从size_t转换为int时没有警告

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.