How can I return a pointer to an element inside an array of structs ?

Question

What am I doing wrong here ?

/*
 * Consider the following pseudo code !
 */
typedef struct foobar {
    unsigned char id, count;
    struct foobar *child;
} foobar;

foobar root = (foobar *) malloc( sizeof(struct foobar) );
root->child = (foobar *) malloc( sizeof(struct foobar) );

root->count++;
root->child[0].id = 1;

root->count++;
root->child[1].id = 2;

root->count++;
root->child[3].id = 3;

root->child[0].child = (foobar *) malloc( sizeof(struct foobar) );

root->child[0].child[0].count++;
root->child[0].child[0].id = 4;

root->child[1].child = (foobar *) malloc( sizeof(struct foobar) );
root->child[0].child[0].count++;
root->child[1].child[0].id = 5;

root->child[0].child[0].count++;
root->child[1].child[1].id = 6;

/* and so on */

/*
 * Function to search for an ID inside the tree,
 * it should call itself in order to go deeper into
 * the childs, but taht's not implemented here
 */
foobar *search( unsigned char id, foobar *start_node = NULL );
foobar *search( unsigned char id, foobar *start_node ) {
    if( start_node == NULL ) {
        unsigned char x;
        for( x = 0; x < root->count; x++ ) {
            if( root->child[ x ].id == id ) {
                foobar *ptr = &root->child[ x ];
                /* If I call ptr->id now, it will return the correct value */
                return &ptr;
            }
        }

    } else { /* not implemented */ }
}

/* Search the array for and ID */
foobar **ptr = this->search( 1 );
/* If I call ptr->id now, it will return memory garbage */

Answer 1

You're returning the address of the pointer you've retrieved. You should be returning the pointer itself.

Answer 2

You only malloc memory for one child, but try to set the id for up to 4 children.

It should be like this:

root->child = (foobar *) malloc( sizeof(struct foobar) * 4 );

Answer 3

root has 4 children (as you access root->child[3]), so you must allocate enough memory:

root->child = (foobar *) malloc( sizeof(struct foobar) * 4 ); //at least 4

Also, you should return the foobar pointer itself, and not a pointer to it (i.e. return ptr; instead of return &ptr;.

Answer 4

You are returning the address of a local variable from function search (return &ptr;). This object will be destroyed as soon as the search function is exited. Trying to use this memory location from outside the function will result in undefined behavior.

Answer 5

I was doing a couple things wrong.. in the code above the lines:

foobar *ptr = &root->child[ x ];
return &ptr;

Should be changed simply to return &root->child[ x ];, this will return a pointer to the memory addr of root->child[ x ].

The line foobar **ptr = this->search( 1 ); will become foobar *ptr = this->search( 1 );, this will allow to access the struct properties using the . char; -> cannot be used and will output garbage. Correct usage example: (*ptr).description.

Many thanks to adamk !