Several answers have pointed at uintptr_t and #include <stdint.h> as 'the' solution. That is, I suggest, part of the answer, but not the whole answer. You also need to look at where the function is called with the message ID of FOO.
Consider this code and compilation:
$ cat kk.c
#include <stdio.h>
static void function(int n, void *p)
{
unsigned long z = *(unsigned long *)p;
printf("%d - %lu\n", n, z);
}
int main(void)
{
function(1, 2);
return(0);
}
$ rmk kk
gcc -m64 -g -O -std=c99 -pedantic -Wall -Wshadow -Wpointer-arith \
-Wcast-qual -Wstrict-prototypes -Wmissing-prototypes \
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE kk.c -o kk
kk.c: In function 'main':
kk.c:10: warning: passing argument 2 of 'func' makes pointer from integer without a cast
$
You will observe that there is a problem at the calling location (in main()) — converting an integer to a pointer without a cast. You are going to need to analyze your function() in all its usages to see how values are passed to it. The code inside my function() would work if the calls were written:
unsigned long i = 0x2341;
function(1, &i);
Since yours are probably written differently, you need to review the points where the function is called to ensure that it makes sense to use the value as shown. Don't forget, you may be finding a latent bug.
Also, if you are going to format the value of the void * parameter (as converted), look carefully at the <inttypes.h> header (instead of stdint.h — inttypes.h provides the services of stdint.h, which is unusual, but the C99 standard says [t]he header <inttypes.h> includes the header <stdint.h> and extends it with
additional facilities provided by hosted implementations) and use the PRIxxx macros in your format strings.
Also, my comments are strictly applicable to C rather than C++, but your code is in the subset of C++ that is portable between C and C++. The chances are fair to good that my comments apply.