printing dereferenced pointer.

[olliepa]

  1 #include <stdio.h>
  2
  3 main()
  4 {
  5     int* x;
  6     x = (int*) malloc(sizeof(int));
  7     scanf("*x: %d", x);
  8     printf("*x: %d\n", *x);
  9 }

After I compile:

[oadams@lister] pointers [1:119] a.out
4
*x: 134613344

Why is this? If I change the *x to an x in the printf statement I get exactly the same output.

[kermit]

Much to think about here. Your most immediate problem is this line:

scanf("*x: %d", x);

The bit in red should not be there. The '4' that you entered never got into the variable x, and so the output you got is just garbage that happened to be in that area of memory.

You should be thinking of a couple of other issues as well. First, main returns an int.

int main(void)
{

    ...
 
   return 0;
}

Also, you cast the return value of malloc:

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

One question - why? The cboard FAQ has a decent entry on that subject here. There is also some good reading about the matter in the comp.lang.c FAQ:
7.6 -- Why am I getting ``warning: assignment of pointer from integer lacks a cast'' for calls to malloc?
7.7 -- Why does some code carefully cast the values returned by malloc to the pointer type being allocated?
7.7b -- What's wrong with casting malloc's return value?

[MK27]

First:

• the return type of main is int
• malloc is part of stdlib.h*
• you do not cast malloc calls in C

But your problem is you are asking scanf to read this as input:
*x: [a number]
Try typing that and you will notice the program does work. But what you wanted was this:

printf("*x: ");
scanf("%d", x);

*it may "work anyway" (or it may not, how are you to tell?)

[olliepa]

Wow, thanks for the good responses!

I've been reading "The C Programming Language" By K&R.

In it they often just write the main function without a type declaration or a void argument. I use this and it works. Do I really have to specify the return type,argument type and then go 'return 0;' if all goes well?

EDIT: Oh so casting malloc is totally unnecessary?

[MK27]

I've been reading "The C Programming Language" By K&R.

In it they often just write the main function without a type declaration or a void argument. I use this and it works. Do I really have to specify the return type,argument type and then go 'return 0;' if all goes well?

Those two goofs I guess we set a higher standard here.

Well, you can do whatever you like, but it is supposed to int main. The return value is for the operating system.

[kermit]

Wow, thanks for the good responses!

I've been reading "The C Programming Language" By K&R.

In it they often just write the main function without a type declaration or a void argument. I use this and it works. Do I really have to specify the return type,argument type and then go 'return 0;' if all goes well?

The C language has changed a bit since K&R wrote that book. A (small) bit of historical perspective from the board FAQ might be helpful for you - read about it here.

Then you may as well have at look at the following sections of the comp.lang.c FAQ (Keeping in mind that the following are more general in nature, and not specifically dealing with your questions; I posted them so that you might have a broad view of some of the issues surrounding the declaration of main. That they have found their way into the FAQ is indicative of the widespread dissemination of false information regarding this matter):

1.25b
11.12a
11.12b
11.13
11.14a
11.14b
11.15
11.16

EDIT: Oh so casting malloc is totally unnecessary?

Unless you are using an ancient compiler, then yes, it is completely, absolutely, totally not necessary.

[laserlight]

Unless you are using an ancient compiler, then yes, it is completely, absolutely, totally not necessary.

Or if you intend your code to be compilable as C++ as well.

[olliepa]

*it may "work anyway" (or it may not, how are you to tell?)

It does 'work anyway'. But I don't know how that could possibly be the case if I havn't imported the library that has the function in it. How can it be?

Thanks a lot for the links, kermit. Much appreciated.

[tabstop]

It does 'work anyway'. But I don't know how that could possibly be the case if I havn't imported the library that has the function in it. How can it be?

Thanks a lot for the links, kermit. Much appreciated.

You have "imported" the library -- it's the same library that allows you to print stuff to the screen, and, well, do anything at all. The C runtime (which includes all the standard functions) is linked automatically whenever you create an executable file. What the header file does is allows the compiler to know what the function should be -- if you don't have it, everything will be turned into ints whether that's what you want or not.