data structures

This is a discussion on data structures within the C Programming forums, part of the General Programming Boards category; Code: #include<stdio.h> #include<stdlib.h> struct foo { unsigned int a:16; unsigned int b:8; unsigned int c:4; } *fooptr = NULL; int ...

  1. #1
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    data structures

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    
    struct foo
    {
        unsigned int a:16;
        unsigned int b:8;
        unsigned int c:4;
    } *fooptr = NULL;
    
    int main( void )
    {
        if( (fooptr = malloc( sizeof *fooptr )) == NULL )
            printf( "malloc failed, that's bad\n" );
        else
        {
            fooptr->a = 1;
            fooptr->b = 2;
            fooptr->c = 3;
            free( fooptr );
        }
        return 0;
    }
    In this sorta of example i just wanted to know how to assign a whole number when you split up the int into a b c but if i know the whole unsigned int could i just assign it without
    fooptr->a = 1;
    fooptr->b = 2;
    fooptr->c = 3;

  2. #2
    ATH0 quzah's Avatar
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    You wouldn't. Your structure doesn't allow for that. You are restricting the size of the varaible's allowed values here. Also, you aren't using a union, so it's not an "either or" sort of thing. It's a structure, that typically means it has multiple members.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
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    So theres not shortcut to defining the members all together its just a whole bunch of multiple definitions

  4. #4
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    Is this what you're looking for?
    Code:
    #include <stdlib.h>
    #include <stdio.h>
    
    struct foo 
    {
        unsigned int a:16;
        unsigned int b:8;
        unsigned int c:8;
    };
    
    struct bar 
    {
        union
        {   
            struct foo f;
            unsigned int i;
        };  
    };
    
    int main(void)
    {
        struct bar b1; 
    
        b1.i = 0x01234567;
    
        printf("a = %x\n", b1.f.a);
        printf("b = %x\n", b1.f.b);
        printf("c = %x\n", b1.f.c);
    
        return 0;
    }
    bit∙hub [bit-huhb] n. A source and destination for information.

  5. #5
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    I figured it out I wanted something like this to define the members in one go

    *(unsigned int *)&fooptr = 0x234324;

  6. #6
    ATH0 quzah's Avatar
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    That assumes of course that your structure is packed the way you expect it to be packed...


    Quzah.
    Hope is the first step on the road to disappointment.

  7. #7
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    You should malloc for sizeof foo (the struct) instead of the pointer's size.

  8. #8
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    Quote Originally Posted by nonoob View Post
    You should malloc for sizeof foo (the struct) instead of the pointer's size.
    He's not using the size of the pointer, he is using the size of the pointer's type. In other words, the way he has it is correct.
    bit∙hub [bit-huhb] n. A source and destination for information.

  9. #9
    ATH0 quzah's Avatar
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    Actually he isn't using anything. That's my code example from another thread.


    Quzah.
    Hope is the first step on the road to disappointment.

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