UB49

Author: PeiHong Dai

Definition

• An expression having signed promoted type is left-shifted and either the value of the expression is negative or the result of shifting would not be representable in the promoted type (6.5.7).
• 左移具有带符号提升类型的表达式时，表达式的值是负数或移位结果不能用提升类型表示

Description

• If \(E1\) has a signed type and nonnegative value, and \(E1 \times 2^{E2}\) is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined. (P85)

Code

UB49.c

#include  <stdio.h>
int main(void)
{
    char src = 100;
    int shift = 29;
    printf("%d\n", src << shift);
    return 0;
}

Configurations

gcc 7.4.0

OS: Microsoft Windows 11 22H2

gcc -v : gcc version 7.4.0, x86_64-w64-mingw32

compile and run commands: gcc -o UB49.exe UB49.c && ./UB49.exe

clang 6.0.0

OS: Microsoft Windows 11 22H2

clang -v : clang version 6.0.0, x86_64-w64-mingw32

compile and run commands: clang -o UB49.exe UB49.c && ./UB49.exe

Behaviors

gcc 7.4.0

compilation successful
output:
-2147483648

clang 6.0.0

compilation successful
output:
-2147483648

Advice

• When shifting types (especially signed) with promotions, double check for boundary violations.

• Table of common Promotions in C

• Original	Type Usually Promoted To
  char	int
  bit-field	int
  enum	int
  unsigned char	int
  short	int
  unsigned short	int
  float	double
  array of anything	pointer to anything