Steven Yong's Weblog

So lastime I wrote about how to modify constant data using reflection in Java, now I'm going to try to modify constant data in C, using pointer.

Consider code below:

int i = 0;
const int ci = 0;
printf("ci initial value %d\n", ci); /* print 0 */

int* pi = &i; /* a pointer to int */
const int* pci = &ci; /* a pointer to const int */

//ci = 1; /* try to assign new value to const, compile error */
//*pci = 1; /* try to assign new value to const, compile error */

pi = (int*)pci; /* cast pointer to const to pointer, success */
*pi = 1; /* no compile error, weird */

printf("ci value %d\n", ci); /* result? */

What result? Answer is undefined and compiler-dependant. On GCC C compiler, constant data gets modified, but on Visual C++ compiler, constant value remains and no runtime error, that means the line *pi = 1 has no effect at all.

There are at least 3 basic ways to assign value to array element in C.

Say we have an integer array defined like this:

int an_array[3] = {1,2,3};

We can change the value of each element using following three ways (there might me more ways).

1) Most common one, using array index.

an_array[0] = 10;
an_array[1] = 20;
an_array[2] = 30;

2) Using pointer, since array is implemented by pointer anyway.

*an_array = 10;
*(an_array+1) = 20;
*(an_array+2) = 30;

3) Using address casting, assuming integer type use 4 bytes on your machine. This method cast the pointer integer value to integer pointer and goes from there.

int addr = an_array;
*(int*)addr = 10;
*(int*)(addr+4) = 20; (change to addr+8 if your int is 8 bytes long)
*(int*)(addr+8) = 30; (change to addr+16 if your int is 8 bytes long)