[C-prog-lang-l] Arithmetic conversions and sizes of types in limits.h

[Walter Herold Veedla]

Hello,

I was thinking about the integer promotion topic that we talked about 
this Monday and I think I have an interesting observation that was not 
explicitly mentioned in class as well as a question.

I dug around in limits.h 
(https://opensource.apple.com/source/xnu/xnu-201/EXTERNAL_HEADERS/machine/limits.h.auto.html) 
and noticed the asymmetric definition of INT_MAX and INT_MIN:

#define  __INT_MAX__  2147483647
#define  INT_MIN  (-INT_MAX-1)

Which puzzled me for a bit and made me think of the question why we do 
not define

#define  INT_MIN -2147483648

instead? After all, this is the value you get when you print out 
INT_MIN. My hunch is that what is mentioned in notes5.md under integer 
promotion (but this is not integer *promotion*, is it?) is also 
happening here and the value 2147483648 does not fit the value of 
integer due to the asymmetrical range of two's complement and is first 
converted to long, before the unary minus is applied. Is that correct?


Secondly, whereas INT_MAX and INT_MIN are defined in this asymmetric 
way, CHAR_MIN, CHAR_MAX, SHRT_MIN, SHRT_MAX are not, see:

#define  CHAR_MAX  127
#define  CHAR_MIN  (-128)
#define  SHRT_MAX  32767
#define  SHRT_MIN  (-32768)

This is really strange, furthermore when printing the sizes of these 
quantities they do not correspond to the sizes of the types, but even 
CHAR_MAX is 4 bytes. See the following output:

cc -std=c99 -Wall -Wextra -pedantic limits_test.c && ./a.out         
|    1 #include <stdio.h>
sizeof(short)   = 2                                                  
|    2 #include <limits.h>
sizeof(int)     = 4                                                  |    3
sizeof(long)    = 8                                                  
|    4 int
INT_MAX = 2147483647  (4 bytes)                                      
|    5 main(void)
8 = sizeof(-2147483648) |    6 {
4 = sizeof(-INT_MAX-1) |    7         printf("sizeof(short)\t= %zu\n", 
sizeof(short));
|    8         printf("sizeof(int)\t= %zu\n", sizeof(int));
|    9         printf("sizeof(long)\t= %zu\n", sizeof(long));
SHRT_MAX = 32767  (4 bytes)                                          |   10
4 = sizeof(-32768) |   11         // The below is -INT_MAX-1 written 
directly
4 = sizeof(-SHRT_MAX-1) |   12         printf("INT_MAX = %d  (%zu 
bytes)\n", INT_MAX, sizeof(INT_MAX));
                                                                      | 
13         printf("%zu = sizeof(-2147483648)\n", sizeof(-2147483648));
                                                                      | 
14         printf("%zu = sizeof(-INT_MAX-1)\n", sizeof(-INT_MAX-1));
CHAR_MAX = 127  (4 bytes)                                            |   15
4 = sizeof(-32768) |   16         printf("\n\n");
4 = sizeof(-SHRT_MAX-1) |   17
                                                                      | 
18         printf("SHRT_MAX = %d  (%zu bytes)\n", SHRT_MAX, 
sizeof(SHRT_MAX));
                                                                      | 
19         printf("%zu = sizeof(-32768)\n", sizeof(-32768));
                                                                      | 
20         printf("%zu = sizeof(-SHRT_MAX-1)\n", sizeof(-SHRT_MAX-1));
                                                                      | 21
                                                                      | 
22         printf("\n\n");
                                                                      | 23
                                                                      | 
24         printf("CHAR_MAX = %d  (%zu bytes)\n", CHAR_MAX, 
sizeof(CHAR_MAX));
                                                                      | 
25         printf("%zu = sizeof(-32768)\n", sizeof(-32768));
                                                                      | 
26         printf("%zu = sizeof(-SHRT_MAX-1)\n", sizeof(-SHRT_MAX-1));
                                                                      | 27 }

What is going on here?

If for some reason the output of the program arrives mis-formatted, I am 
also attaching a .txt file of the output with this e-mail.


Anyways, I'd be happy to hear Your thoughts.

Kind regards

Walter Herold Veedla

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cc -std=c99 -Wall -Wextra -pedantic limits_test.c && ./a.out         |    1 #include <stdio.h>
sizeof(short)   = 2                                                  |    2 #include <limits.h>
sizeof(int)     = 4                                                  |    3
sizeof(long)    = 8                                                  |    4 int
INT_MAX = 2147483647  (4 bytes)                                      |    5 main(void)
8 = sizeof(-2147483648)                                              |    6 {
4 = sizeof(-INT_MAX-1)                                               |    7         printf("sizeof(short)\t= %zu\n", sizeof(short));
                                                                     |    8         printf("sizeof(int)\t= %zu\n", sizeof(int));
                                                                     |    9         printf("sizeof(long)\t= %zu\n", sizeof(long));
SHRT_MAX = 32767  (4 bytes)                                          |   10
4 = sizeof(-32768)                                                   |   11         // The below is -INT_MAX-1 written directly
4 = sizeof(-SHRT_MAX-1)                                              |   12         printf("INT_MAX = %d  (%zu bytes)\n", INT_MAX, sizeof(INT_MAX));
                                                                     |   13         printf("%zu = sizeof(-2147483648)\n", sizeof(-2147483648));
                                                                     |   14         printf("%zu = sizeof(-INT_MAX-1)\n", sizeof(-INT_MAX-1));
CHAR_MAX = 127  (4 bytes)                                            |   15
4 = sizeof(-32768)                                                   |   16         printf("\n\n");
4 = sizeof(-SHRT_MAX-1)                                              |   17
                                                                     |   18         printf("SHRT_MAX = %d  (%zu bytes)\n", SHRT_MAX, sizeof(SHRT_MAX));
                                                                     |   19         printf("%zu = sizeof(-32768)\n", sizeof(-32768));
                                                                     |   20         printf("%zu = sizeof(-SHRT_MAX-1)\n", sizeof(-SHRT_MAX-1));
                                                                     |   21
                                                                     |   22         printf("\n\n");
                                                                     |   23
                                                                     |   24         printf("CHAR_MAX = %d  (%zu bytes)\n", CHAR_MAX, sizeof(CHAR_MAX));
                                                                     |   25         printf("%zu = sizeof(-32768)\n", sizeof(-32768));
                                                                     |   26         printf("%zu = sizeof(-SHRT_MAX-1)\n", sizeof(-SHRT_MAX-1));
                                                                     |   27 }