size of an int

[axneer]

i am newbie in c ,i want to know on what parameters the size of int or char or float is decided , i mean does it depend on compiler only or "compiler and OS" or combination of all above and processor or RAM we are using

like...If we are using a 32 bit processor and OS is 16 bit , then will the size of int be 16 bits ?

[Banfa]

The size of a char is always 1 byte, the C standard guarantees that. The number of bits in a byte is platform dependent.

The size of int should be the size that is most efficient for the platform to process (16 bits on a 16 bit processor etc) but that is not always the case.

For this post the platform should be just the processor but the compiler can have an effect too.

[Arepi]

Hi!
You can take the size of en variable from softwer whit operator sizeof. This give the size of actual variable in bytes.
use is simple:

int a,b;
a=sizeof b;

[Ganon11]

With, of course, parentheses around the argument list:

int a, b;
a = sizeof(b);

[Banfa]

Actually the parentheses are only required if the argument is a type rather than an expression so

int a, b;
a = sizeof b;       // OK
a = sizeof(b);      // OK
a = sizeof(int);    // OK
a = sizeof int;     // NOT OK
 

Personally when using the sizeof operator I try to take the size of an expression rather than a type, because it requires a little less maintenance.

[newb16]

@axneer
It depends only on compiler used ( and its settings ) - old borland C have 16-bit int wherever you run it, and you can as well compile some program with dos extender and run it with 32-bit int on 16-bit dos.

[Ganon11]

@Banfa
What!? Absurd. I didn't know C could look like Perl.

[Banfa]

Remember sizeof is an operator, it is actually more absurd that it must have parentheses in 1 case, I mean you would write -b not -(b) normally wouldn't you?

[JosAH]

@Ganon11
No it isn't absurd; the parentheses disambuigate several matters; have a look:

typedef int t;
 
int main() {
    t u;
    char t= 42;
    double d= sizeof t * 3.14159;
    double e= sizeof (u * 3.14159);
    double f= sizeof (u) * 3.14159;
    printf("%f\n", d);
    printf("%f\n", e);
    printf("%f\n", f);
    return 0;
}
 

Play with the parentheses and see what happens.

kind regards,

Jos

[donbock]

Integer types in Standard C have the following characteristics:
The width of 'char' is at least 8 bits;
the width of 'short' is at least 16 bits;
the width of 'long' is at least 32 bits;
the width of 'long long' is at least 64 bits;
the width of 'int' is greater than or equal to that of 'short' and less than or equal to that of 'long'.

An implementation is expected to select a size for 'int' that has the best performance on that platform and that also meets the preceding width constraints.

Refer to <limits.h> for these details for your particular implementation. This header specifies the number of bits in a 'char' (which is also the number of bits in a "storage unit", see below). Other than that, this header specifies the largest and smallest values that can be stored in each integral type rather than the width of the type in bits. Different implementations may use different encoding schemes for integers, so the same width in bits might corresponds to a different range of values.

The sizeof operator returns the size of a type or data object (in "storage units"). By definition, one storage unit is the amount of memory used to hold a 'char'; that is, sizeof(char) is always one. We like to think that sizeof returns a value in bytes, but that is only typically true. For example, an inefficient compiler implementation might store all integer types (including char) in 64-bit blocks of memory. If so, then sizeof any and all integer types would be one.

[Banfa]

Which C standard are you talking about Don because what you have said is what I understand for C++ but for C89 my understanding is that

char is 1 byte which has CHAR_BIT (defined in limits.h) number of bits that can be <8 (I have heard of definite cases of 7 (and 9) although I can't put my fingers on the platform names)

and that

sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long)

(I realise I haven't mentioned long long)

[Arepi]

@Ganon11
Let me to quote from "Brian W. Kernighan. Dennis M. Ritchie :The C programming language"

url:<URL Removed>
On site 111:

C provides a compile-time unary operator called sizeof that can be used to compute the size
of any object. The expressions
sizeof object
and
sizeof (type name)

[donbock]

My comments were intended to apply to C99. Refer to paragraph 5.2.4.2.1 (Sizes of integer types) of the Standard.

... Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.
 
CHAR_BIT        8
SCHAR_MIN    -127    // -(2^7 - 1)
SCHAR_MAX    +127    // 2^7 - 1
UCHAR_MAX    255    // 2^8 - 1
SHRT_MIN    -32767    // -(2^15 - 1)
SHRT_MAX    +32767    // 2^15 - 1
USHRT_MAX    65535    // 2^16 - 1
LONG_MIN    ...    // -(2^31 - 1)
LONG_MAX    ...    // 2^31 - 1
ULONG_MAX    ...    // 2^32 - 1
LLONG_MIN    ...    // -(2^63 - 1)
LLONG_MAX    ...    // 2^63 - 1
ULLONG_MAX    ...    // 2^64 - 1
 

These CHAR_BIT constraint permits support for a 9-bit char, but appears to prohibit a 7-bit char. The other constraints refer to minimum and maximum values, not number of bits. I can't imagine any integer encoding scheme that satisfies these constraints that wouldn't require at least 16 bits for short, 32 bits for long, or 64 bits for long long.

I know C89 also had minimum values for these parameters, but I don't know what they were. I don't have a copy of C89 nearby.

Regarding the sizeof hierarchy, I agree that every implementation I've ever seen matches the sequence you listed. Notice that here we are talking about a hierarcy of storage allocation, not a hierarchy of maximum values. Suppose a processor provided support for 8-bit, 16-bit, and 32-bit accesses -- but suppose its operation was much faster if all accesses were aligned on 32-bit boundaries. I think the Standard would permit an implementation to allocate 32 bits for 8- and 16-bit variables, even though many of those bits would be unused padding. If so, sizeof would return the same value for char, short, and long. I can't quote chapter and verse from the Standard for this one. We know sizeof a struct includes all pad bytes.

[Banfa]

It is possible that the representation of a short or int or long includes bits or bytes not used to hold the actual value (bits used as padding or bits used for trap values).

However the story for char (or unsigned char at least) is different since the standard expressly dictates that using this type the CPU can contiguously access all bytes of memory. That precludes use of padding bytes in a char type to align the type to an efficient memory boundary for the processor.

[donbock]

@Banfa
Good point. You're right.

[AmberJain]

Hello,
@Arepi
The reply #12 contains a download link to the book "The C programming language". Now that link might violate the copyrights for the book. Admins/mods ahould consider removing that link.

Also, I could not find report button for this thread and so I replied in this thread.

Thanks....
AmbrNewlearner

[boxfish]

The report button is the little red and white triangle with the black line in the top right corner.

[axneer]

The author of my book is saying
"on a 16 bit OS like MS-DOS or windows 3.1 an integer occupies 2 bytes,In case of 32 bit Os like windows 95/98/Me/NT the size of int is 4 bytes"

also he has given a table like

data type range
signed char (-128 to 127)
unsigned char (0 to 255 )
signed int (-32768 to 32767)
like so on------

and said note at the bottom "the sizes and ranges of int,short and long are OS dependent.Sizes in figures are for 16 bit OS"

is he wrong or partially correct?

[newb16]

@axneer
He is correct about ranges for 16- and 8-bit values. But sizes for builtin types are defined by compiler, not OS. If he did not explained it as 'sizes of values accepted by OS API functions' it is wrong - e.g. 32bit program under dos extender under dos.

[Banfa]

@ambrnewlearner
Good catch but by quoting it you doubled the amount of clean-up I had to do :D

[AmberJain]

Hello,

The report button is the little red and white triangle with the black line in the top right corner.

Oh ok...I get it now. ty.

Good catch but by quoting it you doubled the amount of clean-up I had to do :D

OOPS..I almost forgot that. ;)

AmbrNewlearner