Thread: size of struct

  1. #1

    Unhappy size of struct

    struct ip {
    unsigned char ip_length:4;
    unsigned char ip_version:4;
    unsigned char ip_tos;
    unsigned short ip_total_length;
    unsigned short ip_id;
    unsigned short ip_flags;
    unsigned char ip_ttl;
    unsigned char ip_protocol;
    unsigned short ip_cksum;
    unsigned int ip_source;
    unsigned int ip_dest;

    please can anyone please tell why the size of the struct is 24 bytes though if i take ip_length:4 and ip_version:4 out from the structure still the size remains the same .

    i tried playing with the structure a lot and am not sure why the size of 2 integers and one char is 12 bytes


  2. #2
    Skunkmeister Stoned_Coder's Avatar
    Join Date
    Aug 2001
    structures are normally padded so that they fit on paragraph boundaries in memory.
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  3. #3
    Registered User
    Join Date
    Sep 2001
    Fields can only be declared as ints... although if this is something that's giving your compiler pains, I assume it would give a warning.
    Callou collei we'll code the way
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  4. #4
    thanx but i need to define the ip structure field exactly as of 20 bytes but if i use

    unsigned int version:4;
    unsigned int length:4;

    then the structure size goes to 24 bytes but if i use the char then it's ok ...

    the structure is quite like the same as in <netinet/ip.h> in linux .the same is 20bytes in size in linux too for a similar structure .

    i dunno why the compiler is making such a difference when both the OS are 32 bit .

  5. #5
    the hat of redundancy hat nvoigt's Avatar
    Join Date
    Aug 2001
    Hannover, Germany
    >i need to define the ip structure field exactly as of 20 bytes

    Then you have to lookup your compiler specifications in your helpfile. You need to turn of structure padding.

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    When in doubt, read the FAQ.
    Then ask a smart question.

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