access member of a struct in struct

Question

I am a newbie in C. My problem is quite simple. Below is my code. I expect it to increase req_id by 1 and then pint out 1. However, the result is 0.

typedef uint32_t req_id_t;

typedef struct view_stamp_t{
    req_id_t req_id;
}view_stamp;

struct consensus_component_t{
    view_stamp highest_seen_vs;
};
typedef struct consensus_component_t consensus_component;

static void view_stamp_inc(view_stamp vs){
    vs.req_id++;
    return;
};

int main()
{
    consensus_component* comp;
    comp = (consensus_component*)malloc(sizeof(consensus_component));
    comp->highest_seen_vs.req_id = 0;
    view_stamp_inc(comp->highest_seen_vs);
    printf("req id is %d.\n", comp->highest_seen_vs.req_id);
    free(comp);
    return 0;
}

The given code doesn't compile because of `comp->highest_seen_vs.view_id = 1;`. Also, you forgot to `free`. — Spikatrix, Feb 18 '16 at 06:50
[Don't cast malloc in C](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, Feb 18 '16 at 06:51
@CoolGuy Thanks for your reply. That's a dummy variable I forgot to delete. — HuangJie, Feb 18 '16 at 06:52
Passing by reference or pointer should fix one of the problems. — gilgamash, Feb 18 '16 at 06:54

score 4 · Answer 1 · answered Feb 18 '16 at 06:54

When you call functions in C, parameters are passed by value, not by reference. So vs in view_stamp_inc is a copy of comp->highest_seen_vs. Incrementing req_id in the copy has no effect on the original structure.

You need to pass the address of the structure.

static void view_stamp_inc(view_stamp *vs) {
    vs->req_id++;
    return;
}

...

view_stamp_inc(&comp->highest_seen_vs);

score 2 · Answer 2 · answered Feb 18 '16 at 06:53

2

To change the original object passed as an argument to a function it should be passed to the function by reference.

For example

static void view_stamp_inc(view_stamp *vs){
    vs->req_id++;
};

//...

view_stamp_inc( &comp->highest_seen_vs );

answered Feb 18 '16 at 06:53

Vlad from Moscow

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access member of a struct in struct

2 Answers2