sizeof

This is a discussion on sizeof within the C Programming forums, part of the General Programming Boards category; Hi, My question is at what level is "sizeof" operator resolved? to elaborate, if I have sizeof(int), what decides the ...

  1. #1
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    Question sizeof

    Hi,
    My question is
    at what level is "sizeof" operator resolved?

    to elaborate,

    if I have sizeof(int), what decides the size of int? the CPU? if yes then would the sizeof(int) give the same value when executed in the same machine say once in linux and then in windows in a dual boot system?

    if compiler decides it then can it be played around? (Just a figment of my imagination.)

    thanks,
    Anoop.

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    Registered User Nutshell's Avatar
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    It's definitely determined by the hardware thats for sure, and i also believed ( not 100% ) that it's the CPU.

    Not sure for your 2nd question.

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    The Instruction Set Architecture (ISA) decides it. These are the instructions that the CPU understands and are specific to the CPU.

    Now it is possible to define diferent sizes, but then you also have to define the operations. An example of this was before a float was standardized, different implementations existed for it. These had to be defined, usually in assembly.

    If you look in limits.h file, you should see a lot if conditional logic for defining the sizes of primatives based on the processor and/or ISA.

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    sizeof

    Does that mean that every time the compiler encounters the sizeof operator, it has to switch to the kernel mode and interact with the machine. That would be too costly.
    And since if limits.h defines the limit then that means we can manipulate it and make a program crash during runtime.

    I could be terribly wrong in my assumptions so please excuse them. Its just that it has baffled me.
    Thanks
    Anoop.

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    When ever the sizeof operator is encountered it refers to the value defined in limits.h.

    Now if you changed limits.h it could cause compile and runtime errors with the application, but would not affect the operating system. Basically, limits.h is one of those files that you should not edit.

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    Registered User Nutshell's Avatar
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    so i guess basically u could have different values in limits.h in different systems.

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