CODEWITHSUNDEEP
ctypes
Austin
Austin

Reputation: 7349

Is there a way to specify int size in C?

I'm trying to check some homework answers about overflow for 2's complement addition, subtraction, etc. and I'm wondering if I can specify the size of a data type. For instance if I want to see what happens when I try to assign -128 or -256 to a 7-bit unsigned int.

Upvotes: 1

Views: 3259

Answers (4)

viraptor
viraptor

Reputation: 34205

For an arbitrary sized value, you can use bitfields in structs. For example for a 7-bit value:

struct something {
   unsigned char field:7;
   unsigned char padding:1;
};
struct something value;
value.field = -128;

Upvotes: 1

waterjuice
waterjuice

Reputation: 849

On further reading I see you wanted bit sizes that are not normal ones, such as 7 bit and 9 bit etc. You can achieve this using bitfields

struct bits9
{
    int x : 9;
};

Now you can use this type bits9 which has one field in it x that is only 9 bits in size. 

struct bits9    myValue;

myValue.x = 123;

Upvotes: 7

waterjuice
waterjuice

Reputation: 849

Using built in types you have things like:

 char      value1;      // 8 bits
 short     value2;      // 16 bits
 long      value3;      // 32 bits
 long long value4;      // 64 bits

Note this is the case with Microsoft's compiler on Windows. The C standard does not specify exact widths other than "this one must be at least as big as this other one" etc. If you only care about a specific platform you can print out the sizes of your types and use those once you have figured them out.

Alternatively you can use stdint.h which is in the C99 standard. It has types with the width in the name to make it clear

int8_t    value1;    // 8 bits
int16_t   value2;    // 16 bits
int32_t   value3;    // 32 bits
int64_t   value4;    // 64 bits

Upvotes: 1

chasep255
chasep255

Reputation: 12185

The smallest size you have have is char which is an 8 bit integer. You can have unsigned and signed chars. Take a look at the stdint.h header. It defines a int types for you in a platform independent way. Also there is no such thing as an 7 bit integer.

Upvotes: 0

Related Questions