Problems with const in c when having typedef and array [duplicate]

Question

I have the following code:

typedef float vec3_t[3];

void f(const vec3_t v[2]){
    // do stuff
}

int main(int argc, char * argv[]){
    vec3_t v[2];
    v[2][1] = 1;
    f(v);
    return 0;
}

which will not compile using

gcc main.c -std=gnu99 -O0 -o main

but give the error

main.c: In function ‘main’:'
main.c:293:5: warning: passing argument 1 of ‘f’ from incompatible pointer type [enabled by default]
     f(v);
     ^
main.c:286:6: note: expected ‘const float (*)[3]’ but argument is of type ‘float (*)[3]’
 void f(const vec3_t v[2]){
      ^

If I on the other hand remove the const requirement in the function f. It all works well. I can not figure out what is wrong??

As mentioned in my dupe link, this is probably a flaw in C type system. I think this highlights why hiding pointers or arrays behind typedef is not a good idea. — user694733
– user694733, Commented May 4, 2016 at 14:01
It is not an exact duplicate, because the linked question was regarding array pointers. In this case there are 2D arrays. The answer will be pretty much the same though. — Lundin
– Lundin, Commented May 4, 2016 at 14:30
Anyway, I would strongly recommend to not hide arrays behind typedefs. If you need a distinct type, put the array inside a struct. Then make an array of such structs. Coincidentally, that would have solved this problem too. — Lundin
– Lundin, Commented May 4, 2016 at 14:36
@user694733: Ok, but actually this issue does not arise from the typedef. Type out the float[3] and you get the same compiler warning. — Robert
– Robert, Commented May 4, 2016 at 14:39

Brian Zammit · Accepted Answer · 2016-05-04 14:01:32Z

0

Why not cast the parameter to const?

typedef float vec3_t[3];

void f(const vec3_t v[2]){
    // do stuff
}

int main(int argc, char * argv[]){
    vec3_t v[2];
    v[2][1] = 1;
    f((const vec3_t*)v);
    return 0;
}

answered May 4, 2016 at 14:01

Brian Zammit

1896 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Jonathan Leffler Over a year ago

Reaching for a cast is not a good idea. The compiler complains for a reason. Unless you know more than your compiler, simply casting is likely to lead to trouble, sooner or later.

Lundin Over a year ago

Implementing a safe "const cast" through C11 _Generic was one possible solution discussed in the linked duplicate, see for example the answer by chux. If it is a good idea or not, I'm not sure. But it would be 100% safe, unlike the raw cast here.

Collectives™ on Stack Overflow

Problems with const in c when having typedef and array [duplicate]

1 Answer 1

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Linked

Related