"C variable type sizes are machine dependent." Is it really true? signed & unsigned numbers ;
- by claws
Hello,
I've been told that C types are machine dependent. Today I wanted to verify it.
void legacyTypes()
{
/* character types */
char k_char = 'a';
//Signedness --> signed & unsigned
signed char k_char_s = 'a';
unsigned char k_char_u = 'a';
/* integer types */
int k_int = 1; /* Same as "signed int" */
//Signedness --> signed & unsigned
signed int k_int_s = -2;
unsigned int k_int_u = 3;
//Size --> short, _____, long, long long
short int k_s_int = 4;
long int k_l_int = 5;
long long int k_ll_int = 6;
/* real number types */
float k_float = 7;
double k_double = 8;
}
I compiled it on a 32-Bit machine using minGW C compiler
_legacyTypes:
pushl %ebp
movl %esp, %ebp
subl $48, %esp
movb $97, -1(%ebp) # char
movb $97, -2(%ebp) # signed char
movb $97, -3(%ebp) # unsigned char
movl $1, -8(%ebp) # int
movl $-2, -12(%ebp)# signed int
movl $3, -16(%ebp) # unsigned int
movw $4, -18(%ebp) # short int
movl $5, -24(%ebp) # long int
movl $6, -32(%ebp) # long long int
movl $0, -28(%ebp)
movl $0x40e00000, %eax
movl %eax, -36(%ebp)
fldl LC2
fstpl -48(%ebp)
leave
ret
I compiled the same code on 64-Bit processor (Intel Core 2 Duo) on GCC (linux)
legacyTypes:
.LFB2:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
movb $97, -1(%rbp) # char
movb $97, -2(%rbp) # signed char
movb $97, -3(%rbp) # unsigned char
movl $1, -12(%rbp) # int
movl $-2, -16(%rbp)# signed int
movl $3, -20(%rbp) # unsigned int
movw $4, -6(%rbp) # short int
movq $5, -32(%rbp) # long int
movq $6, -40(%rbp) # long long int
movl $0x40e00000, %eax
movl %eax, -24(%rbp)
movabsq $4620693217682128896, %rax
movq %rax, -48(%rbp)
leave
ret
Observations
char, signed char, unsigned char, int, unsigned int, signed int, short int, unsigned short int, signed short int all occupy same no. of bytes on both 32-Bit & 64-Bit Processor.
The only change is in long int & long long int both of these occupy 32-bit on 32-bit machine & 64-bit on 64-bit machine.
And also the pointers, which take 32-bit on 32-bit CPU & 64-bit on 64-bit CPU.
Questions:
I cannot say, what the books say is wrong. But I'm missing something here. What exactly does "Variable types are machine dependent mean?"
As you can see, There is no difference between instructions for unsigned & signed numbers. Then how come the range of numbers that can be addressed using both is different?
I was reading http://stackoverflow.com/questions/2511246/how-to-maintain-fixed-size-of-c-variable-types-over-different-machines I didn't get the purpose of the question or their answers. What maintaining fixed size? They all are the same. I didn't understand how those answers are going to ensure the same size.
