Const in C gives unpredictable result

Question

Possible duplicate:
constants and pointers in C

I have this little piece of code. I am using the gcc compiler:

#include <stdio.h> int main() { const int a=10; int *d; d=&a; *d=30; printf("%d %d\n",a,*d); return 0; } 

It gives a warning when compiling:

"assign qualifiers from target pointer type"

But there is no mistake. Output: 30 30

Then is it worth it to ignore the goal of supporting a constant variable whose value is fixed during program execution (please correct me if I am wrong)?

Answer 1

const not a guarantee. This is only a promise. Promises can be broken. The compiler warns when this happens, but is not required otherwise to prevent this, as there may be times when traversing a constant can be useful.

Answer 2

The above behavior is undefined. As this example shows, you can get other values ​​as well. The explanation for this conclusion is that compilers optimize the code and replace constant variables with their values. So we can see the difference when we do printf("%d %d %u %u\n",a,*d,&a,d); , which would actually be modified for optimization as printf("%d %d %u %u\n",10,*d,&a,d);

Do not rely on any of these outputs. Actual behavior is not defined by the standard.

Answer 3

You should not do this, and the compiler gives you this warning for some reason. However, C / C ++ will allow you to do anything you want. You must write clean code.

You can reflect the constant of this value using:

 int const * d = &a; 

Then, changing what d indicates, gives warnings and errors.