How to read value from struct from GList?

Question

I can't read back the good value from GList.

I declare a struct, and initialize its a variable with a number in the for cycle then add to g_list_append.

The g_list_nth_data returns a pointer; I try to cast it to my struct type and read its member, but I get 9.

What am I doing wrong?

#include "glib.h"
#include <stdio.h>



typedef struct MyType
{
    int a;
}mytype_t;



int main(int argc, char* argv[])
{
    printf("start\n");

    mytype_t myt;

    GList *l = NULL;

    for(int i=0;i<10;i++)
    {
        myt.a = i;
        l = g_list_append(l, &myt);
    }


    printf("length: %d\n", g_list_length(l));

    printf("5th item a value: %d\n", ((mytype_t*)g_list_nth_data(l,4))->a );



    return 0;
}

output:
    start
    length: 10
    5th item value: 9

Answer 1

You need to allocate a copy of mytype_t on the heap for each instance of it which you append to the linked list. Observe the value of &myt in each g_list_append() call: it will be identical for each value of i, since it’s referring to the same chunk of stack memory.

You can do that with:

mytype_t *myt = g_new0 (mytype_t, 1);
myt->a = i;
l = g_list_append (l, myt);

in your loop.

Note that calling g_list_append() in a loop has a cost of O(N^2), as it has to iterate over the length of the list each time to find the end. In general, using a linked list is a bad idea, and a pointer array (GPtrArray in GLib) is usually a much more appropriate data structure.