Change address of struct in C

Question

Let's say that I was given a struct and I need to assign all of it's attributes to a particular address. The code below is giving me a conditional error, but i'm not trying to evaluate it.

struct header block_o_data;
block_o_data.a = 1;
block_o_data.b = 2;
void* startingAddress = sbrk(0);
&block_o_data = *address;

Please let me know what im doing wrong.

You never change the address of an object in C during its entire life-time. The address is an object's identity. — 5gon12eder, Mar 04 '16 at 17:04

score 3 · Answer 1 · edited Mar 04 '16 at 17:05

3

In the assignment to block_o_data, you're taking its address and trying to assign a value to it. The address of a variable is not an lvalue, meaning the expression cannot appear on the left side of an assignment.

You need to declare a pointer to a struct, then assign it the address of where the values actually live:

struct header *block_o_data;
void* startingAddress = sbrk(0);
block_o_data = startingAddress;

edited Mar 04 '16 at 17:05

Jonathan Leffler

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answered Mar 04 '16 at 17:03

dbush

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Does this still work if I assign values to block_o_data before I try to set an address to it? – user287474 Mar 04 '16 at 17:05

score 1 · Accepted Answer · answered Mar 04 '16 at 17:12

1

Suppose you have a struct like this:

struct mystruct {
    int a;
    char b;
};

then you probably need something like this:

// A pointer variable supposed to point to an instance of the struct
struct mystruct *pointer;

// This is a general address represented by void*
void *addr = some_function(0);

// Cast that general address to a pointer varibale pointing to
// an instance of the struct
pointer = (struct mystruct *) addr;

// Use it!
printf("%d", pointer->a);

answered Mar 04 '16 at 17:12

frogatto

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what does the "->" operator mean? – user287474 Mar 04 '16 at 17:18
so would this be valid: "pointer->a = 1" in terms of assigning data to pointer? – user287474 Mar 04 '16 at 17:21

Change address of struct in C

2 Answers2