Everything is by value in C.

gistfile1.c

#include <stdio.h>


void foo (int *p)
{
	printf ("\t1. p is pointing to: %p\n", p);
	p = 0;
	printf ("\t2. p is pointing to: %p\n", p);
}

int main (int argc, char **argv)
{
	int my_stuff = 1234;

	int *my_p = &my_stuff;

	printf ("1. my_p is pointing to: %p\n", my_p);
	foo (my_p);
	printf ("2. my_p is pointing to: %p\n", my_p);

	return 0;
}

Output on my machine is:

1. my_p is pointing to: 0x7fff5fbff4bc
   1. p is pointing to: 0x7fff5fbff4bc
   2. p is pointing to: 0x0
2. my_p is pointing to: 0x7fff5fbff4bc