UB03

author: Xiangzhi Liu.

Definition

Token concatenation produces a character sequence matching the syntax of a universal character name (5.1.1.2).

Description

universal character name 语法定义如下

• \uxxxx
• or \Uxxxxxxxx

其中 x 代表 16 进制数字。例如 \uabcd 或者 \U12345678 满足正确的语法要求。

Token concatenation 指预处理中的 ## 运算符。

Code

UB3.c

#include<stdio.h>
#define C(x,y) x##y
#define X(x) #x
#define Y(x) X(x)

int main()
{
    printf("%s\n",Y(C(\u,0040)));
    return 0;
}

Configurations

Configuration 1.

gcc version 4.9.2

Target: x86_64-w64-mingw32

Behaviors

Configuration 1.Configuration 2.

Compiled and run successfully with no warning.

Output: @

Compiled and run successfully with no warning.

Output: @

Advice

预处理 token 中不要包含不完整的 universal character name