Implenting sizeOf() operator in c

thambuzk
thambuzk used Ask the Experts™
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Can any one help me in implementing the same sizeof() fuction in C.It should take the input like of
Sizeof(2)---should return 2 bytes(Integer)
sizeof(2.5)--should return 8 bytes(Double)
sizeof(2.5f)--should return 4 bytes(float)
and also it should take variable names declared in the program.
like
sizeof(x)
if x is an integer or float or a struct variable it should give me the proper results.
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Commented:
hi,

 you can directly use 'sizeof' macro.
It does take variable, data type or actual data  as an argument and returns the size.

-Rohan
Commented:
The sizeof macro only applies to compile time variables, types and constants.

To determine the size of a program variable, type or constant, use the macro directly, as suggested by Rohan.

To print the size of input values is another matter. A simple solution for the problem as stated:

       if input is a decimal number followed by an "f"
           print sizeof(float)
       if input is a decimal number
           print sizeof(double)
       if input is an integer
           print sizeof(integer)

In other words determine the type based on the input string and use sizeof() to display the size of that type.


Author

Commented:
1)see i am not intrested in the build in sizeof in c,i want my own implementation for that.
2)if condition won't work out with program variables
3)sizeof function accepts constants too.

wht i think is if we can get into the actual memory area  and declare a pointer for the same, increment the pointer if it is integer then it will increment by 2 bytes and if it is float then by 4bytes and so on...But the problem comes i don't know wht the user is gona input integer,or float or struct variable so i have to use a void pointer,which won't work out.
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Commented:
What are you trying to accomplish? Where you write "user input", do you mean input from the command line or stdin? Or do you intend to call your sizeof function at arbitrary points in your program?

If you are trying to determine size of types for user (CLI) input, then my suggestion is valid for that use. You parse the input to determine the type, and then call the built-in sizeof for that type. Note that you are converting an input string to its numeric equivalent. You only need to ensure that the input buffer is sufficiently sized.

--Eric
cup

Commented:
By constant, do you mean a constant in input text or a binary pattern in memory?  If the user input is

"Sizeof(2.0f)"

just look for the . If there is one, the answer is 8.  If an f follows the answer is 4.  If neither then the answer is 2.

If it is a binary pattern in memory then you'll have a hard time distinguishing between a char and an unsigned long long.  At run-time, all the program sees is just a bunch of binary patterns.  If you take the address of the first global variable, then you can get at all the other global stuff but you can't get to the items on the stack.  You'd probably have to examine the varargs macro closely to figure that one out.  Note that it is different for every compiler.
#define sizeof(x) &(x+1) - &(x)
Sorry...
#define sizeof(x) (&x+1) - &x

This will work only for variables

Commented:
You can't roll your own sizeof(). sizeof() can take an arbitrary expression of arbitrary type, so it certainly can't be a C function.  The pointer arithmetic version won't even work for all variables; what if the variable has the register storage class? For that matter, a type is a valid operand of sizeof; a macro can't distinguish between the name of a (possibly typedef) type and the name of a variable; it doesn't have access to the required symbol table information.
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