What is the reason for a behavior change observed in a C program in VS2019 (compared to VS2017)?

Question

Given is the following well defined C90/C99 program test.c (please assume C, not C++!) :

#include <stdio.h>
void* p1; 
void* p2; 
int i[2][2]={{1,2},{3,4}}; 
int main(void) { 
  p1 = (void*)&i[0]; 
  p2 = (void*)&*(int(*)[2])p1; 
  printf("%d", p1==o2); 
  return 0; 
}

Compiling this in a "x64 Native Tools Command Prompt for VS 2017" (cl.exe Version 19.16.27051 for x64) with "cl test.c" and executing the resulting program with ".\test.exe" induces the expected output "1".

In contrast, compiling this in a "x64 Native Tools Command Prompt for VS 2019" (cl.exe Version 19.29.30157 for x64) with "cl test.c" and executing the resulting program with ".\test.exe" induces the unexpected output "0".

When compiling instead with option '/Za' ("Disable Language Extensions"), also VS 2019 produces the expected output "1".

Which language extension induces this changed, unexpected behavior in VS 2019?

Answer 1

Hi, @schroeder

This problem also exists in VS2022. Tested with cl version 19.22, the output is 1. cl 19.23 outputs 0.

Please report the problem to Developer Community and post link here so that we can follow up.

Best regards,

Minxin Yu

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