Question

In the make_quad() function below, how do I set the default values for the vertex_color array in the quad_t structure?

/* RGBA color */
typedef struct {
    uint8_t r,g,b,a;
} rgba_t;

/* Quad polygon - other members removed */
typedef struct {
    rgba_t vertex_color[ 4 ];
} quad_t;

Elsewhere, a function to make and init a quad:

quad_t *make_quad() {
    quad_t *quad = malloc( sizeof( quad_t ) );
    quad->vertex_color = ??? /* What goes here? */
    return ( quad );
}

Obviously I can do it like this:

quad->vertex_color[ 0 ] = { 0xFF, 0xFF, 0xFF, 0xFF };
...
quad->vertex_color[ 3 ] = { 0xFF, 0xFF, 0xFF, 0xFF };

but this:

quad->vertex_color = {
    { 0xFF, 0xFF, 0xFF, 0xFF },
    { 0xFF, 0xFF, 0xFF, 0xFF },
    { 0xFF, 0xFF, 0xFF, 0xFF },
    { 0xFF, 0xFF, 0xFF, 0xFF }
};

…results in “error: expected expression before ‘{‘ token“.

EDIT: Fixed a couple typos

Answer 1

There were a number of errors in your code, I’ve corrected them all here. quad_t was already defined in <sys/types.h> so I changed it to myquad_t.

Basically, you can’t statically init memory that’s dynamically allocated at run-time, so what you can do is create a global which is initialized at compile time which you then use memcpy to copy into your dynamically allocated memory.

Here is an explanation of designated initializers.

#include    <stdlib.h>
#include    <string.h>
#include    <stdint.h>

/* RGBA color */
typedef struct rgba {
    uint8_t r,g,b,a;
} rgba_t;

/* Quad polygon - other members removed */
typedef struct quad {
    rgba_t vertex_color[ 4 ];
} myquad_t;

static const myquad_t init_quad = {
     .vertex_color[ 0 ... 3 ] = { 0xFF, 0xFF, 0xFF, 0xFF } 
    };    

myquad_t *make_quad() {
    myquad_t *q = malloc( sizeof( myquad_t ) );
    memcpy(q , &init_quad, sizeof( myquad_t ) );
    return ( q );
}    

int main(void) {

    myquad_t * q = make_quad();

    return 0;
}