Question

I am new to C and wondering how to do some pointer stuff. Specifically here I am wondering how you can pass a pointer into a function and "get a value out of the function". Sort of like this (semi-pseudocode):

assign_value_to_pointer(void* pointer) {
if (cond1) {
pointer = 10;
} else if (cond2) {
pointer = "foo";
} else if (cond3) {
pointer = true;
} else if (cond4) {
pointer = somestruct;
} else if (cond5) {
pointer = NULL;
} else if (cond6) {
// unknown type!
pointer = flexiblearraymember.items[index];
}
}

main() {
void* pointer = NULL;

assign_value_to_pointer(&pointer);

if (cond1) {
assert(pointer == 10);
} else if (cond2) {
assert(pointer == "foo");
} else if (cond3) {
assert(pointer == true);
} else if (cond4) {
assert(pointer == somestruct);
} else if (cond5) {
assert(pointer == NULL);
}
}
Put another way:

p = new Pointer()
assign_a_value(p)
assert(p.value == 10) // or whatever
Basically it is passing the pointer into the function, the function is assigning a value to the pointer, and then you can use that value outside of the function when it returns. You may not know what kind of value you are getting from the function (but that can be handled by extending this to use structs and such), hence the void pointer. The main goal though is just passing a pointer into some function and having it absorb some value.

Wondering how to do this properly in C with a quick example implementation. Doesn't have to cover every case just enough to get started.

I would like to use this to implement stuff like passing in a NULL error object to a function, and if there is an error, it sets the pointer of the error to some error code, etc.

I don't think this should be a broad question, but if it is, it would be helpful to know where to look for a more thorough explanation or examples in source code.

Answer 1

In C, you can pass a pointer into a function by using the pointer's address and modify the value being pointed to inside the function. By using the '*' symbol in the function parameter list and dereferencing the pointer inside the function, you can access and modify the value of the pointer.

Step-by-step explanation:

In C, you can pass a pointer into a function by using the pointer's address, and then modify the value being pointed to inside the function. This allows you to 'get a value out of the function'.

A pointer is passed to a function by using the '*' symbol before the variable name in the function parameter list. Inside the function, you can dereference the pointer using the '*' symbol to access the value being pointed to.

Here's an example:

void assign_value_to_pointer(void** pointer) { *pointer = "foo"; }

In this example, the function 'assign_value_to_pointer' takes a double pointer as a parameter and assigns the address of the string 'foo' to the value being pointed to by the double pointer.