This post only documents a simple program to show the effects of * and & in C and does not attempt to explain what pointers are.
For a quick read, check out The 5-Minute Guide to C Pointers.
Program:
#include <stdio.h>
int main()
{
// lval (left-hand side value - write) = rval (right-hand side value - read)
int i = 5;
int *j = &i;
int **k = &j;
printf( // *i invalid - error: indirection requires pointer operand ('int' invalid)
"\n i=%d\n &i=%p\n *&i=%d\n &*&i=%p\n",
i, &i, *&i, &*&i
);
printf(
"\n j=%p\n &j=%p\n *j=%d\n *&j=%p\n &*j=%p\n *&*j=%d\n &*&j=%p\n",
j, &j, *j, *&j, &*j, *&*j, &*&j
);
printf( // &&k invalid - error: use of undeclared 'k'
"\n k=%p\n &k=%p\n *k=%p\n *&k=%p\n &*k=%p\n &*&k=%p\n *&*k=%p\n **k=%d\n &**k=%p\n **&k=%p\n &**&k=%p\n",
k, &k, *k, *&k, &*k, &*&k, *&*k, **k, &**k, **&k, &**&k
);
printf("\n");
return 0;
}
To run:
- Save the program as
test.c. - In the terminal, run
gcc -o runtest test.cto compile it. The-ospecifies the name of the output file. On Mac machines, Clang replaces GCC. - Type
./runtestto run the compiled executable. - Just for fun: To generate the equivalent x86 assembly language for the program (GAS syntax), type
gcc -S test.cin the terminal, which will generatetest.s. To ignore warnings during compilation, e.g. for the purposes of security testing, run GCC with the-woption.
Sample Output:
i=5 &i=0x7fff50883bc8 *&i=5 &*&i=0x7fff50883bc8 j=0x7fff50883bc8 &j=0x7fff50883bc0 *j=5 *&j=0x7fff50883bc8 &*j=0x7fff50883bc8 *&*j=5 &*&j=0x7fff50883bc0 k=0x7fff50883bc0 &k=0x7fff50883bb8 *k=0x7fff50883bc8 *&k=0x7fff50883bc0 &*k=0x7fff50883bc0 &*&k=0x7fff50883bb8 *&*k=0x7fff50883bc8 **k=5 &**k=0x7fff50883bc8 **&k=0x7fff50883bc8 &**&k=0x7fff50883bc0