CODEWITHSUNDEEP
coffsetof
accasio
accasio

Reputation: 185

Using offsetof with a float in c

the code works fine for an int but when I want to use a float it fails unless I cast the structure as a character pointer. Here's what it looks like:

struct test
{
    float a;
    float b;
};

void stuff(int offset, test* location);

int main()
{
    test *t;
    t = (test*)malloc(sizeof(test));

    char choice = '\0';

    //Find the byte offset of 'a' within the structure
    int offset;
    printf("Edit a or b?");
    scanf("%c", &choice);
    switch (toupper(choice))
    {

    case 'A':
        offset = offsetof(test, a);
        stuff(offset, t);
        break;
    case 'B':
        offset = offsetof(test, b);
        stuff(offset, t);
        break;
    }
    printf("%f %f\n", t->a, t->b);
    return 0;
}

void stuff(int offset, test* location)
{
    float imput;
    printf("What would you like to put in it? ");
    scanf("%f", &imput);
    *(float *)((char *)location + offset) = imput;
    //*(float *)(location + offset) = imput   Will Not Work
}

*(float *)(location + offset)= imput Will not work for a float but casting location and the offset as an int pointer will.

I've tried looking online but I couldn't find much about the problem.

Upvotes: 4

Views: 451

Answers (1)

alain
alain

Reputation: 12047

This is because pointers have 'units', the size of the objects they point to.

Lets assume you have a pointer p that points to, say, address 1000.

if you have

int* p = 1000;
p += 10;

p will point to 1040 on 32bit machines, because an int has a size of 4 bytes.

But if you have

char* p = 1000;
p += 10;

p will point to 1010.

That's why

*(float *)((char *)location + offset) = imput;

works, but

*(float *)(location + offset) = imput   Will Not Work

doesn't.

Upvotes: 5

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