No warning with conversion to int from size_t - c

Question

Consider this program:
#include <stdio.h>
int main() {
int xr = 2;
int ya = 3;
size_t zu = 4;
xr = zu;
xr = (size_t) ya;
xr = sizeof ya;
return xr;
}
Compiling yields a warning:
conversion to ‘int’ from ‘size_t’ may alter its value [-Wconversion]
xr = zu;
^
but only this warning. As size_t and sizeof both return unsigned data types,
I would expect to see 3 warnings. What is going on here?

Answer 1

cast and sizeof are C operators, not functions. If you use this example:
#include <stdint.h>
size_t mik() {
return 1;
}
int main() {
return mik();
}
You get a warning as expected:
conversion to ‘int’ from ‘size_t’ may alter its value [-Wconversion]
return mik();
^
being operators, the only warnings you can expect are overflow:
char nov[UINT16_MAX];
// large integer implicitly truncated to unsigned type [-Woverflow]
uint8_t osc = sizeof nov;
or improper use of cast:
uint32_t nov = 1;
// conversion to ‘uint32_t’ from ‘int’ may
// change the sign of the result [-Wsign-conversion]
uint32_t osc = (int32_t) nov;
// conversion to ‘uint8_t’ from ‘short unsigned int’ may alter its value [-Wconversion]
uint8_t pap = (uint16_t) nov;
now improper use of cast is actually what was done in the question too:
xr = (size_t) ya;
Here is the difference:
if the target type is signed, the behavior is implementation-defined (which
may include raising a signal)

Answer 2

I would say the compiler is smart enough. Consider the below program :
int xr = -2;
int ya = -3;
size_t zu = 4;
xr = zu;
zu = ya;
/* Added test case
* A negative 'ya' when converted to size_t should change its value.
*/
printf("zu : %zu , but is this what you expected\n",zu);
xr = (size_t) ya;
xr = sizeof ya;
Compilation gives :
gcc -Wall -Wconversion 38257604.c -o 38257604
38257604.c: In function ‘main’:
38257604.c:11:3: warning: conversion to ‘int’ from ‘size_t’ may alter its value [-Wconversion]
xr = zu;
^
38257604.c:12:3: warning: conversion to ‘size_t’ from ‘int’ may change the sign of the result [-Wsign-conversion]
zu = ya;
^
38257604.c:8:7: warning: variable ‘xr’ set but not used [-Wunused-but-set-variable]
int xr = -2;
^
Moral
in C, you're given the responsibility of type-checking. So better take heed of
Type Size
Sign of the value, important in the case where both types, in question, have same size.
Default compiler flags, for example -Wsign-conversion is not enabled by default.
when you write statements involving type conversions.