about the two warning of sprintf()

when I write code in my project, there is an int type argument just can not cast to string, so I write anther small program to test the sprintf() as follow,(the itoa() is copy from other place to compare it result ), it's really shit when I find
the problem

/* sprint.c write by vinco at 2011-07-29
*
*/
#include<stdio.h>
#include<string.h>

const char * itoa(int i);

int main()
{
char var[32];
int i=100;
char* p=NULL;

#if 0

sprintf(var,"%s",i);
printf("var=%s\n",var);

#else

p = itoa(i);
printf("itoa(%d)=%s\n",i,p);

#endif

return 0;
}
const char * itoa(int i)
{
static char buf[256];
sprintf(buf,"%d",i);
return buf;
}

1. #if 0

now call itoa(), waring as following , even so, it can be executed correctly :

root@vinco:/home/vinco# gcc sprint.c -o sprint
sprint.c: In function ‘main’:
sprint.c:22: warning: assignment discards qualifiers from pointer target type
root@vinco:/home/vinco# ./sprint
itoa(100)=100
root@vinco:/home/vinco#

2. #if 1

now call "sprintf(var,"%s",i);" waring as following , it cann't be executed normaly:

root@vinco:/home/vinco# gcc sprint.c -o sprint
sprint.c: In function ‘main’:
sprint.c:17: warning: format ‘%s’ expects type ‘char *’, but argument 3 has type ‘int’
root@vinco:/home/vinco# ./sprint
Segmentation fault
root@vinco:/home/vinco#

3. analyzed as follow:

in case #if 0:

sprint.c:22: warning: assignment discards qualifiers from pointer target type

means itoa() return type "const char * " ,it's assigned to a variable whose type is "char * ",it 's warning without doubt when compile it . to avoid it ,you can modify it as following:

const char* p=NULL;

in case #if 1:



it's reall incomprehensible to some extent if you change the value of variable i to zero(line 12):

int i=0;

compile and execute it , it seem that the "error" doesn't that much:

root@vinco:/home/vinco# gcc sprint.c -o sprint
sprint.c: In function ‘main’:
sprint.c:17: warning: format ‘%s’ expects type ‘char *’, but argument 3 has type ‘int’
root@vinco:/home/vinco# ./sprint
var=(null)
root@vinco:/home/vinco#

it can be executed ,but only get "var=(null)" !!!

---------------------------------------------------------------------------------------------

int sprintf ( char * str, const char * format, ... );

---------------------------------------------------------------------------------------------

after half an hour, I abruptly realize that the "format " must be unifrom with the types of argment list ,after modified line 17:

sprintf(var,"%d",i);

it 's perfect !!!