Thread: Pointers

Hi,

I'm trying to understand how pointers work. It seems I can deference and sometimes I cannot.

Code:

main(int argc, char *argv[]) {
    FILE *f = fopen("/home/9three/Desktop/test.txt", "a");

    if (f == NULL) {
        printf("Unable to open file for writting.");
        return 1;
    }
    fprintf(*f, "Testing..\n");

    return 0;
}

If you notice I'm passing a pointer to fprintf. This doesn't compile and the error is "expecting FILE type". But "f" was declared as FILE type to begin with.

So "f" is a pointer and variable at the same time? If I pass *f then I'm passing a pointer. But if I pass "f" then I'm just passing a variable with a specific type? eehhh.. what?

T *p; // T is type int ,double ,float,FILE ..etc
then p is pointer to T, and *p is type T.

Originally Posted by 9three

Hi,

I'm trying to understand how pointers work. It seems I can deference and sometimes I cannot.

Code:

main(int argc, char *argv[]) {
    FILE *f = fopen("/home/9three/Desktop/test.txt", "a");

    if (f == NULL) {
        printf("Unable to open file for writting.");
        return 1;
    }
    fprintf(*f, "Testing..\n");

    return 0;
}

If you notice I'm passing a pointer to fprintf. This doesn't compile and the error is "expecting FILE type". But "f" was declared as FILE type to begin with.

So "f" is a pointer and variable at the same time? If I pass *f then I'm passing a pointer. But if I pass "f" then I'm just passing a variable with a specific type? eehhh.. what?

Pointers are a type of variable, yes.

The fopenf() function expects a pointer. When you hand it *f you are handing it the value at the address held in the pointer, which it doesn't know what to do with.

Code:

file *f;

fopen (f,"blablabla.txt")...

C has this thing about variables being used the way they're declared...

int *x ... is a pointer to an integer.

then

*x is the data at that address.

Someplace along the way they decided that would be easier...
I rather tend to disagree. I've always found it thoroughly confusing!

Originally Posted by CommonTater

Pointers are a type of variable, yes.
int *x ... is a pointer to an integer.

then

*x is the data at that address.

When you say *x is the data at that address, do you mean this:

Code:

int *x = 12345;

//x contains 12345 ? or does *x contains 12345 ?
//or does *x contain the address only?


//wtf?

Originally Posted by 9three

When you say *x is the data at that address, do you mean this:

Code:

int *x = 12345;

//x contains 12345 ? or does *x contains 12345 ?
//or does *x contain the address only?


//wtf?

He means exactly what he says. x is the address, *x is the data at that address (i.e. the address stored in x).

Originally Posted by 9three

When you say *x is the data at that address, do you mean this:

Code:

int *x = 12345;

//x contains 12345 ? or does *x contains 12345 ?
//or does *x contain the address only?


//wtf?

x contains the address of 12345... not the number itself.

*x is shorthand for "the data at the address in x"

Code:

int *x = malloc(sizeof(int));
*x = 12345;

printf("address = %d  value = %d", x, *x);

free(x);

Try it... you'll see...

Originally Posted by Programmer_P

Unless C is different than C++ with pointers, the statement:

Code:

int* x = zero_value;

does not assign x the value of zero_value. It assigns to x the memory address of zero_value, which would not be 0.

This is false, in both C and C++. To assign the memory address, you would need to use &zero_value.

Just a pointer (no pun intended!) you can use the assert() macro for quickly testing things. Just include <assert.h>

Code:

#include <assert.h>
#include <stdio.h>

int main(){
  FILE *ptr = fopen("~/test.txt", "a+");
  assert(ptr != NULL);
  fprintf(ptr, "Testing a line.\n");

  return 0;
}

Faster testing means faster coding. Though you may want to check for errors in a more meaningful way later.. if you're just testing stuff asserts rock.. plus they rock for complex pointer stuff.

Hi,

I'm not new to programming so we can skip the beginner stuff. I come from the Java world!

@CommonTater

Your code does not compile. It was suppose to be %p, no bigge!

After running your code it does clear it up a bit. But then it leads to another question...

What about this:

Code:

main(int argc, char *argv[]) {
    printf("%s", argv[1]);
    printf("%c", *argv[1]);
}

In the first code block, in order for me to access the data I need to use *x. But in the second code block, if I want the entire data I need to use argv[1] (no dereferencing). Dereferencing gives me the first character of the argument passed with the file.

Can someone explain that? Why such inconsistency ?


btw, does anyone have any resource on getting the address? Such as *argv[1] + 3, it should give me another address.