General misunderstanding over C typedef struct, and relationship to pointers

Question

I think I'm very much confused over how I should be declaring structs and typedefs in C. I've been reading over - and according to this answer, what I'm doing should be correct. I'm trying to declare a simple type (essentially just a vector at the moment), and use it with pointers, but something is very wrong, and I keep getting the error (other compiler output included, including command):

gcc main.c -o simulator -lm -Wall -std=c99
main.c: In function ‘main’:
main.c:20:3: error: incompatible type for argument 1 of ‘init_agent’
   init_agent(agent_list[i]);
   ^
main.c:9:6: note: expected ‘struct agent *’ but argument is of type ‘agent’
 void init_agent(agent *a)
      ^
make: *** [main] Error 1
[Finished in 0.0s with exit code 2]

My code is as follows:

#include <stdlib.h>

typedef struct agent_s
{
    float x;
    float y;
}agent;

void init_agent(agent *a)
{
    a->x = (float)(rand()%100)/10;
    a->y = (float)(rand()%100)/10;
}

int main(int argc, char** argv)
{
    int agent_count = 10;
    agent* agent_list = malloc(sizeof(agent)*agent_count);
    for(int i = 0;i<agent_count;i++)
        init_agent(agent_list[i]);

    return 0;
}

I can't for the life of me work out what's wrong. I think I've done everything correctly, but the error is making me think I've done something wrong in declaring the type, or possibly the way I've declared the array.

Slight edit: I'm tired, and probably not making much sense - essentially I'd like to be able to create an agent "object", similar to c++ objects, and be able to manipulate them simply. I realise that I could just use c++, but I'm attempting to learn more about C, and so I feel like I'd be cheating somewhat.

`agent_list[i]` is an `agent`. The function wants an `agent*`. Pass `&agent_list[i]`. — Daniel Fischer, Jun 22 '13 at 19:58

score 3 · Accepted Answer · answered Jun 22 '13 at 19:58

3

the [] subscription operator dereferences pointers. What you need instead is

init_agent(&agent_list[i]);

or the equivalent

init_agent(agent_list + i);

i. e. the address of the ith item in the list, and not the struct itself.

answered Jun 22 '13 at 19:58

Yup - just realised that as well, thanks though! I think the real problem here though is that I'm an idiot, and I need some sleep. – AdamHarries Jun 22 '13 at 20:01
1

@AdamHarries Just take a nap :P I was surprised to see you making this mistake because your code looks much cleaner and much more organized (indentation, proper use of `malloc()`, etc.) than that of a typical beginner who would make such a mistake. – Jun 22 '13 at 20:03
Have done now. Thanks for the compliment! I've been using C for about 5 years now off and on, so I'm pretty ashamed to have made such a trivial mistake. – AdamHarries Jun 23 '13 at 13:33

General misunderstanding over C typedef struct, and relationship to pointers

1 Answers1