What belongs in an educational tool to demonstrate the unwarranted assumptions people make in C/C++?
I'd like to prepare a little educational tool for SO which should help beginners (and intermediate) programmers to recognize and challenge their unwarranted assumptions in C, C++ and their platforms.
Examples:
I figured that a small test program could be run on various platforms, which runs the "plausible" assumptions which are, from our experience in SO, usually made by many inexperienced/semiexperienced mainstream developers and record the ways they break on diverse machines.
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 how the most inconspicuous expression break on a different machine, if it has a undefined or implementation defined behavior. .
To achieve this I would like to ask you:
Here's the current version for the test toy:
#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("..%sn 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%% mainstreamn",100-(100*count)/total);
return 0;
}
Oh, and I made this community wiki right from the start because I figured that people want to edit my blabber when they read this.
UPDATE Thanks for your input. I've added a few cases from your answers and will see if I can set up a github for this like Greg suggested.
UPDATE : I've created a github repo for this, the file is "gotcha.c":
Please answer here with patches or new ideas, so they can be discussed or clarified here. I will merge them into gotcha.c then.
The order of evaluation of subexpressions, including
+ , - , = , * , / ), with the exception of: && and || ), ?: ), and , ) is Unspecified
For example
int Hello()
{
return printf("Hello"); /* printf() returns the number of
characters successfully printed by it
*/
}
int World()
{
return printf("World !");
}
int main()
{
int a = Hello() + World(); //might print Hello World! or World! Hello
/** ^
|
Functions can be called in either order
**/
return 0;
}
sdcc 29.7/ucSim/Z80
We like to think that:
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..19-2 short<int
but 'sizeof(short)<sizeof(int)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
..25 pointer arithmetic works outside arrays
but '(diff=&var.int2-&var.int1, &var.int1+diff==&var.int2)' is false.
From what I can say with my puny test cases, you are Stop at 0x0013f3: (106) Invalid instruction 0x00dd
printf crashes. "O_O"
gcc 4.4@x86_64-suse-linux
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..14 i++ is strictly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..26 sizeof() does not evaluate its arguments
but '(i=10,sizeof(char[((i=20),10)]),i==10)' is false.
From what I can say with my puny test cases, you are 79% mainstream
gcc 4.4@x86_64-suse-linux(-O2)
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..14 i++ is strictly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..26 sizeof() does not evaluate its arguments
but '(i=10,sizeof(char[((i=20),10)]),i==10)' is false.
From what I can say with my puny test cases, you are 82% mainstream
clang 2.7@x86_64-suse-linux
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..14 i++ is strictly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..21a Function Arguments are evaluated right to left
but '(gobble_args(0,ltr_fun(1),ltr_fun(2),ltr_fun(3),ltr_fun(4)),ltr_result==4321)' is false.
ltr_result is 1234 in this case
..25a pointer arithmetic works outside arrays
but '(diff=&p1-&p2, &p2+diff==&p1)' is false.
..26 sizeof() does not evaluate its arguments
but '(i=10,sizeof(char[((i=20),10)]),i==10)' is false.
From what I can say with my puny test cases, you are 72% mainstream
open64 4.2.3@x86_64-suse-linux
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..21a Function Arguments are evaluated right to left
but '(gobble_args(0,ltr_fun(1),ltr_fun(2),ltr_fun(3),ltr_fun(4)),ltr_result==4321)' is false.
ltr_result is 1234 in this case
..25a pointer arithmetic works outside arrays
but '(diff=&p1-&p2, &p2+diff==&p1)' is false.
..26 sizeof() does not evaluate its arguments
but '(i=10,sizeof(char[((i=20),10)]),i==10)' is false.
From what I can say with my puny test cases, you are 75% mainstream
intel 11.1@x86_64-suse-linux
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..14 i++ is strictly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..21a Function Arguments are evaluated right to left
but '(gobble_args(0,ltr_fun(1),ltr_fun(2),ltr_fun(3),ltr_fun(4)),ltr_result==4321)' is false.
ltr_result is 1234 in this case
..26 sizeof() does not evaluate its arguments
but '(i=10,sizeof(char[((i=20),10)]),i==10)' is false.
From what I can say with my puny test cases, you are 75% mainstream
Turbo C++/DOS/Small Memory
We like to think that:
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..16 malloc()=NULL means out of memory
but '(malloc(0)!=NULL)' is false.
..19-2 short<int
but 'sizeof(short)<sizeof(int)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
..25 pointer arithmetic works outside arrays
but '(diff=&var.int2-&var.int1, &var.int1+diff==&var.int2)' is false.
..25a pointer arithmetic works outside arrays
but '(diff=&p1-&p2, &p2+diff==&p1)' is false.
From what I can say with my puny test cases, you are 81% mainstream
Turbo C++/DOS/Medium Memory
We like to think that:
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..10 void* can store function pointers
but 'sizeof(void*)>=sizeof(void(*)())' is false.
..16 malloc()=NULL means out of memory
but '(malloc(0)!=NULL)' is false.
..19-2 short<int
but 'sizeof(short)<sizeof(int)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
..25 pointer arithmetic works outside arrays
but '(diff=&var.int2-&var.int1, &var.int1+diff==&var.int2)' is false.
..25a pointer arithmetic works outside arrays
but '(diff=&p1-&p2, &p2+diff==&p1)' is false.
From what I can say with my puny test cases, you are 78% mainstream
Turbo C++/DOS/Compact Memory
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..16 malloc()=NULL means out of memory
but '(malloc(0)!=NULL)' is false.
..19-2 short<int
but 'sizeof(short)<sizeof(int)' is false.
..20 ptrdiff_t and size_t have the same size
but '(sizeof(ptrdiff_t)==sizeof(size_t))' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
..25 pointer arithmetic works outside arrays
but '(diff=&var.int2-&var.int1, &var.int1+diff==&var.int2)' is false.
..25a pointer arithmetic works outside arrays
but '(diff=&p1-&p2, &p2+diff==&p1)' is false.
From what I can say with my puny test cases, you are 75% mainstream
cl65@Commodore PET (vice emulator)
alt text http://i34.tinypic.com/2hh0zmc.png
I'll be updating these later:
Borland C++ Builder 6.0 on Windows XP
..04 a char is signed
but 'CHAR_MIN==SCHAR_MIN' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09 overshifting is *always* okay
but '(1<<BITS_PER_INT)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..16 malloc()=NULL means out of memory
but '(malloc(0)!=NULL)' is false.
..19-3 int<long
but 'sizeof(int)<sizeof(long)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
From what I can say with my puny test cases, you are 71% mainstream
Visual Studio Express 2010 C++ CLR, Windows 7 64bit
(must be compiled as C++ because the CLR compiler does not support pure C)
We like to think that:
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..14 i++ is structly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..19-3 int<long
but 'sizeof(int)<sizeof(long)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
From what I can say with my puny test cases, you are 78% mainstream
MINGW64 (gcc-4.5.2 prerelase)
-- http://mingw-w64.sourceforge.net/
We like to think that:
..05 int has the size of pointers
but 'sizeof(int)==sizeof(void*)' is false.
..05a long has at least the size of pointers
but 'sizeof(long)>=sizeof(void*)' is false.
..08 overshifting is okay
but '(1<<bits_per_int)==0' is false.
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..14 i++ is structly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..17 size_t is unsigned int
but 'sizeof(size_t)==sizeof(unsigned int)' is false.
..19-3 int<long
but 'sizeof(int)<sizeof(long)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
From what I can say with my puny test cases, you are 67% mainstream
64 bit Windows uses the LLP64 model: Both int and long are defined as 32-bit, which means that neither is long enough for a pointer.
avr-gcc 4.3.2 / ATmega168 (Arduino Diecimila)
The failed assumptions are:
..14 i++ is structly left to right
..16 malloc()=NULL means out of memory
..19-2 short<int
..21 Evaluation is left to right
..22 floating point is always IEEE
The Atmega168 has a 16 bit PC, but code and data are in separate address spaces. Larger Atmegas have a 22 bit PC!.
gcc 4.2.1 on MacOSX 10.6, compiled with -arch ppc
We like to think that:
..09a minus shifts backwards
but '(t=-1,(15<<t)==7)' is false.
..13 The smallest bits come always first
but '(t=0x1234,0x34==*(char*)&t)' is false.
..14 i++ is structly left to right
but '(i=0,a[i++]=i,a[0]==1)' is false.
..15 structs are packed
but 'sizeof(char_int)==(sizeof(char)+sizeof(int))' is false.
..19-3 int<long
but 'sizeof(int)<sizeof(long)' is false.
..22 floating point is always IEEE
but 'STDC_IEC_559_is_defined' is false.
From what I can say with my puny test cases, you are 78% mainstream
A long time ago, I was teaching C from a textbook that had
printf("sizeof(int)=%dn", sizeof(int));
as a sample question. It failed for a student, because sizeof yields values of type size_t , not int , int on this implementation was 16 bits and size_t was 32, and it was big-endian. (The platform was Lightspeed C on 680x0-based Macintoshes. I said it was a long time ago.)