Does printf function in c calls argument by reference or by value?
Everything in C is passed by value.
Even things that look like they're passed by reference (i.e., pointers to variables so that you can change the underlying variables) are in fact the values of the pointers being passed by value.
There's no pass by reference in C, everything is by value or pointer-to-address by value
call by reference is not applicable in c.
As in c we can't create alias of any variable,so eiher call by value or call by pointer mechanism is supported here..
Related
What does the compiler do when we define a function? And is the function name a pointer? Or maybe it's something else.
I wish I knew something about function names.
What does a function name stand for in C
The function name stands for the function.
What does the compiler do when we define a function?
The compiler remembers the name of the function, and it generates code to implement the function. It also remembers information from the declaration of the function, such as the return type and the types of the parameters, if a prototype form was used.
And is the function name a pointer? Or maybe it's something else.
The name of a function designates a function.
When a function name is used in an expression, it is automatically converted to a pointer to the function, with a couple of exceptions discussed below. Furthermore, effectively the only use for a function name is as a pointer to the function. Because of this, a programmer might think of the function name as a pointer, but it is not actually a pointer.
One exception to the automatic conversion is when a function name is used with sizeof. However, using sizeof on a function is usually an error (it is not defined by the C standard and can work only if a C implementation defines it as an extension), so this exception is never used in ordinary practice. The other exception is when a function name is used with unary &, as in &sin. In this case, we are explicitly taking the address of the function, which produces a pointer to the function, so there is no need for automatic conversion to a pointer to the function.
When you call a function, as with sin(x), the “function-call operator,” ( … ), takes a pointer to the function as its first operand. So, due to the automatic conversion, sin(x) is effectively (&sin)(x), which says to call the function whose address is &sin and pass it the argument x.
The fact that the function-call operator takes a pointer to a function is why we can use pointers for functions, as in double (*p)(double) = sin; printf("sin(.5) = %g.\n", p(.5));.
Specifics matter. Especially when talking about how something works, and even more so when we consider why something works. Currently, as I understand it, EVERYTHING in C is passed by value. NOTHING is passed by reference. Some programmers mention that arrays in C are passed by reference.
But as per my limited understanding,
Even if we pass an array to a function like this void traverse(int arr[4]);, it is actually being taken in as a copy of the pointer variable storing the location in memory of the first element in that array. It is then dereferenced inside the function, but the initial value being passed is actually a local variable. Since memory allocated to arrays in the program stack would be contiguous, the compiler is able to make square bracket notation work as well as pointer arithmetic.
This and passing by reference are not the same thing to me. I would think this is an important distinction.
But on the other hand, we can then just say that everything in computing is passed by value, since something like Java would do the same in a more subtle manner. And it is actually just simulating a pass by reference. Please advise.
At the level of bits in the computer, arguments can only be passed by value. Bits representing some argument are written to a processor register or memory location designated as the place to pass an argument. Passing by reference is a construct built upon passing by value by using an address as the value that is passed. Passing an address can be implemented automatically or manually. Both methods are pass-by-reference.
When we pass some entity by passing its address instead of passing its value directly, that is called pass by reference. This terminology long antedates the creation of “references” in C++. In assembly language, when we load the address of some thing into a register to pass it to a function, that was, and is, called pass by reference. The C standard specifies a pointer provides a reference to an entity (C 2018 6.2.5 20). So, when we have a pointer to an object, we have a reference to an object, and when we pass the pointer to a function, we are passing a reference to the object to the function.
Some languages automated pass-by-reference. FORTRAN passes everything by reference except for some special syntax for calling routines outside FORTRAN. However, whether passing-by-reference is implemented as an automatic feature of the programming language, by a programmer manually loading an address in assembly language, or by a programmer manually requesting an address with a language operator such as C’s &, when a reference to an object is passed, then the object is passed by reference.
C++ created a new type that it called a “reference,” but this was a new use of the word. The C++ meaning of “reference” applies to C++ only. It does not change the existing use of that word outside the context of C++. Outside of C++, “reference” has its ordinary English meaning of providing information on another thing.
Regarding your specific question about passing an array in C,
in C, an array argument is automatically converted to the address of its first element, and this address is typically used to access the entire array. So the array is in fact passed by reference. Describing this as an automatic conversion to a pointer is merely documenting the details. The effect is the same: The function is given access to the object the caller designated by providing a reference to it.
Further, any dispute over the meaning of “pass by reference” is merely one about terminology, not about the actual mechanisms used in the computer.
I was recently reading about the usage of const keyword as function arguments in C and the way to use that has been mentioned in When and for what purposes should the const keyword be used in C for variables and been accepted as correct answer. In this post, one point mentions that
Never use const in a function prototype for a parameter passed by
value. It has no meaning and is hence just 'noise'.
I used this way and it works for me but I am not sure why that is a noise for parameters passed by value and yet not a noise for the parameters passed by reference (more aptly the pointer values in C as there is not concept of pass by value and pass by reference in C). So, by this explanation when I pass a pointer as a function argument and use a const keyword; I have to do this for both the declaration in the header file and the definition in the C file but I need not use the const keyword for a non-pointer argument in the declaration (header file) and only use it while defining the function in the C file.
Any explanations?
The statement you quote is a bit misleading, because in C, all arguments are passed by value.* I suppose it is trying to distinguish between the arguments themselves and, for the special case of arguments that are pointers, their referents.
In any event, the point is that const-qualifying a function parameter in the function declaration conveys no information whatever to callers. Regardless of such qualification, the function cannot modify the caller's copy of any argument anyway, because arguments are passed by value.
*Note, however, that arrays are never passed at all. In function call expressions, as in most contexts, array values "decay" to pointers, and those pointers are passed by value. This produces an effect similar, but not identical, to what you would have if arrays were passed by reference.
It's the rule. If in the declaration of a function, you don't mark your parameters const, you can mark them const in the definition.
Some folk like to mark as many parameters const as possible in the definition since it can guard against unintentional modification of the function parameters; which could introduce bugs. Personally I don't do this but plenty of houses (including a large bank headquartered in Scotland) insist on the style.
Posting this here because I wrote it for a code review and figured it was worth preserving:
Adding const to a value-type parameter in the function declaration is useless. The caller won't care whether the parameter is modified or not because the caller's data won't be affected in any way. The parameter is a separate object than the argument that was passed in, constructed by copy [or by move for rvalue arguments into move-constructible parameters (since C++11)].
Adding const to a value-type parameter in the function definition marks the separate, function-local object as const. This is essentially the same as marking a local variable 'const', and value-type parameters should usually be treated as local variables when determining whether the value-type should be const.
The C & C++ standards demand that compilers & linkers be smart enough to match the following declaration with its definition:
// Declaration
void Foo(int bar);
// Definition
void Foo(const int bar){
printf("%i", bar);
}
I'm learning C, I was looking at https://github.com/mruby/mruby/blob/master/src/load.c and this line made me very confused:
mrb_irep* read_irep_record_1
On line 40.
I can see that this is a pointer of some sort.
What I'd like to know is the following
What does this do?
How do you use them?
What are these called?
How do they work?
How can I replicate this in a program?
I've only this used in C projects, is it recommended to use these in C++? Can you do this in C++?
I searched a bit on Stackoverflow for pointer functions but couldn't find anything like this.
Thanks in advance!
That line is simply declaring a function that returns a pointer to mrb_irep. For example, what does a function declared as int foo() return? Well it returns an int, as we see in the declaration. Similarly, a function declared as mrb_irep* read_irep_record_1(...) returns a variable of type mreb_irep*, or a pointer to a struct called mreb_irep.
I've just discovered variadic functions in C and have defined one as a general notification typedef, that as well as a pointer to a text string can optionally have whatever arguments sent along with it- useful as a generic debug function for instance where I want all the output string manipulation in one place.
Since I want my C files to be as generic as possible I have static variables that contain pointers to possible callbacks in higher code, populated in an init call. Since the pointers may be null if higher code isn't interested, I'd normally have a local wrapper that only calls through the pointer if it's not null. But I'm having trouble figuring out how to forward this fuzzy thing represented by '...' and simply calling the function with '...' in the argument list gives a syntax error.
Is there any way to do this, or am I stuck with having a dummy local handler and having init set null callbacks to a pointer to that?
You can't pass on the variadic arguments. You have to fetch them into a va_list and pass this to the inner function.
Take a look at this Question at the C FAQ. It defines a variadic error function that wants to forward to printf. This is just your use case.
In the same FAQs, it is generally recommended to have a version taking va_list for every (or most) variadic functions