What belongs in an educational tool to demonstrate the unwarranted assumptions people make in C/C++?
我想为 SO 准备一个小小的教育工具，它可以帮助初学者(和中级)程序员识别和挑战他们在 C、 C + + 和他们的平台中的不必要的假设。
例子:
- “整数缠绕”
- "everyone has ASCII"
- “我可以把函数指针储存在虚空中”
我认为一个小型的测试程序可以在不同的平台上运行，这些平台运行“合理的”假设，根据我们在 SO 中的经验，这些假设通常是由许多缺乏经验/经验不足的主流开发人员做出的，并记录他们在不同机器上的崩溃方式。
The goal of this is not to prove that it is "safe" to do something (which would be impossible to do, the tests prove only anything if they break), but instead to demonstrate to even the most uncomprehending individual 如果最不明显的表达式具有未定义的或实现定义的行为，那么它在不同的机器上是如何破坏的。.
为了实现这一目标，我想问你们:
- 这个想法怎样才能改进呢？
- 哪些测试是好的，它们应该是什么样子的？
- 您是否会在您可以使用的平台上运行测试并发布结果，以便我们最终得到一个平台数据库，它们之间的差异以及为什么允许这种差异？
下面是测试玩具的最新版本:
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <stddef.h>
int count=0;
int total=0;
void expect(const char *info, const char *expr)
{
printf("..%s\n but '%s' is false.\n",info,expr);
fflush(stdout);
count++;
}
#define EXPECT(INFO,EXPR) if (total++,!(EXPR)) expect(INFO,#EXPR)
/* stack check..How can I do this better? */
ptrdiff_t check_grow(int k, int *p)
{
if (p==0) p=&k;
if (k==0) return &k-p;
else return check_grow(k-1,p);
}
#define BITS_PER_INT (sizeof(int)*CHAR_BIT)
int bits_per_int=BITS_PER_INT;
int int_max=INT_MAX;
int int_min=INT_MIN;
/* for 21 - left to right */
int ltr_result=0;
unsigned ltr_fun(int k)
{
ltr_result=ltr_result*10+k;
return 1;
}
int main()
{
printf("We like to think that:\n");
/* characters */
EXPECT("00 we have ASCII",('A'==65));
EXPECT("01 A-Z is in a block",('Z'-'A')+1==26);
EXPECT("02 big letters come before small letters",('A'<'a'));
EXPECT("03 a char is 8 bits",CHAR_BIT==8);
EXPECT("04 a char is signed",CHAR_MIN==SCHAR_MIN);
/* integers */
EXPECT("05 int has the size of pointers",sizeof(int)==sizeof(void*));
/* not true for Windows-64 */
EXPECT("05a long has at least the size of pointers",sizeof(long)>=sizeof(void*));
EXPECT("06 integers are 2-complement and wrap around",(int_max+1)==(int_min));
EXPECT("07 integers are 2-complement and *always* wrap around",(INT_MAX+1)==(INT_MIN));
EXPECT("08 overshifting is okay",(1<<bits_per_int)==0);
EXPECT("09 overshifting is *always* okay",(1<<BITS_PER_INT)==0);
{
int t;
EXPECT("09a minus shifts backwards",(t=-1,(15<<t)==7));
}
/* pointers */
/* Suggested by jalf */
EXPECT("10 void* can store function pointers",sizeof(void*)>=sizeof(void(*)()));
/* execution */
EXPECT("11 Detecting how the stack grows is easy",check_grow(5,0)!=0);
EXPECT("12 the stack grows downwards",check_grow(5,0)<0);
{
int t;
/* suggested by jk */
EXPECT("13 The smallest bits always come first",(t=0x1234,0x34==*(char*)&t));
}
{
/* Suggested by S.Lott */
int a[2]={0,0};
int i=0;
EXPECT("14 i++ is strictly left to right",(i=0,a[i++]=i,a[0]==1));
}
{
struct {
char c;
int i;
} char_int;
EXPECT("15 structs are packed",sizeof(char_int)==(sizeof(char)+sizeof(int)));
}
{
EXPECT("16 malloc()=NULL means out of memory",(malloc(0)!=NULL));
}
/* suggested by David Thornley */
EXPECT("17 size_t is unsigned int",sizeof(size_t)==sizeof(unsigned int));
/* this is true for C99, but not for C90. */
EXPECT("18 a%b has the same sign as a",((-10%3)==-1) && ((10%-3)==1));
/* suggested by nos */
EXPECT("19-1 char<short",sizeof(char)<sizeof(short));
EXPECT("19-2 short<int",sizeof(short)<sizeof(int));
EXPECT("19-3 int<long",sizeof(int)<sizeof(long));
EXPECT("20 ptrdiff_t and size_t have the same size",(sizeof(ptrdiff_t)==sizeof(size_t)));
#if 0
{
/* suggested by R. */
/* this crashed on TC 3.0++, compact. */
char buf[10];
EXPECT("21 You can use snprintf to append a string",
(snprintf(buf,10,"OK"),snprintf(buf,10,"%s!!",buf),strcmp(buf,"OK!!")==0));
}
#endif
EXPECT("21 Evaluation is left to right",
(ltr_fun(1)*ltr_fun(2)*ltr_fun(3)*ltr_fun(4),ltr_result==1234));
{
#ifdef __STDC_IEC_559__
int STDC_IEC_559_is_defined=1;
#else
/* This either means, there is no FP support
*or* the compiler is not C99 enough to define __STDC_IEC_559__
*or* the FP support is not IEEE compliant. */
int STDC_IEC_559_is_defined=0;
#endif
EXPECT("22 floating point is always IEEE",STDC_IEC_559_is_defined);
}
printf("From what I can say with my puny test cases, you are %d%% mainstream\n",100-(100*count)/total);
return 0;
}
我从一开始就建立了这个社区维基因为我觉得人们看到这篇文章的时候会想编辑我的胡言乱语。
感谢您的参与。我从你的回答中添加了一些案例，看看我是否能像格雷格建议的那样建立一个 github。
UPDATE: I've created a github repo for this, the file is "gotcha.c":
请在这里回答补丁或新的想法，以便他们可以在这里讨论或澄清。那我就把它们合并到 Gotcha.c 里。
