CODEWITHSUNDEEP
cheaderstructinitializationgnu
user548800
user548800

Reputation: 71

How to initialize a a struct of structs inside a header file in C?

I am trying to port some old code over from a 20 year old DOS system to a GNU Linux system. In several of their header files, (which are included all over the place), they have structs of structs that they declare and initialize. I am getting warnings when I compile with the way the legacy code was written. Any tips on how I can get this to work with staying inside the same header file?

The following is a simplified example I made of what they are doing.

struct A
{

    struct B  temp1;
    struct C  temp2;
};

struct B
{

    int temp3;
    int temp4; 
    int temp5;
};

struct C
{

    int temp6;
    int temp7;
    int temp8;
};


//These are the variables in how they are related to the initialization below

//struct A test_one = {{temp3,temp4,temp5},{temp6,temp7,temp8}};

struct A test_one = {{1,2,3},{4,5,6}};

Upvotes: 4

Views: 23073

Answers (3)

EnabrenTane
EnabrenTane

Reputation: 7466

You shouldn't instantiate any structures in header files. If you do a different instance will be created in each C file you include the header in which is usually not the desired effect.

In a C file to do this you would have to do the following.

void foo(){
struct A parent;
struct B child_b;
struct C child_c;

child_b.temp3 = 3;
child_b.temp4 = 4;
child_b.temp5 = 5;

child_c.temp6 = 6;
child_c.temp7 = 7;
child_c.temp8 = 8;

parent.temp1 = child_b;
parent.temp2 = child_c;
}

I would strong consider making helper functions similar to this

void initB(struct B* s, int x, int y, int z){
    s->temp3 = x;
    s->temp4 = y;
    s->temp5 = z;
}

If you would like the keep the array initialization syntax then consider using a union.

Upvotes: 6

AnT stands with Russia
AnT stands with Russia

Reputation: 320381

The code you posted is not compilable, since it is illegal to use incomplete type to declare struct members. I assume that you simply misarranged your struct definitions: the definitions for B and C should go first.

Having said that, the only warning this code can generate is the "warning" from linker that might complain about multiple definitions of the same object test_one, if the header file is included in multiple translation units. In C this is technically illegal, although many compilers allow it as a popular compiler extension.

So, what "warnings" are you getting?

Upvotes: 0

mvladic
mvladic

Reputation: 1189

Declare struct B and C before A, i.e. :

struct B { int temp3; int temp4; int temp5; };
struct C { int temp6; int temp7; int temp8; };
struct A { struct B temp1; struct C temp2; };

Upvotes: 1

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