assignment makes pointer from integer without a cast [enabled by default]

shifter · 09-18-2012, 01:10 PM

Hi all, I've a warning message after compile time.
The warning is:

Code:

file.c: In function ‘main’:
file.c:16:10: warning: assignment makes pointer from integer without a cast [enabled by default]

My code is the following:

Code:

void main() {

  FILE *fp;

  typedef char STRING[20];

  struct s_type {
    STRING nome;
    STRING cognome;
  } utente;

  if (fp = fopen("test", "w")  == NULL) {
    printf("Impossibile aprire il file\n");
    exit(1);
  }

}

Where is the problem?

johnsfine · 09-18-2012, 01:27 PM

The problem is that you did not include the header required to declare fopen.

C lets you use external functions without declaring them. But when you do so, the compiler makes assumptions about those functions, which may be incorrect.

Depending on architecture details, those incorrect assumptions may cause compile time warnings or compile time errors or run time errors.

In this case, the error would be a serious run time error on an architecture (such as 64 bit Linux) in which pointers are a different size than integers. But is just on warning on 32 bit Linux or other architectures where a pointer and integer are more compatible.

If you type
man fopen
it should give you info including the fact that you need this include when using fopen

Code:

#include <stdio.h>

King_DuckZ · 09-18-2012, 01:42 PM

Allora, you don't mix Italian and C, it's so unelegant!

So let's see, the problem is that comparison has precedence over assignment (http://www.swansontec.com/sopc.html), so what this line is doing?

Code:

fp = fopen("test", "w")  == NULL

It first evaluates the comparison between fopen's return value and NULL, which yields an integer (1 or 0 arguably), then it says:

Code:

fp = 1

And that's your warning, il tuo puntatore è sminchio a bestia it means!

That's the int being coerced into a pointer... try to read from fp and you should experience a beautiful crash! Mamma mia!!

Use parentheses to set the correct precedence:

Code:

if ((fp = fopen("test", "w")) == NULL) {

johnsfine · 09-18-2012, 01:48 PM

Quote:

Originally Posted by King_DuckZ

the problem is that comparison has precedence over assignment

I guess I didn't read the first post carefully enough.

I agree with the above answer. (You still should include the right header to declare fopen, but the above issue is more serious).

shifter · 09-18-2012, 02:20 PM

Thank you very much King :-))

NevemTeve · 09-19-2012, 02:41 AM

Note: use these flags at compilation: -W -Wall -Wextra -pedantic -Werror