybe your compiler puts variables into certain registers (and so the function will just grab random garbage out of a register).
-
But in the program, our system. It works. I tested it. just the caller.h: extern void A(void); I debugged the code, it stepped into the A(i); in fact, only one A() function in the program.
You shoud correct your caller.h to have the correct prototype for A(). Then #include caller.h in both where you define A() and where you invoke it. This "works", due to C calling conventions, where the caller is responsible for pushing and popping arguments off the stack. In your case the caller doesn't push anything on the stack. When A() executes it will use whatever values are on the stack as its parameters, which in this case could be any value at all.
-
You shoud correct your caller.h to have the correct prototype for A(). Then #include caller.h in both where you define A() and where you invoke it. This "works", due to C calling conventions, where the caller is responsible for pushing and popping arguments off the stack. In your case the caller doesn't push anything on the stack. When A() executes it will use whatever values are on the stack as its parameters, which in this case could be any value at all.
Further to this, in C the declaration
void f();does not declare a function with no parameters, but declares a function with an unknown number of parameters. If you know that a function takes no arguments, you should prototype it asvoid f(void);. Otherwise, the compiler will happily let you do the following:void f();
int main(void)
{
f();
f(15);
f("Hello", "World");
}Using gcc I can compile the above with -Wall -Wextra, and get no warnings. If I add -Wstrict-prototypes, I do get the warning "function declaration isn’t a prototype [-Wstrict-prototypes]. If you're using MS-VSC, there's probably a similar warning flag that you can use.
-
I read a program, a function definition like:
void A( int channel)
{
}While the caller like:
A(); //no parameter here.
I know it's OK. can't find a explanation.
There are only two options... 1. There is a function declarations in an other (probably h) file with a default value
// this is in an external h
void A(int c = 0);// this is the main cpp
int main()
{
A(1);
}// this is in an external cpp
void A(int c)
{
// empty
}2. There is an overloaded version of A somewhere else
Skipper: We'll fix it. Alex: Fix it? How you gonna fix this? Skipper: Grit, spit and a whole lotta duct tape.
-
There are only two options... 1. There is a function declarations in an other (probably h) file with a default value
// this is in an external h
void A(int c = 0);// this is the main cpp
int main()
{
A(1);
}// this is in an external cpp
void A(int c)
{
// empty
}2. There is an overloaded version of A somewhere else
Skipper: We'll fix it. Alex: Fix it? How you gonna fix this? Skipper: Grit, spit and a whole lotta duct tape.
Kornfeld Eliyahu Peter wrote:
void A(int c = 0);
Do C functions allow this?
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
-
Kornfeld Eliyahu Peter wrote:
void A(int c = 0);
Do C functions allow this?
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
Of course not - but that only means that OP does not know the difference between C and C++
Skipper: We'll fix it. Alex: Fix it? How you gonna fix this? Skipper: Grit, spit and a whole lotta duct tape.
-
Of course not - but that only means that OP does not know the difference between C and C++
Skipper: We'll fix it. Alex: Fix it? How you gonna fix this? Skipper: Grit, spit and a whole lotta duct tape.
Kornfeld Eliyahu Peter wrote:
Of course not
Right. So then why would you suggest to him that such a function was a possible explanation for his question?
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
-
Kornfeld Eliyahu Peter wrote:
Of course not
Right. So then why would you suggest to him that such a function was a possible explanation for his question?
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles
As I told - I'm not sure OP knows the difference between C and C++ and what language is the project in (a leftover from someone else probably)... Taking in count, that it is part of code from a working system, and that the mentioned compiler can compile both C and C++, and the abilities of C and C++ I put my bet on OP's mistake... In fact if we stick to the C version, it can not work at all (C has only the varargs option, but that forces at least one named parameter, which we have not)...
Skipper: We'll fix it. Alex: Fix it? How you gonna fix this? Skipper: Grit, spit and a whole lotta duct tape.
-
It's possible that the compiler patches the call with a default argument. What is the value in channel? I'm guessing it is 0. Anyway this is not valid in standard C and will surely not compile in GCC or Microsoft C compilers.
«_Superman_» _I love work. It gives me something to do between weekends.
_Microsoft MVP (Visual C++) (October 2009 - September 2013)
